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Original Research

Tests of Muscle Power Output Assess Rapid Movement Performance When Normalized for Body Size

Nedeljkovic, Aleksandar1; Mirkov, Dragan M1; Markovic, Srdjan1; Jaric, Slobodan2

Author Information
Journal of Strength and Conditioning Research: August 2009 - Volume 23 - Issue 5 - p 1593-1605
doi: 10.1519/JSC.0b013e3181a9ebce
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Abstract

Introduction

Testing of physical abilities has been widely popular and extensively used to assess muscle function, provide normative values for various groups of subjects, evaluate the success of training and rehabilitation procedures, prevent injuries, and evaluate the performance for sport- and work-related activities (3,49,25). Simplicity, reliability, and validity have made physical ability tests popular in areas such as sports medicine, athletics, physical education, physical medicine and rehabilitation, and ergonomics. In addition to age, gender, body composition, skill, and various lifestyle factors like physical activity and training (1,3,45), the body size has proved to be an important attribute that has profound consequences on the outcome of a number of physical ability tests (3,24,34).

The structure of physical abilities might be considered as a hypothetical construct for which validity is necessary to be experimentally demonstrated (33). Safrit (41) emphasized that physical abilities represent a multidimensional construct, which cannot be appropriately presented with a single measure. A comprehensive testing performed during the 1950s and 1960s represents benchmark studies on the structure of physical abilities (15). Regarding muscle strength in particular, this structure suggests “static strength” (exerting static forces and lifting submaximal weights), “explosive strength” (running and jumping), and “dynamic strength” (push-ups and squats) as independent physical abilities. A number of comprehensive testing batteries have been designed taking into account either this or other similar structures of physical abilities (33,42), including the comprehensively used Eurofit test battery (13). Of particular importance for the present study could be 2 groups of tests that assess presumably independent physical abilities: the muscle power and rapid movement performance.

From the mechanical aspect, power represents either a rate of mechanical work done or a product of force acting upon an object and the object's velocity. At the level of a single isolated muscle, the muscle power is generally believed to depend on a number of both “biological factors” (e.g., muscle size, percentage of fast twitch motor units, and muscle architecture) and mechanical factors (e.g., muscle length and speed of shortening (14,34)). Regarding the later ones, due to the effect of speed of contraction, muscle strength (i.e., ability to exert maximum force under given conditions) and muscle power are only moderately related. Namely, the maximum force can be exerted against heavy external resistance (and, therefore, during slow and long-lasting movements), whereas maximum muscle power is recorded against moderate loads, which is often represented solely by weight and inertia of one's own body (11,12,32). A number of specific approaches have been used for estimating the power-generating capacity of human skeletal muscle (46). The most often employed approach has been the direct assessment of muscle power. That approach is based on the assessment of maximum muscle power output either from measured or from assessed muscle force or work done during complex movements, such as cycling (e.g., Wingate test), running (e.g., Margaria step test), jumping (e.g., Sargent jump on a force platform), or during isolated joint rotations performed against an isokinetic apparatus (46). Note that these tests usually require expensive laboratory instrumentation and also additional data processing, and their outcome is represented as power (i.e., calculated in watts).

Another important group of tests of physical abilities infrequently related with muscle power are the tests of rapid movement performance (e.g., running velocity, jump height, or speed of throwing and kicking of light objects). The outcome of these tests is usually reported by various kinematic variables that presumably reveal “higher performance” (e.g., higher running speed, shorter movement time, or higher vertical jump). Although often obtained from the same movements as the above-mentioned tests of direct assessment of muscle power, the outcomes of the tests of direct assessment of muscle power and the tests of rapid movement performance proved to be only moderately related (2,27,35). For example, Aragon-Vargas and Gross (2) showed that peak power and mean power (i.e., the tests of direct assessment of muscle power) share only 46 and 43% of common variance with the vertical jump height (being a test of rapid movement performance), whereas Winter (50) suggested that the jumping performance is based on muscle's capability to develop impulse rather than muscle power. As a result, the tests of direct assessment of muscle power and the tests of rapid movement performance are usually separately conducted within the comprehensive test batteries because they presumably assess partly independent physical abilities.

A possibility is that 2 above-mentioned groups of tests could be closely related originates from the phenomenon that the outcomes of the tests of direct assessment of muscle power and rapid movement performance are differently related with body size. Although the role of body size has been studied for decades (19,34), the normalization for body size had been often inconsistently applied when presenting the data from routine physical ability tests (23,25). Therefore, most of the data presented through the scientific literature have been body size dependent, whereas the relationships among different tests have been confounded by the effect of body size (25). It has been generally accepted that, similarly to the tests of muscle strength, the outcome of the tests based on the direct assessment of muscle power is positively related with body size. Specifically, the simplest models based on the presumption of geometric similarity predict the allometric parameter equals 2/3 (i.e., the tested power increases proportionally to body mass on power 2/3 (3,31)), whereas according to some more elaborate models, the same parameter should be about 3/4 (34,36,48). However, most of the experimental studies report the values closer to 2/3 or even lower (29,30). From the other side, the tests of rapid movement performance (e.g., duration of discrete point-to-point movements, maximum jump height, and maximum running speed) seem to be body size independent. For example, our recent study (30) demonstrated that the power calculated from the ground reaction force recorded during different types of maximum vertical jumps (representing the direct assessment of muscle power) was strongly related with body size, whereas the height of the same jumps (representing rapid movement performance) was mainly body size independent. Moreover, our later study (31) demonstrated that the jump height and muscle power calculated from the same jump, properly normalized for body size, assess the same physical ability as they belonged to the same principal components after had been factor analyzed.

In the present study, we extended the approach of Markovic and Jaric (31) to a broad range of physical ability tests. The aim of the study was to assess the effect of body size on the structure of physical abilities. In particular, we hypothesized that the tests of rapid movement performance assess the same physical ability as the tests of direct assessment of muscle power properly normalized for the effect of body size. This finding could be of considerable importance not only for designing and interpreting routine batteries of physical ability tests but also for better understanding of some essential properties of human movements.

Methods

Experimental Approach to the Problem

The entire experimental procedure was based on a fixed design. The reasons for avoiding randomization were that (a) we evaluated a single group of subjects; (b) most of the tests were not expected either to demonstrate transfer of practice or to cause fatigue; and (c) due to a long duration and the associated fatigue, some particular tests had to be performed at the end of the experimental sessions (see further text for details). The testing was conducted within 3 consecutive testing sessions with 2 or more days of rest among them. The sessions included 23 physical ability tests based on muscle strength assessment and on either direct assessment of muscle power or assessment of rapid movement performance. Within the first testing session, the maximum isometric strength in half squat, bench press, and shoulder press and the standing ball kick, seated chest pass, squat jump (SJ), 15-second repeated rebound jump, and Wingate test on bicycle ergometer were tested. The second session included measurements of 1 repetition maximum (1RM) of the concentric half squat, bench press, and shoulder press, as well as countermovement jump, sprint 10 m (standing start), and sprint 20 m (flying start). Maximum power output in SJ and concentric bench and shoulder press throw and the drop jump (DJ), Margaria staircase test, and Wingate test on rowing ergometer were tested within third testing session. All tests within the same testing session were performed in a randomized order. The only exceptions were 2 Wingate tests that were always performed at the end of testing sessions due to prolonged recovery demands. After the last testing session, the body size and composition measurements were taken.

A standard 10-minute warming up and stretching procedure preceded each testing session. After both a detailed verbal explanation and demonstration of each test, all subjects performed one-practice trial, although they have already been familiar with most of the tests through their regular activity courses. Two trials of all physical ability tests (except in the 2 Wingate and 3 1RM tests) were recorded with 2-minute rest periods between them, and a better result was stored for further analysis. The pause between 2 consecutive tests was approximately 5 minutes.

Subjects

Male sport and physical education students (n = 111; age 18-24 years) participated in this study. All subjects were physically active through their standard academic program that included on average 12 activity classes per week. A number of subjects also reported regular sport activity outside the academic program, although none of them were a professional athlete. The study was approved by Ethical Committee of the Faculty of Sport and Physical Education, University of Belgrade. Before testing, all participants received a complete explanation regarding the purpose and procedures of the study and possible risks. They also signed an informed consent document according to Helsinki Declaration. Neither medical problems nor recent injuries were reported by any of the subjects.

Procedures

Tests of Muscle Strength

Maximum isometric strength in half squat (Fmax HSquat)

Isometric tests of muscle function have been reported to have high test-retest reliability (49). The maximum voluntary force was measured by a strain gauge dynamometer (KKM-1; AB Bofors, Stockholm, Sweden; the linearity and reliability better than 0.4 and 0.5%, respectively). The signal was recorded at a rate of 200·s−1, low-pass filtered (5 Hz), and stored on a computer disc for off-line analysis. A modified Smith machine was provided with one additional upper bar that was attached to the lower one over a force transducer sensitive on a compression. The lower bar was rested on the shoulders and knees were flexed. The knee angle was 90° (measured manually with a goniometer). The metal stops precluded bar lifting. The subjects were instructed to achieve the maximum force of knee extensors against the lower bar as soon as possible and to retain it for 4 seconds. A digital display provided feedback information regarding the exerted force.

Maximum isometric strength in bench press (Fmax BenchP)

The subjects lied supine on the bench. The lower bar was held at self-selected width but not narrower than shoulder width. The elbow angle was 90°, and the bar was placed above the chest in the level of nipples. The testing procedure was the same as for testing maximum isometric strength in half squat.

Maximum isometric strength in shoulder press (Fmax ShoulderP)

The subjects sat down on the vertical shoulder press bench. The lower bar was held at self-selected width but not narrower than shoulder width. As a result, the elbow angle was 90° and was placed above the shoulders behind the neck. The testing procedure was the same as for testing maximum isometric strength in half squat.

One repetition maximum of concentric half squat (1RM HSquat)

One repetition maximum measurements demonstrated high test-retest reliability (1). This test was performed using the modified Smith machine. The shoulders were in contact with a bar supported by the bottom metal stops, and the starting knee angle was 90°. The subjects were instructed to perform a concentric leg extension against the resistance determined by the weight plates added to the bar. Additional specific warm-up consisted of performing 10, 8, and 6 repetitions at 40, 50, 60%, respectively, of perceived maximum based on the maximum isometric strength test results. Three subsequent lifts were then made to determine the 1RM with 5 minutes of rest between lifts.

One repetition maximum of concentric bench press (1RM BenchP)

The subjects lied supine on the bench. The bar was positioned 2 cm above the subject's chest at the level of nipples supported by the bottom metal stops. The bar was pressed from the subject's chest against a maximum load. Both the procedure and sequence of the warm-up and testing trials were the same as for testing the 1RM of concentric half squat.

One repetition maximum of concentric shoulder press (1RM ShoulderP)

The subjects sat down on the vertical shoulder press bench. The bar was positioned 2 cm above the subject's shoulders behind the neck supported by the bottom metal stops. Both the procedure and sequence of the warm-up and testing trials were the same as for testing the 1RM of concentric half squat.

Tests of Direct Assessment of Muscle Power

Wingate test on bicycle ergometer (WAnT bic)

The subjects were instructed to pedal as fast as possible for 30 seconds at the resistance load of 95 g·kg−1. The test was initiated after few initial revolutions, which were performed to avoid starting wheel inertia. Power was calculated and displayed by ergometer computer, and the subject was verbally informed regarding his result. The collected data were averaged to calculate the mean power (Mean Pow: mean work output over the 30-second period) and the peak power (Peak Pow: the highest power output within a 5-second period). High test-retest reliability for both mean and peak power has been reported (7).

Wingate test on rowing ergometer (WAnT row)

Additional Wingate test protocol identical to the previous one was performed on a Concept II rowing ergometer (40).

Margaria staircase test (Margaria ST)

The subjects stood 2 m from the stairs (0.165 m height) and were instructed to run as fast as possible 2 steps at a time up a staircase. The photocells were placed on the 8th and the 12th stairs (28). The test results were calculated using the following equation: P = (m·g·h)/T, where P represents the calculated power output (in watts), m is body mass of the subjects (kg), g = 9.81 m·s−2, h represents the total vertical height of stairs climbed (meters), and T represents the measured time (seconds).

15-second repeated rebound jump test (15sRRJump)

This test was performed using a timing mat. While keeping their hands akimbo and reducing the contact with the ground, the subjects were instructed to perform as many of the highest jumps as possible over 15-second period (6). The maximum power output was calculated and displayed by computer software (Jumper-FiTRONiC, version 1.20).

Maximum power output in squat jump, concentric bench, and shoulder press throw (MPowO HSquat, MPowO BenchP, and MPowO ShoulderP, respectively)

These tests were performed using a modified Smith machine. The Smith machine is a testing device that allows for safe performance of dynamic exercise while recording relevant kinematic data (49). The subjects were instructed to perform an explosive SJ, an explosive bench, or a shoulder press throw from the starting positions, identical to the corresponding 1RM test. The bar was loaded with 40% of the previously determined 1RM because the same relative loading should provide the maximum power output (37,44). The bar displacements were measured using a 3-dimensional analyzing kinematic system (Qualisys-240 Hz) where a marker was placed at the end of the bar. The concentric phase was determined as the time interval between the movement initiation and the instant of the highest vertical velocity of the marker. The maximum power output was calculated just over the concentric phase by multiplying the force (weights × 9.81 m·s−2) and velocity (distance/time).

Drop jump

The test was performed on a timing mat. The subjects dropped from a 30-cm high box. They were instructed to perform jump upon landing with the minimum ground contact time and maximum height. The hands were held akimbo. The maximum power output and the jump height were calculated and visually displayed by computer software (Jumper-FiTRONiC, version 1.20).

Tests of Rapid Movement Performance

Squat jump

The test was performed on the timing mat. The knee angle at starting position was 90°, and the hands were held akimbo (26). To provide no additional countermovement before the jump, a 3-dimensional kinematic analysis system (Qualisys-240 Hz) was used. The marker was placed on the trochanter major, and the trials were repeated if any downward movement was recorded. The jump height was calculated and visually displayed by the computer software.

Countermovement jump

The test was performed using the timing mat. The subjects were instructed to perform an unconstrained maximum vertical jump from standing upright position with hands held akimbo that includes the initial countermovement (26). The jump height was calculated and displayed by the computer software.

Sprint 10 m (standing start) and Sprint 20 m (flying start)

Three photocells were positioned at the start, 10 and 30 m apart. The subjects were instructed to run as fast as possible over the entire distance of 30 m. As a result, a 10 m (corresponds to acceleration) and the following 20 m (corresponds to maximum speed) of the running time was measured. The standing start was used and the time was initiated when the first photocell infrared ray was cut.

Standing ball kick (SBKick)

The subjects were instructed to kick a soccer ball (0.450 kg) with their dominant leg without a run-up. The subjects stood with the nondominant leg aside the stationary ball and, after the natural counterswing of the kicking leg, kicked the ball as fast as possible toward the target. The maximum foot velocity was measured using the 3-dimensional kinematic analysis system. The marker was placed at the lateral malleolus of the kicking leg.

Seated chest pass (SCPass)

The subjects were sitting on the floor with their head, shoulders, and lower back positioned against the wall behind. They were instructed to throw a soccer ball (0.450 kg) positioned in front of their chest with both hands as fast as possible forward without the head, shoulders, and hips moving from the wall (9). The maximum hand velocity was measured by placing a marker of the 3-dimensional kinematic analysis system at the radial styloid process.

Body Size and Composition Assessments

Body size and composition measures were taken according to the procedures recommended by the International Society for the Advancement of Kinanthropometry (38). Body height and mass of all subjects (without shoes and wearing only shorts) were measured to the nearest 0.1 kg and 0.1 cm, respectively. The cross-hand technique was applied for measuring chest, upper arm, and thigh girths by using an anthropometric tape to 0.1 cm. The skinfold measurements were taken from triceps, subscapular, biceps, supraspinale, abdominal, front thigh, and medial calf to the nearest 0.2 mm by using a skinfold caliper (John Bull, British Indicators Ltd, West Sussex, UK). To reduce measurement variation, the same experienced investigator examined all subjects. Each measurement was taken in triplicate, and the average value was taken for further analysis. The sum of 7 skinfolds was used to indicate the level of fatness in the subjects (17).

Statistical Analyses

To obtain muscle strength and directly assessed muscle power data (S) independent of body size (M), we applied a theoretical approach (3,19,25,34) that presumes a standard allometric relationship. Specifically, the approach was based on the simple theory of geometric similarity (3,19), suggesting that both the muscle strength and directly assessed muscle power should be proportional to the muscle cross-sectional area, which is proportional to body mass raised to the power of 2/3 (i.e., the allometric parameter is b = 2/3 = 0.67). Therefore, we calculated the normalized (i.e., body size independent) indices of both muscle strength and power (Sn) using the formula Sn = S/M0.67. Although the elaborate theoretical models recommend the allometric parameter b = 0.75, the parameter we used is closer to most of the experimental findings (see Introduction for details). On the other hand, both the same theory and the experimental data (29) suggest that rapid movement performance does not depend on body size (i.e., b = 0).

Descriptive statistics was calculated for all experimental data as mean and SD. A Pearson product moment method was used to assess both the relationship between 2 consecutive trials of each particular test (excluding 2 Wingate and 3 1RM tests) and the relationships among the tests of muscle strength, directly assessed muscle power, rapid movement performance, and various indices of body size. Paired samples t-test was employed for detection of possible learning and fatigue effects on 2 consecutive trials of each particular test. Statistical significance was set at p ≤ 0.05. The corresponding intercorrelation matrices of all selected variables were factorized using a principal components factor analysis (PCA (39)). The data were analyzed using SPSS (version 10.0). The number of significant principal components in the factor pattern matrix extracted by the PCA was determined by the scree test, which plots the eigenvalues of each component in the initial solution, and only the components on the steep slope were retained (8). The original factor pattern matrix was rotated to improve the simple structure of the matrix. This rotation was oblique and used a Promax criterion with Kaiser normalization. The final outcomes of each PCA were communalities and factor loadings for each manifest variable, eigenvalues, and percentage of variance explained by each rotated principal component. Statistical significance was set at p ≤ 0.05.

Results

The subjects' mean (SD) body height; mass; girths of chest, upper arm, and thigh; and body fat percentage were 182.9 (6.7) cm; 79.7 (8.7) kg; 99.3 (4.6) cm, 30.8 (2.2) cm, 58.9 (3.9) cm; and 7.2% (3.0%), respectively. Table 1 shows descriptive data of the applied muscle strength, directly assessed muscle power, and rapid movement performance tests, as well as both the relationship and differences between 2 consecutive trials. Although some tests revealed small but significant differences between the first and the second trial, note that virtually all tests demonstrated a high reliability (i.e., r > 0.80).

Table 1
Table 1:
The outcome of the applied muscle strength (I), directly assessed muscle power (II), and rapid movement performance tests (III).†‡

Intercorrelations among the body size indices and outcomes of the applied physical ability tests calculated before (above diagonal) and after (below diagonal) the normalization for body size are depicted in Table 2. The applied normalization was associated with a decrease in the correlation coefficients between the indices of body size and both the muscle strength and the directly assessed muscle power measures. This finding suggests that the applied normalization successfully accounted for the presumed effect of body size on the selected muscle strength and directly assessed muscle power measures. Another consequence of the applied normalization was an increase in most of the correlation coefficients observed between directly assessed muscle power and rapid movement performance measures.

Table 2
Table 2:
Intercorrelations among the results of muscle strength (I), directly assessed muscle power (II), rapid movement performance tests (III), and body size measures (IV) before (above diagonal) and after (below diagonal) the normalization of physical ability tests for body size (r > 0.19, p < 0.05).*
Table 2
Table 2:
(continued) Intercorrelations among the results of muscle strength (I), directly assessed muscle power (II), rapid movement performance tests (III), and body size measures (IV) before (above diagonal) and after (below diagonal) the normalization of physical ability tests for body size (r > 0.19, p < 0.05).*

The main finding of this study is related to the differences between the outcomes of 2 PCAs applied before and after the normalization for body size performed on muscle strength and directly assessed muscle power. Specifically, the first PCA (Table 3) revealed 3 principal components or factors, which accounted for 57.5% of variance of all selected manifest variables. The highest correlations (i.e., “factor loading”) with the first principal component (Table 3) were demonstrated by 5 of 6 muscle strength tests and by 2 directly assessed muscle power tests (MPowO BenchP and MPowO ShoulderP). Upper arm girth was also correlated with this principal component. The second principal component was loaded by 4 of 5 selected indices of body size and by 5 of 10 directly assessed muscle power tests (all Wingate tests and Margaria ST). One muscle strength test (Fmax HSquat) and 1 rapid movement performance test (SCPass) also loaded the second principal component. The correlations of 2 directly assessed muscle power tests (15sRRJump and DJ power) with this principal component are very close to the highest one. The third principal component was loaded by 6 of 7 rapid movement performance tests and by 3 directly assessed muscle power tests (15sRRJump, MPowO Hsquat, and DJ power).

Table 3
Table 3:
PCA conducted before the normalization of the results of muscle strength (I) and directly assessed muscle power tests (II) for body size and the results of rapid movement performance tests (III) and body size (IV) are also depicted.*

The second PCA (Table 4) was applied on the same set of data where the muscle strength and directly assessed muscle power tests were normalized for body size. The results also revealed 3 principal components, which explained 52.9% of variance of all selected manifest variables. The first component was loaded by 8 of 10 tests of the direct assessment of muscle power and by 6 of 7 tests of rapid movement performance. The second principal component was loaded by all muscle strength tests and by 2 directly assessed muscle power tests (MPowO BenchP and MPowO ShoulderP). Finally, the third principal component was loaded by all selected indices of body size and by 1 rapid movement performance test (SCPass). However, of the utmost importance for the present study is that the applied normalization caused the outcomes of most of the directly assessed muscle power and rapid movement performance tests to load the same principal component.

Table 4
Table 4:
PCA conducted after the normalization of the results of muscle strength (I) and directly assessed muscle power tests (II) for body size (seeTable 3 for other details).*

Discussion

In the present study, we evaluated the relationship between 2 groups of tests that had been considered to assess partly different physical abilities. The findings were mainly in line with the hypothesized correspondence between the tests of rapid movement performance and the tests of direct assessment of muscle power properly normalized for the effect of body size. However, before discussing the obtained results, several potentially important methodological aspects should be emphasized.

When studying the relationship among various human physical abilities, the results obtained are known to be affected by a number of confounding factors: age, gender, body composition, level of physical activity, and skill (1,3,23,45). Therefore, it should be stressed that we tested a relatively homogenous group who were not only of the same age and gender but also physically highly active and already familiar with the tested tasks. The percent fat was also rather low and comparable to trained male athletes (47). Moreover, the results obtained from physical ability testing are comparable with the results recorded in elite athletes (16).

An important methodological issue in our study is related to the use of PCA for data analysis. It has been suggested that a particular attention should be paid to an appropriate selection of variables, the number of variables, the number of subjects tested (39). Specifically, more than 10 moderately to strongly interrelated variables should be analyzed using PCA, while the number of subjects tested should be at least 10 times higher than the number of variables. In our study, we used 28 manifest variables measured on 111 subjects. Although the number of subjects is below the above-mentioned methodological recommendations, the literature review indicated proportionally equal or even smaller number of subjects per variable in most of the similar studies (4,10,18,21,22,43). Moreover, the variables were moderately to strongly interrelated (Table 2), suggesting that their selection was appropriate. Finally, the data shown in Table 1 indicate a high test-retest reliability of applied physical ability tests, which is in line with previous findings (1,20,49), although a high face validity of these tests has been also generally accepted (1,7,13,16,38,42). Therefore, from the methodological perspective, we believe that the present study provides both a valid and reliable set of data that allows for testing the stated research hypothesis.

The main finding of the present study is related to the observed differences in the results of PCA before and after the normalization of applied physical ability tests for body size (e.g., body mass; Tables 3 and 4). Although both the number of principal components were equal (i.e., 3) and the percentage of variables' variance explained by principal components extracted before and after normalization for body size were comparable, their factor structure differed considerably. Before the normalization (Table 3), the first component was predominantly loaded by muscle strength tests, which correspond to the ability to generate a high muscle force. However, the second principal component revealed the structure that seems hard to interpret because it was mainly loaded by 5 of 10 tests of direct assessment of muscle power and by almost all body size indices. Taking into account that the direct assessment of muscle power is also positively related with body size, one could speculate that the observed complex structure of this principal component is probably mediated by body size. The effect of body size on the complexity of structure of physical abilities has been also recorded in our previous study (31). The third principal component was mainly loaded by the tests of rapid movement performance. From the perspective of the main aim of the study, the main implication of the first PCA is that the tests of direct assessment of muscle power and the tests of rapid movement performance mainly belong to different principal components and, therefore, are likely to assess partly independent physical abilities.

The second PCA was applied to the set of data where the assessed muscle strength and power data were normalized for the effect of body size (Table 4). In contrast to the previous structure, the present one seems to be both simpler and easier to interpret. With very few exceptions, the first principal component included virtually all tests of the direct assessment of muscle power and rapid movement performance, whereas the second and third components included the muscle strength tests and body size indices, respectively. The obtained structure of principle components has several important implications. First, the outcomes of both the muscle strength and muscle power tests seem to be independent of body size, which suggests appropriateness of the applied normalization procedure. Second, muscle strength and power proved to be partly independent, which is in line with a number of previous studies (see Introduction for details). However, the most important finding is that the outcomes of the tests of direct assessment of muscle power and rapid movement performance were grouped within the same principle component. This finding is not only in line with our previous findings obtained from the tests of vertical jumps (31) but also strongly supports the hypothesized correspondence of these 2 groups of tests of physical abilities. Specifically, it seems that the tests of rapid movement performance could assess the same physical ability as the tests of direct assessment of muscle power properly normalized for the effect of body size.

The obtained findings could have several potentially important implications. From the practical aspect, it seems that the comprehensive batteries of physical performance tests could be considerably simplified because they may not need to include the tests from both groups of the discussed tests of physical abilities. For example, one could replace the tests of direct assessment of muscle power (usually requiring both a complex laboratory equipment and normalization procedures) with the tests of rapid movement performance. However, from the theoretical aspect, the implications could be even more important. Regarding the above-discussed Fleishman's (15) structure of “strength,” the results supported the distinction between the “static strength” (corresponds to our muscle strength tests) and “explosive strength” (corresponds to our tests of rapid movement performance). However, our results suggest that the Fleishman's explosive strength corresponds to the ability of exerting high external power normalized for body size.

The correspondence of muscle power and rapid movement performance could have potentially important implications for understanding some important aspects of adaptation of the muscular system and for the design of the muscular system per se. For example, the correspondence of muscle power and movement performance could justify the importance of the training procedures that target enhancing muscle power to improve movement performance. From the other side, functional performance tests could be used to assess the power-producing ability of the neuromuscular system. Furthermore, one could also speculate whether the obtained findings also support our recent hypothesis concerning the design of the muscular system. Namely, based on the finding that both the loading and unloading of human body are associated with a decrease in the muscular mechanical output (e.g., momentum or average power), we hypothesized that the muscular system could be designed to produce the maximum mechanical output when loaded with weight and inertia of one's own body (32). From that perspective, the finding that the maximum power output corresponds to maximum performance of rapid movement, such as running and jumping, should not be considered as a surprising one.

We identified 3 potential limitations of the present study. First, we applied the same allometric parameter (i.e., b = 0.67) when normalizing all applied strength and power tests, although the experimental data reveal quite inconsistent findings across the variety of the tests and population (4,5,23,29,36,30). The main reason was that we intended to evaluate a “standard approach” (despite some potential shortcomings; see previous sections) that could justify a general recommendation regarding the method of normalization (3,24,25). Nevertheless, future studies should be focused on possible differences among the allometric parameters of particular tests that could be caused by the specific role of inertial and gravitational resistance, participation of the body segments in the tested movements that the tested outcome does not account for, and so on (1,11,12,32). Second, the multivariate statistical method (i.e., PCA) employed in this study could heavily depend on various methodological issues related to the selection of both the subjects and variables. Although we believe that the PCA was applied in the present study in a methodologically acceptable way (see first subsection of Discussion for details), further complementary studies based on other methodological approaches are needed to explore the studied phenomenon. Finally, some systematic exceptions in the structure of principal components obtained after the applied normalization procedure could be speculated. Specifically, the results suggest that the subject's ability to produce muscle power might not be generalized across the arm and leg muscle groups (also suggested by Fleishman (15)) because some tests based on action of arm muscles loaded different components from most of the remaining tests of the same group (i.e., MPowO BenchP, MPowO ShoulderP, and SCPass; Tables 3 and 4). Regarding the SCPass test in particular, the difference from the hypothesized structure could be caused by using of a ball of the constant mass, which could have provided an advantage to the heavier subjects because they were throwing a relatively lighter ball.

Practical Applications

Despite the aforementioned limitations, we could conclude that our findings provide additional evidence regarding the studied structure of human physical abilities. Specifically, it seems that the tests of rapid movement performance could assess the same physical ability as the tests of direct assessment of muscle power normalized for the effect of body size. Both groups of tests have been broadly popular in various human movement-related fields. Therefore, the finding should be of considerable importance for both designing and simplification of a number of batteries of physical ability tests because both groups of the tests do not necessarily need to be represented. A particular advantage could be that the tests of direct assessment of muscle power that usually require complex equipment and elaborate data processing can be replaced by the much simpler tests of rapid movement performance.

Acknowledgments

This study was supported in part by a Serbian Research Council grant (145082).

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Keywords:

mass; scaling; strength; structure

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