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Role of Body Size in the Relation Between Muscle Strength and Movement Performance

Jaric, Slobodan

Exercise and Sport Sciences Reviews: January 2003 - Volume 31 - Issue 1 - p 8-12
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JARIC, S. Role of body size in the relation between muscle strength and movement performance. Exerc. Sport Sci. Rev., Vol. 31, No. 1, pp. 8–12, 2003. Body size relates differently on the outcome of various tests of muscle strength and movement performance. Thus the estimation of functional performance from tests of muscle strength, and vice versa , should be based on the normalization of both muscle strength and movement performance.

University of Delaware, Newark

Accepted for publication: June 3, 2002.

Address for correspondence: Slobodan Jaric, Human Performance Lab, Department of Health and Exercise Sciences, University of Delaware, 547 South College Avenue, Newark, DE 19716 (E-mail: jaric@UDel.edu).

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INTRODUCTION

Muscle strength is usually defined as the maximum force or torque of a particular muscle group developed during maximal voluntary contraction under a given set of conditions. Movement performance tests refer to assessment of abilities to perform various functional tasks under standardized movement conditions (see Table 1 for examples of muscle strength and movement performance tests). Both muscle strength and movement performance tests have been frequently applied in athletic, ergonomic, and physical rehabilitation assessments. The purpose of testing muscle strength has usually been to assess muscle function, to provide normative values for various groups of subjects, and to evaluate performance capabilities for sport- and work-related activities (1,7,14). The tests of movement performance have been employed to assess the ability to perform various functional tasks (1,2), but also to assess muscle function, such as the muscle ability to exert force or torque (3). Therefore, the presumed relation between strength and performance has frequently represented the main rationale for testing either muscle strength or movement performance. In particular, the relation between muscle strength and movement performance has been also often interpreted as external validity of the tests of muscle strength (14).

TABLE 1

TABLE 1

It has been known for some time that various indices of both muscle strength and movement performance can be related to body size (for review, see (1) and (10)). This relation has usually been investigated using an allometric approach (4,13). For example, the assumed relation between muscle strength S and body mass m (or any other index of body size) was:where q indicates the allometric coefficient, whereas p is a parameter (for interpretation of p see further text). The log-transform provides a regression model where log p and q are the intercept and slope, respectively, of the relation between muscle strength and body mass:MATHThe regression model applied on a scatter of experimentally recorded data provides the value of the allometric parameter q. Applied within equation 1, this value enables calculation of the normalized strength Sn that represents the body-size–independent index for particular subject population and particular strength test:Note that the parameter p of the equation 1 corresponds to the normalized strength Sn. The same approach can be used when assessing the relation between a particular index of movement performance, such as the maximum weight lifted, and body size.

Both this method and the particular values of the allometric parameter q have been recommended in the literature as a way to derive indices of either muscle strength (9,12,13) or movement performance (3,4,15) that are independent of body size. However, the normalization for body size has been applied inconsistently in the literature (7,8). Even extensive reviews dealing with various methodological problems associated with strength testing have neglected the possible confounding effects of body size (14).

The purpose of the review is to identify potential problems that arise when normalizing such data and to propose a more systematic approach for normalizing the results of these tests. First, it will be demonstrated that some tests of muscle strength and movement performance can require different normalization methods. Second, it will be emphasized that estimation of functional performance from tests of muscle strength, or vice versa, requires an appropriate normalization of muscle strength and movement performance.

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NORMALIZATION OF MUSCLE FORCE AND TORQUE

Although the relation between muscle strength and body size has been studied often, the normalization of the measured strength relative to body size has been performed inconsistently. These limitations include the use of different values of the allometric coefficient, the application of several normalization methods to the same set of strength data, and the failure to apply any normalization at all (7). As a consequence, the results of different studies cannot be compared due to differences in the normalization procedures. Furthermore, some strength tests require different types of normalization, such as the measurement of muscle force with a dynamometer and the recording of muscle torque with a standard isokinetic apparatus.

Under the presumption of “geometric similarity” of human subjects (for details, see (1,4,10)), all body linear dimensions L change proportionally to body size, all areas (such as the cross-sectional area of muscle) change proportionally to L2, and all volumes or masses (including body mass m) change proportionally to L3. An alternative formulation of these relations is that all areas should be proportional to m2/3, whereas all linear dimensions should be proportional to m1/3. When muscle strength is tested with a dynamometer (D, see Figure 1), the presumed relations suggest that the recorded force Fr should be proportional to muscle force Fm, because the leverage of the limb (b/a) does not change with body size. The muscle force is expected to be proportional to the cross-sectional area of the muscle, which also means proportional to L2 or to m2/3. The conclusion is that the allometric parameter (see equation 3) used to normalize strength with respect to body mass, the most common index of body size, when it is measured as a muscle force should be q = 2/3 = 0.67.

Figure 1

Figure 1

When strength is assessed as the muscle torque recorded with a standard isokinetic apparatus (Figure 2), the torque T changes with body size not only due to the change in muscle cross-sectional area (proportional to m2/3), but also due to the lever arm a of the muscle. As with other linear dimensions, the lever arm changes proportionally to m1/3, which gives the product m2/3·m1/3 = m1. Consequently, the allometric parameter to normalize strength relative to body mass when it is measured as muscle torque should be q = 1.

Figure 2

Figure 2

Although the concept of distinguishing between tests of muscle force and muscle torque has only recently been suggested (8,15), previous experimental findings underscore this distinction. For example, the literature (7,8) suggests that the allometric parameters for various measurements of muscle force have been q = 0.45–0.87, whereas those for muscle torque have been within the range q = 0.74–1.31. These results indicate that the allometric parameter is generally greater for torque compared with force. However, even some extensive reviews on the methodology of muscle-strength testing neglect this distinction (14).

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NORMALIZATION OF MOVEMENT PERFORMANCE

Although various features of movement performance are related to body size (for review, see (1) and (10)), normalization for body size has been rarely applied when presenting results of routine tests of movement performance (7). What could add to the complexity of this problem is that the outcome of different movement performance tests can be related in different ways to body size, as described subsequently.

A number of performance tasks are based on exerting the maximum force against external objects. Examples include determining the maximum load that can be lifted in a standard lifting task, assessing the maximum force that can be exerted during a handgrip task, and evaluating performance on manual material-handling task in ergonomics. Both everyday experience and experimental results (3,4,12,15) suggest that the observed index of performance (i.e., the maximum force exerted, or the maximum load lifted) increases with body size. Accordingly, this force should be related to body size in the same manner as the tested muscle force discussed within the previous section. Therefore, the allometric parameter (see equation 3) used to normalize performance of functional movement tasks based on exertion of external force could be close to q = 2/3. It should be noted that the standard tests of muscle power could also belong to this group (1), although more complex scaling methods suggest somewhat higher values (10).

An important group of functional movement tests is based on performing rapid movements of either body limbs (e.g., kicking, throwing) or the entire body (such as jumping and sprinting). The literature suggests that there is a weak relation, if any, between movement velocity and body size within a wide range of body sizes across species (10); this relation becomes negligible within the relatively narrow range of sizes for the human body (1). Furthermore, performance in athletics is generally consistent with this conclusion because the fastest sprint times, the longest jumps, and the longest ball kicks are not expected from either the largest or the smallest athletes. Therefore, the allometric parameter used to normalize performance of functional tasks involving rapid movements should be close to q = 0, because no normalization is required.

Finally, a number of functional tests are based on muscle actions intended to support body weight under strength-demanding conditions. Examples include actions to move the body (e.g., sit-ups, pull-ups, chin-ups, one-leg raises) and to sustain particular postures (e.g., keep the back extended in a horizontal position, hold a pose in yoga, maintain a balance position in gymnastics). Because the capacity to exert muscle force increases at a lesser rate than body weight, these types of tests should be negatively related to body size. This relation is readily apparent when the effect of body dimensions is examined across species (see (10) for review), and by the experiential observation that gymnasts and acrobats tend to have a smaller stature compared with their peers. Because the allometric coefficients for muscle force (q = 2/3) and body weight (increases proportionally to m1, hence q = 1) are different, the allometric parameter used to normalize performance on tasks based on overcoming body weight should be q = −1/3.

A number of studies underscore the dissociation in the relation between body size and performance on various movements. For example, lighter subjects have lesser capabilities in functional tests based on overcoming an external load (e.g., bench-press, hip-sled, medicine-ball throw, two-hand lift), whereas they are more successful when supporting their own weight (e.g., pull-ups, sit-ups, maintaining a posture) (2,11). A simple scaling approach suggests similar results (1). The correlation analysis of ergonomics tests based on exerting forces against external objects (push, pulls, manual material handling) and those based on lifting one’s own body (squats, lift-ups, one-leg lifts) suggests that they belong to different “movement abilities” (6). The same result was reported in the classical studies of Fleishman (see (5) for review) for the tests of “static strength” (exerting external forces), “dynamic strength” (push-ups, chin-ups), and “explosive strength” (running, jumping). It seems, therefore, that some elements of Fleishman’s “structure of movement abilities” (5) represent an artifact of inappropriately applied normalization for body size.

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MUSCLE STRENGTH AND MOVEMENT PERFORMANCE

The normalization methods (Table 1) are expected to present the body-size–independent indices of muscle strength and movement performance. These standardized procedures will facilitate the comparison of data obtained in different studies.

The most important point of the review, however, concerns the strength-performance relation. As stated previously, a frequent reason for testing muscle strength is to assess functional capabilities with various movements, and the converse is also a common experimental strategy. As depicted in Figure 3, both muscle strength and movement performance tests could be related to body size. Moreover, this relation could be based on different values of the allometric parameter q. As a consequence, the selection of appropriate normalization methods is required for both muscle strength and movement performance. The normalization of various indices of muscle strength and movement performance for body size was discussed separately in the preceding sections and is summarized in Table 1. Therefore, this section examines how these separate normalizations can be reduced to a single factor when assessing the strength-performance relation.

Figure 3

Figure 3

For example, let us assume that the maximum voluntary forces of the elbow flexor muscles and shoulder extensor muscles are measured to assess the ability of a particular subject population to perform the chin-up exercise. These tests correspond to “muscle force” and “supporting body weight” groups of tests depicted in Table 1. To relate body-size–independent measures of the tested strength and performance (Pn ∼ Sn), equations 4 and 8 (see Table 1) must be used:MATHwhich reduces to:MATHThis result suggests that the recorded maximum voluntary forces should be divided by body mass to estimate the number of chin-ups that subjects of different size can perform.

This approach also applies in the converse direction. For example, let us assume that the muscle torques acting about the shoulder and elbow joints are assessed from a functional test of pushing an implement under standardized working conditions. To relate the muscle torque to the tested performance (i.e., Sn ∼ Pn),equations 5 and 6 (see Table 1) should be applied. The relation between normalized strength and normalized performance provides:MATHwhich reduces to:MATHThis result suggests that the estimated torque for subjects of different size is approximately equal to the performance on the test multiplied by body mass raised to the 1/3rd power.

This approach can also be applied to relate two different strength or movement performance tests. For example, let us assume that the ability for manual material handling (e.g., pushing, pulling, and lifting various objects) is assessed from results of push-ups, squats, and chin-ups. Because these tests respectively correspond to “exertion of external force” (P1) and “supporting body weight” (P2) groups of movement performance tests (Table 1), equations 6 and 8 provide:MATHwhich reduces to:MATHThis result suggests that the recorded number of push-ups, squats, and chin-ups should be multiplied by body mass to estimate the ability of manual material handling in subjects of different size.

Despite a number of limitations with this approach (see next section), these examples illustrate how the normalization of body size should be applied when assessing the relations between muscle strength and movement performance. However, these examples also illustrate what happens when the appropriate normalization procedures are not applied to both the tests of muscle strength and muscle performance. In short, the obtained relations remain confounded with the body size effect. Interestingly, none of the studies cited within a recent literature review (7) considered the role of body size in both muscle strength and movement performance tests, whereas other reviews on the same topic do not mention the effect of body size at all (e.g., (14)).

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LIMITATIONS

Most of the normalization methods for body size proposed in the literature have been based on the aforementioned presumption of geometric similarity (1,8,10). The same approach has been used for assessment of the allometric parameters presented in Table 1. However, it is well known that human bodies are not geometrically similar because there are some prominent systematic differences in either body shape or body composition when different groups of subjects are compared (1). Among the well-known examples are prominent differences in body shape between men and women and among children of different ages, and significant differences in both body shape and body composition among some athletic groups (e.g., basketball players, gymnasts, and swimmers). Processes of maturation and aging are associated with different rates of change in body size, muscle strength, and movement performance, whereas both sport selection and athletic training could also affect muscle strength and body size in particular groups of subjects. Finally, the tests of the rate of force development could be more important than tests of maximal muscle force or torque when assessing ability for performing rapid body movements (12,14). Therefore, one could conclude that the relations between muscle strength, movement performance, and body size could be both complex and specific to the subject group. This conclusion implies that different subject populations tested on the same tasks could require different values for the allometric parameters. This conclusion has been supported by a number of studies (1,9). Because routine tests do not provide conditions for assessment of optimal allometric parameters for each particular subject group, muscle strength test, or movement performance tests, the standardized methods, such as those depicted in Table 1, should be used.

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CONCLUSIONS

Although the relations between body size and some standard tests of muscle strength and movement performance represent a well-documented phenomenon in the literature, the normalization for body size has been used both inconsistently and arbitrarily. This problem is exacerbated when muscle strength is tested to assess functional capabilities or when movement performance is tested to estimate muscle strength. The reason is that both muscle strength and movement performance can be influenced by body size. Moreover, these relations can differ for the particular muscle strength and movement performance tests.

This review proposes a more systematic approach for normalizing the effect of body size on the outcomes of both muscle strength and movement performance tests. This approach is expected to provide indices of muscle strength and movement performance that are independent of the body size and to facilitate the assessment of functional performance based on the tests of muscle strength. Moreover, these normalizations can be reduced to a single procedure for particular groups of the related tests. However, further research is needed to determine the exact effect of body size on the various tests of muscle strength and movement performance, as well as to reveal the role of some potentially confounding factors (e.g., body shape and body composition) that could influence the allometric parameters in specific subject populations.

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Acknowledgments

The study was supported in part by grants from The Swedish Sport Research Council (Centrum for Idrottsforskning), the Swedish Council for Work Life Research, and the Serbian Research Council.

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Keywords:

force; torque; allometric scaling; functional test; mass

©2003 The American College of Sports Medicine