Measurement of Neuromuscular Performance Capacities

Neuromuscular Functional Units Purposes of Measuring Selected Neuromuscular Performance Capacities

Range of Motion and Extremes of Motion Movement Terminology • Factors Influencing ROM/EOM and ROM/EOM Measurements • Instrumented Systems Used to Measure ROM/EOM • Key Concepts in Goniometric Measurement • Numerical Notation Systems

Strength

Strength Testing and Muscle Terminology • Factors Influencing Muscle Strength and Strength Measurement • Grading Systems and Parameters Measured • Methods and Instruments Used to Measure Muscle Strength • Key Concepts in Measuring Strength

Speed of Movement

Speed of Movement Terminology • Factors Influencing Speed of Movement and Speed of Movement Measurements • Parameters Measured • Instruments Used to Measure Speed of Movement • Key Concepts in Speed of Movement Measurement

Endurance

Endurance Terminology • Factors Influencing Neuromuscular Endurance and Measurement of Endurance • Parameters Measured • Methods and Instruments Used to Measure Neuromuscular Endurance • Key Concepts in Measuring Muscle Endurance

Susan S. Smith

Texas Woman’s University

подпись: susan s. smith
texas woman’s university
Reliability, Validity, and Limitations in Testing

Performance Capacity Space Representations

Conclusions

Movements allow us to interact with our environment, express ourselves, and communicate with each other. Life is movement. Movement is constantly occurring at many hierarchical levels including cellular and subcellular levels. By using the adjective “human” to clarify the term “movement”, we are not only defining the species of interest, but also limiting the study to observable performance and its more overt causes. Study of human performance is of interest to a broad range of professionals including rehabili­tation engineers, orthopaedic surgeons, therapists, biomechanists, kinesiologists, psychologists, and so
On. Because of the complexity of human performance and the variety of investigators, the study of human performance is conducted from several theoretical perspectives including, (1) anatomical, (2) purpose or character of the movement (such as locomotion), (3) physiological, (4) biomechanical, (5) psychological, (6) socio-cultural, and (7) integrative. The Elemental Resource Model (ERM), pre­sented at the beginning of this section, is an integrative model which incorporates aspects of the other models into a singular system accounting for the human, the task, and the human-task interface [Kon­draske, 1999].

The purposes of this chapter are to: (1) provide reasons for measuring four selected variables of human performance: extremes/range of motion, strength, speed of movement, and endurance; (2) briefly define and discuss these variables; (3) overview selected instruments and methods used to measure these vari­ables; and (4) discuss interpretation of performance for a given neuromuscular subsystem.

Neuromuscular Functional Units

While the theoretical perspectives listed previously may be useful within specific contexts or within specific disciplines, the broader appreciation of human performance and its control can be gained from the perspective of an integrative model such as the ERM [Kondraske, 1999]. This model organizes performance resources into four different domains. Basic movements, such as elbow flexion, are executed by neuromuscular functional units in the environmental interface domain. Intermediate and complex tasks, such as walking and playing the piano, utilize multiple basic functional units. A person performing a movement operates the involved functional units along different dimensions of performance according to the demands of the task. Dimensions of performance are factors such as joint motion, strength, speed of movement, and endurance. Lifting a heavy box off the floor requires, among other things, a specific amount of strength associated with neuromuscular functional units of the back, legs, and arms according to the weight and size of the box. Reaching for a light weight box from the top shelf of a closet requires that the shoulder achieve certain extremes of motion according to the height of the shelf.

Whereas four dimensions of performance are considered individually in this chapter, they are highly interdependent. For an example, strength availability during a movement is partly dependent on joint angle. Despite interdependence, considering the variables as different dimensions is essential to studying human performance. The components limiting a person’s ability to complete a task can only be identified and subsequently enhanced by determining, for example, that the reason a person cannot reach the box off the top shelf is not because of insufficient range of motion of the shoulder, but because of insufficient strength of the shoulder musculature required to lift the arm through the range of motion. Isolating the subsystems involved in a task and maximally stressing them along one or more “isolated” dimensions of performance is a key concept in the ERM. “Maximally stressing” the subsystems means that the maximum amount of the resource available is being determined. This differs from determining the amount of the resource which happened to be used while performing a particular task. Often the distinction between obtaining maximal performance vs. submaximal performance is in the instructions given to the subject. For example, in measuring speed, we say “move as fast as you can.”

Purposes of Measuring Selected Neuromuscular Performance Capacities

Range of motion, strength, speed of movement, and/or endurance can be measured for one or more of the following purposes:

To determine the amount of the resource available and to compare it to the normal value for that individual. “Normal” is frequently determined by comparisons with the opposite extremity or with normative data when available. This information may be used to develop goals and a program to change the performance.

To assist in determining the possible effects of insufficient or imbalanced amounts of the variable on a person’s performance of activities of daily living, work, sport, and leisure pursuits. In this Case, the amount of the variable is compared to the demands of the task, rather than to norms or to the opposite extremity.

To assist in diagnosis of medical conditions and the nature of movement dysfunctions.

To reassess status in order to determine the effectiveness of a program designed to change the amount of the variable.

To motivate persons to comply with treatment or training regimes.

To document status and the results of treatment or training and to communicate with other involved persons.

To assist in ergonomically designed furniture, equipment, techniques, and environments.

To provide information to combine with other measures of human performance to predict func­tional capabilities.

Range of Motion and Extremes of Motion

Range of motion (ROM) is the amount of movement that occurs at a joint. Range of motion is typically measured by noting the extremes of motion (EOM). The designated reference or zero position must be specified for measurements of the two extremes of motion. For example, to measure elbow (radiohumeral joint) flexion and extension, the preferred starting position is with the subject supine with the arm parallel to the lateral midline of the body with the palm facing upward [Norkin and White, 1995; Palmer and Epler, 1998]. Measurements are taken with the elbow in the fully flexed position and with the elbow in the fully extended position.

Movement Terminology

Joint movements are described using a coordinate system with the human body in an anatomical position. The anatomical position of the body is an erect position, face forward, arms at sides, palms facing forward, and fingers and thumbs in extension. The central coordinate system consists of three cardinal planes and axes with its origin located between the cornua of the sacrum [Panjabi, White, and Brand, 1974]. Figure 148.1 demonstrates the planes and axes of the central coordinate system. The same coordinate system can parallel the master system at any joint in the body by relocating the origin to any defined point.

The sagittal plane is the y, z plane; the frontal (or coronal) plane is the y, x plane; the horizontal (or transverse) plane is the x, z plane. Movements are described in relation to the origin of the coordinate system. The arrows indicate the positive direction of each axis. An anterior translation is +z; a posterior translation is — z. Clockwise rotations are +□, and counterclockwise rotations are — Q

Joints are described as having degrees of freedom (dof) of movement. Dof is the number of indepen­dent coordinates in a system that are necessary to accurately specify the position of an object in space. If a motion occurs in one plane and around one axis, the joint is defined as having one dof. Joints with movements in two planes occurring around two different axes, have two dof, and so on.

Angular movements refer to motions that cause an increase or decrease in the angle between the articulating bones. Angular movements are flexion, extension, abduction, adduction, and lateral flexion (See Table 148.1). Rotational movements generally occur around a longitudinal (or vertical) axis except for movements of the clavicle and scapula. The rotational movements occurring around the longitudinal axis (internal rotation, external rotation, opposition, horizontal abduction, and horizontal adduction) are described in Table 148.1. Rotation of the scapula is described in terms of the direction of the inferior angle. Movement of the inferior angle of the scapula toward the midline is a medial (or downward) rotation, and movement of the inferior angle away from the midline is lateral (or upward) rotation. In the extremities the anterior surface of the extremity is used as the reference area. Because the head, neck, trunk, and pelvis rotate about a midsagittal, longitudinal axis, rotation of these parts is designated as right or left. As can be determined from Fig. 148.1, Axial rotation of the trunk toward the left can be described mathematically as +Qy.

A communications problem often exists in describing motion using the terms defined in Table 148.1. A body segment can be in a position such as flexion, but can be moving toward extension. This confusion is partially remedied by using the form of the word with the suffix,

FIGURE 148.1 Planes and axes are illustrated in anatomical position. The central coordinate system with its origin between the cornua of the sacrum is shown. Source: White III AA, Panjabi MM. 1990. Clinical Biomechanics of the Spine, 2nd ed., p 87. Philadelphia, JB Lippincott Com­pany. With permission.

подпись: 
figure 148.1 planes and axes are illustrated in anatomical position. the central coordinate system with its origin between the cornua of the sacrum is shown. source: white iii aa, panjabi mm. 1990. clinical biomechanics of the spine, 2nd ed., p 87. philadelphia, jb lippincott company. with permission.
-ion, to indicate a static position and using the suffix, — ing, to denote a movement. Thus, an elbow can be in a posi­tion of 90D flexion and also extending.

Factors Influencing ROM/EOM and ROM/EOM Measurements

The ROM and EOM available at a joint is determined by morphology and the soft tissues surrounding and cross­ing a joint, including the joint capsule, ligaments, ten­dons, and muscles. Other factors such as age, gender, swelling, muscle mass development, body fat, passive insufficiency (change in the ROM/EOM available at one joint in a two-joint muscle complex caused by the posi­tion of the other joint), and time of day (diurnal effect) also affect the amount of motion available. Some persons, because of posture, genetics, body type, or movement habits, normally demonstrate hypermobile or hypomo­bile joints. Dominance has not been found to significantly affect available ROM. See discussion in Miller [1985]. The shapes of joint surfaces, which are designed to allow movement in particular directions, can become altered by disease, trauma, and posture, thereby, increasing or decreasing the ROM/EOM. Additionally, the soft tissues crossing a joint can become tight (contracted) or over­stretched altering the ROM/EOM.

The type of movement, active or passive, also affects ROM/EOM. When measuring active ROM (AROM), the person voluntarily contracts muscles and moves the body part through the available motion. When measuring passive ROM (PROM), the examiner moves the body part through the ROM. PROM is usually slightly greater than AROM due to the extensibility of the tissues crossing and comprising the joint. AROM can be decreased because of restricted joint mobility, muscle weakness, pain, unwillingness to move, or inability to follow instructions. PROM is assessed to determine the integrity of the joint and the extent of structural limitation.

Instrumented Systems Used to Measure ROM/EOM

The most common instrument used to measure joint ROM/EOM is a goniometer. The universal goni­ometer, shown in Fig. 148.2a, iS most widely used clinically. A variety of universal goniometers have been developed for specific applications. Two other types of goniometers are also shown in Fig. 148.2.

Table 148.2 lists and compares several goniometric instruments used to measure ROM/EOM. Choice of the instrument used to measure ROM/EOM depends upon the degree of accuracy required, time available to the examiner, the measurement environment, the body segment being measured, and the equipment available.

Non-goniometric methods of joint measurement are available. Tape measures, radiographs, photo­graphy, cinematography, videotape, and various optoelectric movement monitoring systems can also be

Movement Term

Plane

Axis

Description of Movement

Flexion

Sagittal

Frontal

Bending of a part such that the anterior surfaces approximate each other. However, flexion of the knee, ankle, foot, and toes refers to movement in the posterior direction.

Extension

Sagittal

Frontal

Opposite of flexion; involves straightening a body part.

Abduction

Frontal

Sagittal

Movement away from the midline of the body or body part; abduction of the wrist is sometimes called radial deviation.

Adduction

Frontal

Sagittal

Movement towards the midline of the body or body part; adduction of the wrist is sometimes called ulnar deviation.

Lateral flexion

Frontal

Sagittal

Term used to denote lateral movements of the head, neck, and trunk.

Internal (medial)

Horizontal

Longitudinal

Turning movement of the anterior surface of a part towards the midline of

Rotation

The body; internal rotation of the forearm is referred to as pronation.

External (lateral)

Horizontal

Longitudinal

Turning movement of the anterior surface of a part away from the midline

Rotation

Of the body; external rotation of the forearm is referred to as supination.

Opposition

Multiple

Multiple

Movement of the tips of the thumb and little finger toward each other.

Horizontal

Horizontal

Longitudinal

Movement of the arm in a posterior direction away from the midline of the

Abduction

Body with the shoulder joint in 90D of either flexion or abduction.

Horizontal

Horizontal

Longitudinal

Movement of the arm in an anterior direction toward the midline of the

Adduction

Body with the shoulder joint in 90D of either flexion or abduction.

Tilt

Depends

Depends on

Term used to describe certain movements of the scapula and pelvis. In the

On joint

Joint

Scapula, an anterior tilt occurs when the coracoid process moves in an anterior and downward direction while the inferior angle moves in a posterior and upward direction. A posterior tilt of the scapula is the opposite of an anterior tilt. In the pelvis, an anterior tilt is rotation of the anterior superior spines (ASISs) of the pelvis in an anterior and downward direction; a posterior tilt is movement of the ASISs in a posterior and upward direction. A lateral tilt of the pelvis occurs when the pelvis is not level from side to side, but one ASIS is higher than the other one.

Gliding

Depends

Depends on

Movements that occur when one articulating surface slides on the opposite

On joint

Joint

Surface.

Elevation

Frontal

A gliding movement of the scapula in an upward direction as in shrugging the shoulders.

Depression

Frontal

Movement of the scapula downward in a direction reverse of elevation.

Used to measure or calculate the motion available at various joints. These methods are beyond the scope of this chapter.

Key Concepts in Goniometric Measurement

Numerous textbooks [Clarkson and Gilewich, 1989; Norkin and White, 1995; Palmer and Epler, 1998] are available that describe precise procedures for goniometric measurements of each joint. Unfortunately, there is a lack of standardization among these references.

In general, the anatomical position of zero degrees (preferred starting position) is the desired starting position for all ROM/EOM measurements except rotation at the hip, shoulder, and forearm. The arms of the goniometer are usually aligned parallel and lateral to the long axis of the moving and the fixed body segments in line with the appropriate landmarks. In the past, some authors contended that place­ment of the axis of the goniometer should be congruent with the joint axis for accurate measurement [West, 1945; Wiechec and Krusen, 1939]. However, the axis of rotation for joints changes as the body segment moves through its ROM; therefore, a goniometer cannot be placed in a position in line with the joint axis during movement. Robson [1966] described how variations in the placement of the goniometer’s axis could affect the accuracy of ROM measurements. Miller [1985] suggested that the axis problem could be handled by ignoring the goniometer’s axis and concentrating on the accurate alignment of the arms of the goniometer with the specified landmarks. Potentially some accuracy may be sacrificed,

Measurement of Neuromuscular Performance Capacities

FIGURE 148.2 Three types of goniometric instruments used to measure range and extremes of motion are shown: (a) typical 180- and 360-degree universal goniometers of various sizes; (b) a fluid goniometer, which is activated by the effects of gravity; (c) an APM I digital electronic device that works similarly to a pendulum goniometer.

подпись: figure 148.2 three types of goniometric instruments used to measure range and extremes of motion are shown: (a) typical 180- and 360-degree universal goniometers of various sizes; (b) a fluid goniometer, which is activated by the effects of gravity; (c) an apm i digital electronic device that works similarly to a pendulum goniometer.

(b) (c)

подпись: (b) (c)But the technique is simplified and theoretically more reproducible. The subject’s movement is observed for unwanted motions that could result in inaccurate measurement. For example, a subject might attempt to increase forearm supination by laterally flexing the trunk.

Universal goniometer

A protractor-like device with one arm considered movable and the other arm stationary; protractor can have a 180Dor 360Dscale and is usually numbered in both directions; available in a range of sizes and styles to accommodate different joints (See Fig. 148.2a).

Fluid (or bubble) goniometer

A device with a fluid-filled channel with a 360°—scale that relies on the effects of gravity (See Fig. 148.2b); dial turns allowing the goniometer to be “zeroed;” some models are strapped on and others must be held against the body part.

Pendulum goniometer

A scaled, inclinometer-like device with a needle or pointer (usually weighted); some models are strapped on and others must be held against the body part (not shown).

“Myrin” OB goniometer

A fluid-filled, rotatable container consisting of compass needle that responds to the earth’s magnetic field (to measure horizontal motion), a gravity-activated inclination needle (to measure frontal and sagittal motion), and a scale (not shown).

Arthrodial protractor

A large, flat, clear plastic protractor without arms that has a level on the straight edge (not shown). APM I

Computerized goniometer with digital sensing and electronics; can perform either continuous monitoring or calculate individual ROM/EOM from a compound motion function (See Fig. 148.2c).

Electrogoniometer

Arms of a goniometer are attached to a potentiometer and are strapped to the proximal and distal body parts; movement from the device causes resistance in the potentiometer which measures the ROM (not shown).

Inexpensive; portable; familiar devices; size of the joint being measured determines size of the goniometer used; clear plastic goniometers have a line through the center of the arms to make alignment easier and more accurate; finger goniometers can be placed over the dorsal aspect of the joint being measured.

Quick and easy to use because it is not usually aligned with bony landmarks; does not have to conform to body segments; useful for measuring neck and spinal movements; using a pair of fluid goniometers permits distinguishing regional spinal motion.

Inexpensive; same advantages as for the fluid goniometer described above.

Can be strapped on the body part allowing the hands free to stabilize and move the body part; not necessary to align the goniometer with the joint axis; permits measurements in all three planes.

Does not need to conform to body segments; most useful for measuring joint rotation and axioskeletal motion.

Easy to use; provides rapid digital read­out; measures angles in any plane of motion; one hand is free to stabilize and move body segments; particularly easy for measuring regional spinal movements.

More useful for dynamic ROM, especially for determining kinematic variables during activities such as gait; provides immediate data; some electrogoniometers permit measurement in one, two, or three dimensions.

Several goniometers of different sizes may be required, especially if digits are measured; full — circle models may be difficult to align when the subject is recumbent and axis alignment is inhibited by the protractor bumping the surface; the increments on the protractors may vary from 1Q 2Q or 5Q placement of the arms is a potential source of error.

More expensive than universal goniometers; using a pair of goniometers is awkward; useless for motions in the horizontal plane; error can be induced by slipping, skin movement, variations in amount of soft tissue owing to muscle contraction, swelling, or fat, and the examiner’s hand pressure changing body segment contour; reliability may be sacrificed from lack of orientation to landmarks and difficulty with consistent realignment [Miller, 1985].

Some models cannot be “zeroed”; useless for motions in the horizontal plane; same soft tissue error concerns as described above for the fluid goniometer.

Expensive and bulky compared with universal goniometer; not useful for measuring small joints of hand and foot; susceptible to magnetic fields [Clarkson and Gilewich, 1989]; subject to same soft tissue error concerns described above under fluid goniometer.

Not useful for measuring smaller joints, especially those with lesser ROMs; usually scaled in large increments only.

Expensive compared to most other instruments described; device must be rotated perpendicular to the direction of segment motion only; unit must stabilize prior to measurement; excessive delays in recording must be avoided; subject to the same soft tissue error concerns as described above under fluid goniometer.

Aligning and attaching the device is time­consuming and not amenable to all body segments; device and equipment needed to use it are moderately expensive; essentially laboratory equipment; less accurate for measurement of absolute limb position; device itself is cumbersome and may alter the movement being studied.

Numerical Notation Systems

Three primary systems exist for expressing joint motion in terms of degrees. These are the 0-180 System, the 180-0 System, and the 360 System. The 0-180 System is the most widely accepted system in medical applications and may be the easiest system to interpret. In the 0-180 System, the starting position for all movements is considered to be 00 and movements proceed toward 1800 As the joint motion increases, the numbers on the goniometric scale increase. In the 180-0 System, movements toward flexion approach 00 and movements toward extension approach 1800 Different rules are used for the other planes of motion. The 360 System is similar to the 180-0 System. In the 360 System, movements are frequently performed from a starting position of 1800 Movements of extension or adduction which go beyond the neutral position approach 3600 Joint motion can be reported in tables, charts, graphs, or pictures. In the 0-180 System, the starting and ending ranges are recorded separately, as 0G-130Q If a joint cannot be started in the 00 position, the actual starting position is recorded, as 10G-130Q

Strength

Muscle strength implies the force or torque production capacity of muscles. However, to measure strength, the term must be operationally defined. One definition modified from Clarkson and Gilewich [1989] states that muscular strength is the maximal amount of torque or force that a muscle or muscle groups can voluntarily exert in one maximal effort, when type of muscle contraction, movement velocity, and joint angle(s) are specified.

Strength Testing and Muscle Terminology

Physiologically, skeletal muscle strength is the ability of muscle fibers to generate maximal tension for a brief time interval. A muscle’s ability to generate maximal tension and to sustain tension for differing time intervals is dependent on the muscle’s cross-sectional area (the larger the cross-sectional area, the greater the strength), geometry (including the muscle fiber arrangement, length, moment arm, and angle of pennation), and physiology. Characteristics of muscle fibers have been classified based on twitch tension and fatigability. Different fiber types have different metabolic traits. Different types of muscle fibers are differentially stressed depending on the intensity and duration of the contraction. Ideally, strength tests should measure the ability of the muscle to develop tension rapidly and to sustain the tension for brief time intervals. In order to truly measure muscle tension, a measurement device must be directly attached to the muscle or tendon. Whereas this direct procedure has been performed [Komi, 1990], it is hardly useful as a routine clinical measure. Indirect measures are used to estimate the strength of muscle groups performing a given function, such as elbow flexion.

Muscles work together in groups and may be classified according to the major role of the group in producing movement. The prime mover, or agonist, is a muscle or muscle group that makes the major contribution to movement at a joint. The antagonist is a muscle or muscle group that has an opposite action to the prime mover(s). The antagonist relaxes as the agonist moves the body part through the ROM. Synergists are accessory muscles that contract and work with the agonist to produce the desired movement. Synergists may work by stabilizing proximal joints, preventing unwanted movement, and joining with the prime mover to produce a movement that one muscle group acting alone could not produce.

A number of terms and concepts are important toward understanding the nature and scope of strength capacity testing. Several of these terms are defined below; however, there are no universally accepted definitions for these terms.

Dynamic contraction—the output of muscles moving body segments [Kroemer, 1991].

Isometric—tension develops in a muscle, but the muscle length does not change and no movement occurs.

Static—same as isometric.

Isotonic—a muscle develops constant tension against a load or resistance. Kroemer [1991] suggests the term, isoforce, more aptly describes this condition.

Concentric—a contraction in which a muscle develops internal force that exceeds the external force of resistance, the muscle shortens, and movement is produced [O’Connel and Gowitzke, 1972].

Eccentric—a contraction in which a muscle lengthens while continuing to maintain tension [O’Connel & Gowitzke, 1972].

Isokinetic—a condition where the angular velocity is held constant. Kroemer [1991] prefers the term, isovelocity, to describe this type of muscle exertion.

Isoinertial—a static or dynamic muscle contraction where the external load is held constant [Kroemer, 1983].

Factors Influencing Muscle Strength and Strength Measurement

In addition to the anatomical and physiological factors affecting strength, other factors must be considered when strength testing. The ability of a muscle to develop tension depends on the type of muscle con­traction. Per unit of muscle, the greatest tension can be generated eccentrically, less can be developed isometrically, and the least can be generated concentrically. These differences in tension generating capacity are so great that the type of contraction being strength-tested requires specification.

Additionally, strength is partially determined by the ability of the nervous system to cause more motor units to fire synchronously. As one trains, practices an activity, or learns test expectations, strength can increase. Therefore, strength is affected by previous training and testing. This is an important consider­ation in standardizing testing and in retesting.

A muscle’s attachments define the angle of pull of the tendon on the bone and thereby the mechanical leverage at the joint center. Each muscle has a moment arm length, which is the length of a line normal to the muscle passing through the joint center. This moment arm length changes with the joint angle which changes the muscle’s tension output. Optimal tension is developed when a muscle is pulling at a 90D angle to the bony segment.

Changes in muscle length alter the force-generating capacity of muscle. This is called the length-tension relationship. Active tension decreases when a muscle is either lengthened or shortened relative to its resting length. However, after applying a precontraction stretch, or slightly lengthening a muscle and the series elastic component (connective tissue) prior to a contraction causes a greater amount of total tension to be developed [Soderberg, 1992]. Of course, excessive lengthening would reduce the tension-generating capacity.

A number of muscles cross over more than one joint. The length of these muscles may be inadequate to permit complete ROM of all joints involved. When a multijoint muscle simultaneously shortens at all joints it crosses, further effective tension development is prevented. This phenomena is called active insufficiency. For example, when the hamstrings are tested as knee flexors with the hip extended, less tension can be developed than when the hamstrings are tested with the hip flexed. Therefore, when testing the strength of multijoint muscles, the position of all involved joints must be considered.

The load-velocity relationship is also important in testing muscle strength. A load-velocity curve can be generated by plotting the velocity of motion of the muscle lever arm against the external load. With concentric muscle contractions, the least tension is developed at the highest velocity of movement and vice versa. When the external load equals the maximal force that the muscle can exert, the velocity of shortening reaches zero and the muscle contracts isometrically. When the load is increased further, the muscle lengthens eccentrically. During eccentric contractions, the highest tension can be achieved at the highest velocity of movement [Komi, 1973].

The force generated by a muscle is proportional to the contraction time. The longer the contraction time, the greater the force development up to the point of maximum tension. Slower contraction leads to greater force production because more time is allowed for the tension produced in contractile elements to be transferred through the noncontractile components to the tendon. This is the force-time relationship.

Tension in the tendon will reach the maximum tension developed by the contractile tissues only if the active contraction process is of adequate (even up to 300 msec) duration [Sukop and Nelson, 1974].

Subject effort or motivation, gender, age, fatigue, time of day, temperature, occupation, and dominance can also affect force or torque production capacity. Important additional considerations may be changes in muscle function as a result of pain, overstretching, immobilization, trauma, paralytic disorders, neurologic conditions, and muscle transfers.

Grading Systems and Parameters Measured

Clinically, the two most frequently used methods of strength testing are actually non-instrumented tests: the manual muscle test (MMT) and the functional muscle test (See Amundsen [1990] for more infor­mation on functional muscle tests). In each of these cases interval scaled grading criteria are operationally defined. However, a distinct advantage of using instruments to measure strength is that quantifiable units can be obtained, usually force or torque. Torque = force □ the distance between the point of force application and the axis of rotation:

T = Fx. (148.1)

An important issue in strength testing is deciding whether to measure force (a linear quantity) or torque (a rotational quantity). If the point of application of a force is closer to the axis of rotation, the muscle being assessed has a mechanical advantage as compared to when the point of contact is more distal. Therefore, when forces are measured, unless the measurement devices are applied at the same anatomical position for each test, force measurements can differ substantially even though actual muscle tension remains the same. If the strength testing device has an axis of rotation that can be aligned with the anatomical axis of rotation, then torque can be measured directly. When this is not the case, the moment arm can be measured and torque calculated. Force is more typically measured in whole-body exertions, such as lifting. Another issue is whether to measure and record peak or averaged values. However, if strength is defined as maximum torque production capacity, peak values are implied.

In addition to single numerical values, some strength measurement systems display and print force or torque (versus time) curves, angle-torque curves, and graphs. Computerized systems frequently com­pare the “involved” with the “uninvolved” extremity calculating “percent deficits.” As strength is consid­ered proportional to body weight (perhaps erroneously, see Delitto [1990]) force and torque measurements are frequently reported as a peak torque to body weight ratio. This is seemingly to facilitate use of normative data where present.

Methods and Instruments Used to Measure Muscle Strength

There are two broad categories of testing force or torque production capacity: one category consists of measuring the capacity of defined, local muscle groups (e. g., elbow flexors); the second category of tests consists of measuring several muscle groups on a whole-body basis performing a higher level task (e. g., lifting). The purpose of the test, required level of sensitivity, and expense are primary factors in selecting the method of strength testing. No single method has emerged as being clearly superior or more widely applicable. Like screwdrivers, different types and different sizes are needed depending upon job demands.

Many of the instrumented strength testing techniques, which are becoming more standardized clini­cally and which are almost exclusively used in engineering applications, are based on the concepts and methods of MMT. Although not used for performance capacity tests, because of the ease, practicality, and speed of manual testing, it is still considered a useful tool, especially diagnostically to localize lesions. Several MMT grading systems prevail. These differ in the actual test positions and premises upon which muscle grading is based. For example, the approach promoted by Kendall, McCreary, and Provance [1993] tests a specific muscle (e. g., brachioradialis) rather than a motion. The Daniels and Worthingham [Hislop and Montgomery, 1995] method tests motions (e. g., elbow flexion) which involve all the agonists and synergists used to perform the movement. The latter is considered more functional and less time consuming, but less specific. The reader is advised to consult these references directly for more informa­tion about MMT methods. Further discussion of non-instrumented tests is beyond the scope of this chapter.

An argument can be made for using isometric testing because the force or torque reflects actual muscle tension as the position of the body part is held constant and the muscle mechanics do not change. Additionally, good stabilization is easier to achieve, and muscle actions can be better isolated. However, some clinicians prefer dynamic tests, perceiving them as more reflective of function. An unfortunate fact is that neither static nor dynamic strength measurements alone can reveal whether strength is adequate for functional activities. However, strength measurements can be used with models and engineering analyses for such assessments.

Selected instrumented methods of measuring force or torque production capacity are listed and compared in Table 148.3. Table 148.3 is by no means comprehensive. More indepth review and compar­isons of various methods can be found in Amundsen [1990] and Mayhew and Rothstein [1985]. Figure 148.3 Illustrates three common instruments used to measure strength.

Key Concepts in Measuring Strength

Because of the number of factors influencing strength and strength testing (discussed in a previous section), one can become discouraged rather than challenged when faced with the need to measure strength. Optimally, strength testing would be based on the “worst case” functional performance demands required by an individual in his or her daily life. “Worst case” testing requires knowing the performance demands of tasks including the positions required, types of muscle contractions, and so on.

In the absence of such data, current strategy is to choose the instruments and techniques that maximally stress the system under a set of representative conditions that either (a) seem logical based on knowledge of the task, or (b) have been reported as appropriate and reliable for the population of interest. An attempt is made to standardize the testing in terms of contraction-type, test administration instructions, feedback, warm-up, number of trials, time of day, examiner, duration of contraction (usually 4 to 6 seconds), method and location of application of force, testing order, environmental distractions, subject posture and position of testing, degree of stabilization, and rest intervals between exertions (usually 30 seconds to 2 minutes) [Chaffin, 1975; Smidt and Rogers, 1982]. In addition, the subject must be observed for muscle group substitutions and “trick” movements.

Speed of Movement

Speed of movement refers to the rate of movement of the body or body segments. The maximum movement speed that can be achieved represents another unique performance capacity of an identified system that is responsible for producing motion. Everyday living, work, and sport tasks are commonly described in terms of the speed requirements (e. g., repetitions per minute or per hour). For physical tasks, such descriptions translate to translational motion speeds (e. g., as in lifting) as well as rotational motion speeds (i. e., movement about a dof of the joint systems involved). Thus, there is important motivation to characterize this capacity.

Speed of Movement Terminology

Speed of movement must be differentiated from speed of contraction. Speed of contraction refers to how fast a muscle generates tension. Two body parts may be moving through an arc with the same speed of movement; however, if one part has a greater mass, its muscles must develop more tension per unit of time to move the heavier body part at the same speed as the lighter body part.

Speed, velocity, and acceleration also can be distinguished. The terms velocity and speed are often used interchangeably; however, the two quantities are frequently not identical. Velocity means the rate of motion in a particular direction. Acceleration results from a change in velocity over time. General

Instrument/Method

Repetition maximum

Amount of weight a subject can lift a given number of times and no more; one determines either a one repetition maximum (1-RM) or a ten repetition maximum (10-RM). A 1-RM is the maximum amount of weight a subject can lift once; a 10-RM is the amount of weight a subject can lift 10 times; a particular protocol to determine RMs is defined [DeLorme and Watkins, 1948]; measures dynamic strength in terms of weight (pounds or kilograms) lifted.

Hand-held dynamometer

Device held in the examiner’s hand used to test strength; devices use either hydraulics, strain gauges (load-cells), or spring systems (See Fig. 148.3a); used with a “break test” (the examiner exerts a force against the body segment to be tested until the part gives way) or a “make test” (the examiner applies a constant force while the subject exerts a maximum force against it); “make tests” are frequently preferred for use with hand-held dynamometers [Bohannon, 1990; Smidt, 1984]; measures force; unclear whether test measures isometric or eccentric force (this may depend on whether a “make test” or a “break test” is used).

Cable tensiometer

One end of a cable is attached to an immovable object and the other end is attached to a limb segment; the tensiometer is placed between the sites of fixation; as the cable is pulled, it presses on the tensiometer’s riser which is connected to a gauge (See discussion in Mayhew and Rothstein [1985]); measures isometric force.

Strain gauge

Electroconductive material applied to metal rings or rods; a load applied to the ring or bar deforms the metal and a gauge; deformation of the gauge changes the electrical resistance of the gauge causing a voltage variation; this change can be converted and displayed using a strip chart recorder or digital display; measures isometric force.

Isokinetic dynamometer

Constant velocity loading device; several models marketed by a number of different companies; most consist of a movable lever arm controlled by an electronic servomotor that can be preset for selected angular velocities usually between 0D and 500D per second; when the subject attempts to

Advantages/Uses

Requires minimal equipment (weights); inexpensive and easy to administer; frequently used informally to assess progress in strength training.

Similar to manual muscle testing (MMT) in test positions and sites for load application; increased objectivity over MMT; portable; easy to administer; relatively inexpensive; commercially available from several suppliers; adaptable for a variety of test sites; provide immediate output; spring and hydraulic systems are non­electrical; load-cell based systems provide more precise digital measurements.

Mostly used in research settings; evidence presented on reliability when used with healthy subjects [Clarke, 1954; Clarke, Bailey and Shay, 1952]; relatively inexpensive.

Mostly used in research settings; increased sensitivity for testing strong and weak muscles.

Permits dynamic testing of most major body segments; especially useful for stronger movements; most devices provide good stabilization; measures reciprocal muscle contractions; widespread

Disadvantages/Limitations

Uses serial testing of adding weights which may invalidate subsequent testing; no control for speed of contraction or positioning; minimal information available on the reliability and validity of this method.

Stabilization of the device and body segment can be difficult; results can be affected by the examiner’s strength; limited usefulness with large muscle groups; spring-based systems fatigue over time becoming inaccurate; range and sensitivity of the systems vary; shape of the unit grasped by the examiner and shape of the end-piece vary in comfort, and therefore the force a subject or examiner is willing to exert; more valuable for testing subjects with weakness than for less involved or healthy subjects due to range limits within the device (See discussion in Bohannon [1990]).

Requires special equipment for testing; testing is time-consuming and some tests require two examiners; unfamiliar to most clinicians; not readily available; less sensitive at low force levels.

Strain gauges require frequent calibration and are sensitive to temperature variations; to be accurate the body part must pull or push against the gauge in the same line that the calibration weights were applied; unfamiliar to most clinicians; not commercially available; difficult to interface the device comfortably with the subject.

Devices are large and expensive; need calibration with external weights or are “self-calibrating;” signal damping and “windowing” may affect data obtained; angle-specific measurements may not be accurate if a damp is used because torque readings do not relate to the

Accelerate beyond the pre-set machine speed, the machine resists the movement; a load cell measures the torque needed to prevent body part acceleration beyond the selected speed; computers provide digital displays and printouts (See typical device in Fig. 148.3b); measures isokinetic-concentric (and in some cases, isokinetic-eccentric) and isometric strength; provides torque (or occasionally force) data; debate exists about whether data are ratio-scaled or not; accounting for the weight of the segment permits ratio-scaling [Winter, Wells, and Orr, 1981].

Hand dynamometer

Instruments to measure gripping or pinching strength specifically for the hand; usually use a spring scale or strain-gauge system (See typical grip strength testing device in Fig. 148.3c); measures isometric force.

Clinical acceptance; also records angular data, work, power, and endurance-related measures; provides a number of different reporting options; also used as exercise devices.

Readily available from several suppliers; easy to use; relatively inexpensive; widespread use; some normative data available.

Goniometric measurements; joints must be aligned with the mechanical axis of the machine; inferences about muscle function in daily activities from isokinetic test results have not been validated; data obtained between different brands are not interchangeable; adequate stabilization may be difficult to achieve for some movements; may not be usable with especially tall or short persons.

Only useful for the hand; different brands not interchangeable; normative data only useful when reported for the same instrument and when measurements are taken with the same body position and instrument setting; must be recalibrated frequently.

Velocity and acceleration measurements are beyond the intent of this chapter. Reaction speed and response speed are other related variables also not considered.

Factors Influencing Speed of Movement and Speed of Movement Measurements

Muscles with larger moment arms, longer muscle fibers, and less pennation tend to be capable of generating greater speed. Many of the same factors influencing strength, discussed previously, such as muscle length, fatigue, and temperature affect the muscle’s contractile rate. The load-velocity relationship is especially important when testing speed of movement. In addition to these and other physiological factors, speed can be reduced by factors such as friction, air resistance, gravity, unnecessary movements, and inertia [Jensen and Fisher, 1979].

Parameters Measured

Speed of movement can be measured as a linear quantity or as an angular quantity. Typically, if the whole body is moving linearly in space as in walking or running, a point such as the center of gravity is picked, and translational motion is measured. Also, when an identified point on a body segment (e. g., the tip of the index finger) is moved in space, translational movement is observed, and motion is measured in translational terms. If the speed of a rotational motion system (e. g., elbow flexors) is being measured, then the angular quantity is determined. As the focus here is on measuring isolated neuromuscular performance capacities, the angular metric is emphasized. Angular speed of a body segment is obtained by: angular speed = change in angular position/change in time:

□ □ —. (148.2)

□ t

Thus, speed of movement may be expressed in revolutions, degrees, or radians per unit of time, such as degrees per second (deg/s).

Measurement of Neuromuscular Performance Capacities

(b)

FIGURE 148.3 Three types of instrumented strength testing devices are shown: (a) a representative example of a typical hand-held dynamometer; (b) an example of an isokinetic strength testing device; (c) a hand dynamom­eter used to measure grip strength

Another type of speed measure applies to well-defined (over fixed angle or distance) cyclic motions. Here repetitions per unit time or cycles per unit time measures are sometimes used. However, in almost every one of these situations, speed can be expressed in degrees per second or meters per second. The latter units are preferred because they allow easier comparison of speeds across a variety of tasks. The only occasion when this is difficult is when translation motion is not in a simple straight line, such as when a person is performing a complex assembly task with multiple subtasks.

The issue of whether to express speed as maximum, averaged, or instantaneous values must also be decided based on which measure is a more useful indicator of the performance being measured. In addition to numerical reporting of speed data, time-history graphs of speed may be helpful in comparing some types of performance.

Instruments Used to Measure Speed of Movement

When movement time is greater than a few seconds and the distance is known, speed can be measured with a stop watch or with switch plates, such as the time elapsed in moving between two points or over a specified angle. With rapid angular joint movements, switch plates or electrogoniometers with electronic timing devices are required. Speeds can also be computed from the distance or angle and time data available from cinematography, optoelectric movement monitoring systems, and videotape systems. Some dynamic strength testing devices involve presetting a load and measuring the speed of movement.

In addition, accelerometers can be used to measure acceleration directly, and speed can be derived through integration. However, piezoelectric models have no steady-state response and may not be useful for slower movements. Single accelerometers are used to measure linear motion. Simple rotatory motions require two accelerometers. Triaxial accelerometers are commercially available that contain three pre­mounted accelerometers perpendicular to each other. Multiple accelerometer outputs require appropriate processing to resolve the vector component corresponding to the desired speed. Accelerometers are most appropriately used to measure acceleration when they are mounted on rigid materials. Accelerometers have the advantage of continuously and directly measuring acceleration in an immediately usable form. They can also be very accurate if well-mounted. Because they require soft tissue fixation and cabling or telemetry, they may alter performance and further error may be induced by relative motion of the device and tissues. The systems are moderately expensive (See discussion of accelerometers in Robertson and Sprigings [1987]).

Key Concepts in Speed of Movement Measurement

As discussed, maximum speed is determined when there is little stress on torque production resources. As resistance increases, speed will decrease. Therefore, the load must be considered and specified when testing speed. Because speed of movement data are calculated from displacement and temporal data, a key issue is minimizing error which might result from collecting this information. Error can result from inaccurate identification of anatomical landmarks, improper calibration, perspective error, instrument synchronization error, resolution, digitization error, or vibration. The sampling rate of some of the measurement systems may become an issue when faster movements are being analyzed. In addition, the dynamic characteristics of signal conditioning systems should be reported.

Endurance

Endurance is the ability of a system to sustain an activity for a prolonged time (static endurance) or to perform repeatedly (dynamic endurance). Endurance can apply to the body as a whole, a particular body system, or to specific neuromuscular functional units. High levels of endurance imply that a given level of performance can be continued for a long time period.

Endurance Terminology

General endurance of the body as a whole is traditionally considered cardiovascular endurance or aerobic capacity. Cardiovascular endurance is most frequently viewed in terms of V02max. This chapter considers only endurance of neuromuscular systems. Although many central and peripheral anatomic sites and physiologic processes contribute to a loss of endurance, endurance of neuromuscular functional units is also referred to as muscular endurance.

Absolute muscle endurance is defined as the amount of time that a neuromuscular system can continue to accomplish a specified task against a constant resistance (load and rate) without relating the resistance to the muscle’s strength. Absolute muscle endurance and strength are highly correlated. Conversely, strength and relative muscle endurance are inversely related. That is, when resistance is adjusted to the person’s strength, a weaker person tends to demonstrate more endurance than a stronger person. Fur­thermore, the same relationships between absolute and relative endurance and strength are correlated by type of contraction; in other words, there is a strong positive correlation between isotonic strength and absolute isotonic endurance and vice versa for strength and relative isotonic endurance. The same types of relationships exist for isometric strength and isometric endurance [Jensen and Fisher, 1979].

Factors Influencing Neuromuscular Endurance and Measurement of Endurance

Specific muscle fiber types, namely fast-twitch fatigue-resistant fibers (FR), generate intermediate levels of tension and are resistant to short-term fatigue (a duration of about 2 minutes or intermittent stimu­lation). Slow-twitch fibers (S) generate low levels of tension slowly, and are highly resistant to fatigue. Muscle contractions longer than 10 seconds, but less than 2 minutes, will reflect local muscle endurance [Astrand and Rodahl, 1986]. For durations longer than 2 minutes, the S fibers will be most stressed. A submaximal isometric contraction to the point of voluntary fatigue will primarily stress the FR and S fibers [Thorstensson and Karlsson, 1976]. Repetitive, submaximal, dynamic contractions continued for about 2 to 6 minutes will measure the capacity of FR and S fibers. Strength testing requires short duration and maximal contractions; therefore, to differentiate strength and endurance testing, the duration and intensity of the contractions must be considered.

Because strength affects endurance, all of the factors discussed previously as influencing strength, also influence endurance. In addition to muscle physiology and muscle strength, endurance is dependent upon the extensiveness of the muscle’s capillary beds, the involved neuromuscular mechanisms, contrac­tion force, load, and the rate at which the activity is performed.

Endurance time, or the time for muscles to reach fatigue, is a function of the contraction force or load [von Rohmert, 1960]. As the load (or torque required) increases, endurance time decreases. Also, as speed increases, particularly with activities involving concentric muscle contractions, endurance decreases.

Parameters Measured

Endurance is how long an activity can be performed at the required load and rate level. Thus, the basic unit of measure is time. Time is the only measure of how long it takes to complete a task. If we focus on a given variable (e. g., strength, speed, or endurance), it is necessary to either control or measure the others. When the focus is endurance, the other factors of force or torque, speed, and joint angle, can be described as conditions under which endurance is measured. Because of the interactions of endurance and load, or endurance and time, for examples, a number of endurance-related measures have evolved. These endurance-related measures have clouded endurance testing.

One endurance-related measure uses either the number of repetitions that can be performed at 20, 25, or 50 percent of maximum peak torque or force. The units used to reflect endurance in this case are number of repetitions at a specified torque or force level. One difficulty with this definition has been described previously, that is, the issue of relative versus absolute muscle endurance. Rothstein and Rose [1982] demonstrated that elderly subjects with selected muscle fiber type atrophy were able to maintain 50 percent of their peak torque longer than young subjects. However, if a high force level is required to perform the task, then the younger subject would have more endurance in that particular activity [Rothstein, 1982]. Another difficulty is that the “repetition method” can be used only for dynamic activities. If isometric activities are involved, then the time an activity can be sustained at a specified force or torque level is measured. Why have different units of endurance? Time could be used in both cases. Furthermore, the issue of absolute versus relative muscle endurance becomes irrelevant if the demands of the task are measured.

Yet another method used to reflect endurance is to calculate an endurance-related work ratio. Many isokinetic testing devices, such as the one shown in Fig. 148.3b, Will calculate work (integrate force or torque over displacement). In this case, the total amount of work performed in the first five repetitions is compared with the total amount of work performed in the last five repetitions of a series of repetitions (usually 25 or more). Work degradation reflects endurance and is reported as a percentage. An additional limitation of using these endurance ratios is that work cannot be determined in isometric test protocols. Mechanically there is no movement, and no work is being performed.

Overall, the greatest limitation with most endurance-related approaches is that the measures obtained cannot be used to perform task-related assessments. In a workplace assessment, for example, one can determine how long a specific task (defined by the conditions of load, range, and speed) needs to be performed. Endurance-related metrics can be used to reflect changes over time in a subject’s available endurance capacity; however, endurance-related metrics cannot be compared to the demands of the task. Task demands are measured in time or repetitions (e. g., 10) with a given rate (e. g., 1/0.5 h) from which total time (e. g., 5 h) can be calculated. A true endurance measure (versus an endurance-related measure) can serve both purposes. Time reflects changes in endurance as the result of disease, disuse, training, or rehabilitation and also can be linked to task demands.

Methods and Instruments Used to Measure Neuromuscular Endurance

Selection of the method or instrument used to measure endurance depends on the purpose of the mea­surement and whether endurance or endurance-related measures will be obtained. As in strength testing, endurance tests can involve simple, low level tasks or whole-body, higher level activities. The simplest method of measuring endurance is to define a task in terms of performance criteria and then time the performance with a stop watch. A subject is given a load and a posture and asked to hold it “as long as possible” or to move from one point to another point at a specific rate of movement for “as long as possible.”

An example of a static endurance test is the Sorensen test used to measure endurance of the trunk extensors [Biering-Sorensen, 1984]. This test measures how long a person can sustain his or her torso in a suspended prone posture. The individual is not asked to perform a maximal voluntary contraction, but an indirect calculation of load is possible [Smidt and Blanpied, 1987].

An example of a dynamic endurance test is either a standardized or non-standardized, dynamic isoin­ertial (see previous description in the section on strength testing) repetition test. In other words, the subject is asked to lift a known load with a specified body part or parts until defined conditions can no longer be met. Conditions such as acceleration, distance, method of performance, or speed may or may not be controlled. The more standardized of these tests, particularly those which involve lifting capacity, are reported and projections about performance capacity over time are estimated [Snook, 1978]. Ergometers and some of the isokinetic dynamometers discussed previously measure work, and several can calculate endurance-related ratios. These devices could be adapted to measure endurance in time units.

Key Concepts in Measuring Muscle Endurance

Of the four variables of human performance discussed in this chapter, endurance testing is the least developed and standardized. Except for test duration and rest intervals, attention to the same guidelines as described for strength testing is currently recommended.

Reliability, Validity, and Limitations in Testing

Space does not permit a complete review of these important topics. However, a few key comments are in order. First, it is important to note that reliability and validity are not inherent qualities of instruments, but exist in the measurements obtained only within the context in which they are tested. Second, reliability and validity are not either present or absent but are present or absent along a continuum. Third, traditional quantitative measures of reliability might indicate how much reliability a given measurement method demonstrates, but not how much reliability is actually needed. Fourth, technology has advanced to the extent that it is generally possible to measure physical variables such as time, force, torque, angles, and speed accurately, repeatably, and with high resolution. Lastly, clinical generalizability of human performance capacity measures ultimately results from looking at the body of literature on reliability as a whole and not from single studies.

For these types of variables, results of reliability studies basically report that: (1) if the instrumentation is good, and (2) if established, optimal procedures are carefully followed, then results of repeat testing will usually be in the range of about 5 to 20% of each other. This range of repeatability depends on: (1) the particular variable being measured (i. e., repeated endurance measures will differ more than repeated measures of hinge joint EOM), and (2) the magnitude of the given performance capacity (i. e., errors are often in fixed amounts such as 3D for ROM; thus, 3D out of 180D is smaller percentage-wise than 3D out of 20°). One can usually determine an applicable working value (e. g., 5 or 20%) by careful review of the relevant reliability studies. Much of the difference obtained in test-retest results is because of limitations in how well one can reasonably control procedures and the actual variability of the parameter being measured, even in the most ideal test subjects. Measurements should be used with these thoughts in mind. If a specific application requires extreme repeatability, then a reliability study should be conducted under conditions that most closely match those in which the need arises. Reliability discussions specific to some of the focal measures of this chapter are presented in Amundsen [1990], Hellebrandt, Duvall, and Moore [1949], Mayhew and Rothstein [1985], and Miller [1985].

Measurements can be reliable but useless without validity. Most validity studies have compared the results of one instrument to another instrument or to known quantities. This is the classical type of validity testing which is an effort to determine whether the measurement reflects the variable being measured. In the absence of a “gold standard” this type of testing is of limited value. In addition to traditional studies of the validity of measurements, the issue of the validity of the inferences based on the measurements is becoming increasingly important [Rothstein and Echternach, 1993]. That is, can the measurements be used to make inferences about human performance in real life situations? Unfortunately, measurements which have not demonstrated more than content validity are frequently used as though they are predictive. The validity of the inferences made from human performance data needs to be rigorously addressed.

Specific measurement limitations were briefly addressed in Tables 148.2 and 148.3 and in the written descriptions of various measurement techniques and instruments. Other limitations have more to do with interpreting the data. A general limitation is that performance variables are not fixed human attributes. Another limitation is that population data are limited and available normative data are, unfortunately, frequently extrapolated to women, older persons, and so on [Chaffin and Andersson, 1991]. Some nor­mative data suggest the amount of resources such as strength, ROM, speed of movement, and endurance required for given activities; other data suggest the amount available. As previously mentioned, these are two different issues. Performance measurements may yield information about the current status of per­formance, but testing rarely indicates the cause or the nature of dysfunction. More definitive, diagnostic studies are used to answer these questions. Whereas, considerable information exists with regard to measuring performance capacities of human systems, much less energy has been directed to understanding requirements of tasks. The link between functional performance in tasks and laboratory-acquired mea­surements is a critical question and a major limitation in interpreting test data. The ERM addresses several of these limitations by using a multidimensional, individualized, cause-and-effect model.

Performance Capacity Space Representations

In both the study and practice, performance of neuromuscular systems has been characterized along one or two dimensions of performance at a time. However, human subsystems function within a multi­dimensional performance space. ROM/EOM, strength, movement speed, and endurance capacities are
Not only interdependent, but may also vary uniquely within individuals. Multiple measurements are necessary to characterize a person’s performance capacity space, and performance capacity is dependent on the task to be performed. Therefore, both the individual and the task must be considered when selecting measurement tools and procedures [Chaffin and Andersson, 1991].

Measurement of Neuromuscular Performance CapacitiesIn many of the disciplines in which human per­formance is of interest, traditional thinking has often focused on single number measures of ROM, strength, speed, etc. More recent systems engineer­ing approaches [Kondraske, 1999] emphasize con­sideration of the performance envelope of a given system and suggest ways to integrate single mea­surement points that define the limits of perfor­mance of a given system [Vasta and Kondraske,

1997]. Figure 148.4 il Lustrates a three-dimensional performance envelope derived from torque, angle, and velocity data for the knee extensor system. The additional dimension of endurance can be repre­sented by displaying this envelope after performing an activity for different lengths of time. A higher level, composite performance capacity, as is some — FIGURE 148.4 An exampk of a torque-angle-velocity

Times needed, could be derived by computing the performance envelope for the knee extensor system.

I j u i o i — Source: Vasta PJ, Kondraske GV. 1994. A multi-dimen —

Volume enclosed by this envelope. Such represen —

Sional performance space model for the human knee

Tations also facilitate assessment of the given system

Extensor (technical Report 94-001R). p 11. University of

In a specific task; that is, a task is defined as a point

Texas at Arlington, Human Performance Institute, in this space that will either fall inside or outside Arlington, Texas. With permission. the envelope.

Conclusions

In conclusion, human movement is so essential that it demands interest and awe from the most casual observer to the most sophisticated scientists. The complexity of performance is truly inspiring. We are challenged to understand it! We want to reduce it to comprehensible units and then enhance it, reproduce it, restore it, and predict it. To do so, we must be able to define and quantify the variables. Hence, an array of instruments and methods have emerged to measure various aspects of human performance. To date, measurement of neuromuscular performance capacities along the dimensions of ROM/EOM, strength, speed of movement, and endurance represents a giant stride but only the “tip of the iceberg.” Progress in developing reliable, accurate, and valid instruments and in understanding the factors influ­encing the measurements cannot be permitted to discourage us from the larger issues of applying the measurements toward a purpose. Yet, single measurements will not suffice; multiple measurements of different aspects of performance will be necessary to fully characterize human movement.

Defining Terms

Endurance: The amount of time a body or body segments can sustain a specified static or repetitive activity.

Extremes of motion (EOM): The end ranges of motion at a joint measured in degrees.

Muscle strength: The maximal amount of torque or force production capacity that a muscle or muscle

Groups can voluntarily exert in one maximal effort, when type of muscle contraction, movement velocity, and joint angle(s) are specified.

Neuromuscular functional units: Systems (that is, the combination of nerves, muscles, tendons, liga­ments, and so on) responsible for producing basic movements.

Range of motion (ROM): The amount of movement that occurs at a joint, typically measured in

Degrees. ROM is usually measured by noting the extremes of motion, or as the difference between the extreme motion and the reference position.

Speed of movement: The rate of movement of the body or body segments.

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Further Information

Human Performance Measurement, Inc. 1998. APM I Portable Electronic Goniometer: User’s Manual. PO Box 1996, Arlington, TX 76004-1996.

Journals: Clinical Biomechanics, Journal of Biomechanics, Medicine and Science in Sports and Exercise, Physical Therapy.

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Task Force on Standards for Measurement in Physical Therapy. 1991. Standards for tests and measure­ments in physical therapy practice. Phys. Ther. 71:589-622.

Jones, R. D. “Measurement of Sensory-Motor Control Performance Capacities: Tracking Tasks.” The Biomedical Engineering Handbook: Second Edition.

Ed. Joseph D. Bronzino

Boca Raton: CRC Press LLC, 2000

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