THE CONTRIBUTION OF MAXIMAL FORCE

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Journal of Strength and Conditioning Research, 2006, 20(4), 867–873 䉷 2006 National Strength & Conditioning Association

THE CONTRIBUTION OF MAXIMAL FORCE PRODUCTION TO EXPLOSIVE MOVEMENT AMONG YOUNG COLLEGIATE ATHLETES MARK D. PETERSON,1 BRENT A. ALVAR,1

AND

MATTHEW R. RHEA2

Department of Exercise and Wellness, Arizona State University, Mesa, Arizona 85212; 2Department of Physical Education, Southern Utah University, Cedar City, Utah 84720. 1

ABSTRACT. Peterson, M.D., B.A. Alvar, and M.R. Rhea. The contribution of maximal force production to explosive movement among young collegiate athletes. J. Strength Cond. Res. 20(4): 867–873. 2006.—Critical to multidimensional sport conditioning is a systematic knowledge of the interactions between fitness components, as well as the transference relationships to performance. The purpose of this investigation was to examine the relationships between lower body muscular strength and several fundamental explosive performance measures. Fifty-four men and women collegiate athletes were tested to determine (a) lower-body muscular strength (1 repetition maximum barbell back squat), (b) countermovement vertical jump height and peak power output, (c) standing broad jump distance, (d) agility (cone Ttest time), (e) sprint acceleration (m·s⫺2), and (f) sprint velocity (m·s⫺1). Analyses were performed using Pearson r correlations to examine these relationships. Partial correlations tested for relationships between performance measures while controlling for muscular strength. T-tests were performed to assess the difference between men and women. Correlation data demonstrated that significant (p ⬍ 0.01) strong linear relationships were indicated between muscular strength and power, as well as every sport-performance field tests. However, when controlling for strength with partial correlation, each of these relationships appreciably diminished. Significant differences (p ⬍ 0.05) were found between men and women for each of the performance tests. Muscular strength, peak power output, vertical jumping ability, standing broad jump, agility, sprint acceleration, and sprint velocity were all shown to be very highly related. Further examination demonstrated that body mass–adjusted muscular strength is more highly related to performance measures than is absolute muscular strength. Current correlation data provide a quantified look at the interaction between muscular fitness components, as well as the transfer relationship to several athletic-specific performance measures. KEY WORDS. power, performance enhancement, correlations

INTRODUCTION esistance training has been used as a means to augment muscular hypertrophy (7, 12, 25), muscular strength capacity (28, 30), rate of force production, and coordinated movement speed (1, 17). Traditionally, each of these objectives was not thought to be mutually exclusive, and athletes were trained haphazardly to get bigger, stronger, and faster. However, as scientists and practitioners continue to collaborate, more empirical evidence suggests a need for specific training approaches to accommodate the various muscular fitness components. Training to stimulate adaptation within the muscular system exploits several synergistic physiological components that lend to increased force production, including neuromuscular, metabolic, and hormonal-capacity modifications.

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One of the most well known characteristics of muscle tissue functionality is the force-velocity relationship. This relationship exemplifies the interactions between muscular contraction velocity and magnitude of contractionforce, such that a muscle contracts at a speed inversely proportional to the load. Intuitively, the maximal velocity of a given movement is dependent upon the resistance applied to that movement. This trade-off between velocity and force for coordinated movement is easily demonstrated during resistance training, in which extremely high loads are lifted through a range of motion at very low velocities. At some incremental point, the load may be great enough that velocity reaches zero, and an isometric contraction is produced. It is at this point that maximal force is generated. While isometric expressions of force output were traditionally considered to the measurable benchmark for maximal muscular force, most current-day research investigations and sport conditioning practices that conform to the principles of specificity utilize maximal dynamic force (1 repetition maximum [1RM], the maximum weight that can be lifted through an eccentric and concentric range of motion) as the gold standard. Essentially, the principles of specificity are governed by the assumptions that not only are muscular fitness components distinct but also optimal development is highly contingent upon training modality. Although to some extent the principles are based in theory, many professionals successfully use the premises of specificity to prescribe exercise testing and training for sport. One particular aspect, velocity specificity, implies that resistance training produces its greatest strength gains at the velocity that it is performed. As this principle suggests, improvement with high resistance, low velocity movements will not bring about optimal improvement in low resistance, high velocity movements. Likewise, improvement in low resistance, high velocity movements will not elicit optimal enhancement of high resistance, low velocity movements. In reality, muscular power is exhibited by all muscle actions that produce a velocity and may be defined as the rate of muscular force production, throughout a range of motion (14). Muscular power is considered necessary for sports, as well as normal functional ability. An increase in power enables a given muscle to produce the same amount of work in less time or a greater magnitude of work in the same time. Muscular peak power (PP) (i.e., maximal speed strength) has been designated as the maximum potential product of strength and speed and is demonstrated as the highest power output attainable during 867

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a given movement/repetition (21). Given that PP is related both to force generation and movement velocity, muscular PP capacity has been viewed as an exceedingly important testing variable and training objective in most sport conditioning programs, and especially for those sports that involve sprinting and jumping. An area of exercise prescription research that requires further delineation is that which investigates the strength of relationships between maximal dynamic force capacity of the lower body and powerful lower body movements. Establishment of such data is necessary to fully comprehend the underlying strength requirement for superior participation in power-related performance activities, as well as to demonstrate the relative importance of specific training to stimulate optimized muscular strength adaptation. A review of the literature indicates a positive relationship between closed-kinetic-chain lower-body strength and, both vertical jump (VJ) height and standing broad jump distance (2, 6, 8, 20, 23, 37, 40). Other investigations have found significant correlations between muscular strength and peak rate of force production (17), peak muscular power (26), and instantaneous power production (11). To date, much of this research has been conducted with novice study subjects or lesser-trained recreational athletes. Research examining more experienced individuals has found conflicting results (3, 10, 36) and poses a potentially diminished association between the magnitude of dynamic force production and power-related performance measures. A theoretical explanation for this phenomenon may be considered: Greater strength capacities beyond a threshold level will not continue to elicit a subsequent predictable/transferable improvement in muscular power or explosive movement within an elite, homogeneous population. This tenuous belief has been cultivated in recent years and has led many sport conditioning professionals to adopt speed training modalities, while simultaneously de-emphasizing fundamental strength exercises. Ultimately, evidence suggests that muscular strength shares a significant relationship with muscular power and, in some capacity, with sport specific powerful movement. However, there is a shortage of empirical data obtained from trained individuals using biomechanically specific exercises and tests to verify these interactions, and even less research to delineate these relationships among various levels of training statuses (e.g., such as within the hierarchy of athletic rankings). Alternatively, many recent investigations and reviews have been conducted to identify the necessary training intensities and speeds to maximize the expression of muscular power (4, 19, 22, 33). These studies provide valuable information to the strength and conditioning professional with regard to the optimal training load to accentuate peak mechanical power output during a given movement repetition. Unfortunately, most do not incorporate a system of usage for such recommendations. Ostensibly, during the conversion process from science to practice, important practical application is likely lost. As previously mentioned, muscular power and explosive coordinated movement rely on rate of force production, as well as magnitude of force production. Case in point, failure to optimize the basic force producing characteristics of muscle may diminish the developmental potential of muscular power adaptation and expression. It

is important to examine these fitness components in order to clarify the framework of exercise prescription within a periodized, sport-specific model. The purpose of this investigation was therefore to analyze the relationships between slow velocity dynamic muscular strength and several higher velocity auxiliary fitness tests among firstyear collegiate athletes, a population thought to exemplify advanced athleticism despite lacking extensive formalized fitness training. It was hypothesized that test data from lower body muscular strength measures (i.e., as measured by 1RM back squat) would reveal a moderate positive association with explosive movement tests. Identification of these interactions could offer insight into the contribution of muscular strength capacities to the performance of explosive sport-specific measures among young collegiate athletes. Data may also elucidate the degree to which other unidentified physical (e.g., rate of force production) and technique-related variables influence the variances in power/performance profiles.

METHODS Experimental Approach to the Problem

During the normal conditioning schedule for each respective sport, college athletes were tested regularly for capacities of lower body muscular strength and jumping ability on barbell back squat and countermovement VJ, respectively. Tests of variable-distance timed sprints, standing broad jump, and a timed agility protocol were also assessed. Correlation analyses were performed to examine the relationship between each of these measures. The results were then analyzed to determine the degree of relation between each. Subjects

First-year college athletes (n ⫽ 19 men and 36 women) of various sports were recruited as subjects for this investigation (mean age ⫽ 19.4 years; age range ⫽ 18–21 years). Each of the athletes included in the analyses were from one of the following sports: men’s basketball, women’s basketball, women’s volleyball, men’s baseball, and women’s softball. All subjects were involved in regular sport conditioning during the time of the investigation. Subsequently, all subjects were of similar experience with regard to pertinent testing protocols. Inclusion criteria for participation in this study were (a) no pending medical problems and (b) no ankle, knee, or back pathology within the preceding 4 months. All athletes were in good physical condition during the time of testing and cleared by a certified athletic trainer to participate in sport conditioning. Furthermore, each subject signed informed consent documents allowing use of pertinent testing data for research and publication purposes. All procedures were approved by the human subjects review board of Arizona State University. Procedures

Lower body dynamic muscular strength was measured via the 1RM barbell back squat exercise, according to the National Strength and Conditioning Association (NSCA) guidelines for strength testing (18). Each athlete had previously performed this test numerous times in conjunction with his or her normal sport conditioning program, for purposes of monitoring strength development. Hence, each subject had been well familiarized with the proce-

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dures of the test. Athletes were required to perform a nonspecific warm-up of running and to participate in dynamic stretching before executing approximately 10 squat repetitions with a light resistance. The resistance was then progressively increased to amounts estimated to be less than the subject’s 1RM, for several subsequent warm-up sets. Finally, for the 1RM test, the resistance was increased in incremental loads following each successful 1RM attempt, until failure. All 1RM values were determined within 3–5 attempts, in order to ensure reliability. An inclusion criterion for squat depth was to complete the 1RM attempts at a 90⬚ knee angle. For each respective individual, this measurement standard was set using a standard handheld goniometer (Jamar EZ-Read; Sammons Preston Roylan, Bolingbrook, IL) prior to warm-up sets. If this depth of squat was not sufficiently met, the test was not counted. Trained sport conditioning specialists and investigators oversaw the testing process to ensure proper technique and safety. Body masses were taken to enable a standardized evaluation of maximal lower body relative strength. Jumping ability was assessed using a countermovement VJ and horizontal standing broad jump. For VJ testing, standing reach and VJ height were tested using the Vertec apparatus (Sports Imports, Columbus, OH). Each athlete was allowed 3–5 trials in order to achieve maximal jump performance. PP was estimated using the equations developed by Sayers et al. (29): PP (W) ⫽ (60.7) ⫻ (jump height [cm]) ⫹ 45.3 ⫻ (body mass [kg]) ⫺ 2.055. This equation was used to estimate PP output because gender differences do not interfere with the accuracy of PP estimates (29). Further, this method has been verified as a valuable means of assessing lower-body PP and relationship with weightlifting ability among elite athletes (8). Horizontal standing broad jumps were performed with the use of a plastic measuring tape, which was fixed to the floor. Subjects began this testing with their toes behind the 0-centimeter mark of the tape. The distance from the rearmost heel strike to the starting line was used for measurement. Similar to the VJ test, each subject was allowed 3–5 trials in order to achieve maximal jump performance. Subjects were required to complete a nonspecific warm-up of running and dynamic stretching, as well as a specific, submaximal jump warm-up protocol, prior to all jump testing. Three readily used measures of sport-specific physical fitness capacities are acceleration, speed, and agility (27, 38). Variable distance sprint tests were administered to assess acceleration over 20 yards (18.29 meters) and speed over 40 yards (36.58 meters). A timed cone T-test was used to assess agility, in accordance with NSCA protocol (18). Time, in seconds, was recorded for each athlete for the 20-yard (as a split time of the 40-yard), 40-yard, and agility tests. Acceleration (m·s⫺2) was calculated for the 20-yard sprint performance with the equation: Acceleration ⫽ [distance/(time)2]. Subsequently, running velocity (m·s⫺1) was determined by the 40-yard sprint performance and the equation: Velocity ⫽ (distance/time). Light stretching and submaximal sprint/agility trials preceded the respective tests to serve as a warm-up. All tests were executed 3 times, with adequate rest between trials, and the fastest trials were recorded. The aforementioned battery of fitness and performance tests is considered to be well-known and accepted indices within the strength and conditioning profession

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(18) and were deemed to be fundamental competencies for each of the involved sports. Therefore, despite utilizing various types of athletes within the study sample, an assumption was made that general closed-kinetic-chain muscular strength ability would have an equivalent influence upon the battery of biomechanically diverse explosive movements. As this research investigation incorporated a relatively large number of fitness tests, the battery of strength and explosive performance measures was administered over the course of several days, during each of the respective athlete’s off-season training phase. As previously mentioned, each subject was thoroughly familiarized with the experimental testing protocol, having completed the tests several times for his or her strength and conditioning commitment. However, to ensure reliable testing outcome measures, subjects were further familiarized prior to the investigation, which included practice sessions of the exact testing procedure. Test-retest reliabilities for all experimental tests done in this same order demonstrated intraclass correlations of R ⱖ 0.90. Statistical Analyses

Descriptive data (mean and standard deviation values) for the various tests were computed. Independent t-tests with a Bonferroni adjustment were performed to assess the differences between men and women for measures of body mass adjusted strength, as well as for estimated PP output from the VJ test (Sayer’s equation [28]). Pearson product correlation analysis was performed to test the relationships between muscular squat strength (1RM), body mass–adjusted squat strength (1RM/body mass), countermovement VJ height, 20-yard (18.23 meters) sprint acceleration, 40-yard (36.58 meters) sprint velocity, standing broad jump distance, and cone agility T-test time. Reliability was set at p ⱕ 0.05. Additionally, partial correlations analyses were calculated between each of the aforementioned performance measures, while controlling for the affect of muscular strength. Percent covariation was examined between explosive movement tests prior to and following partial correlation analyses to identify the effect of muscular strength.

RESULTS Descriptive statistics may be seen in Table 1. Data are offered for body mass, 1RM squat, body mass–adjusted strength (1RM squat/body mass), countermovement VJ height, standing broad jump distance, agility T-test time, sprint acceleration, and sprint velocity. T-tests demonstrated significant (p ⬍ 0.03) differences for body mass– adjusted strength, as well as for estimated PP output, between male and female subjects. Significant (p ⬍ 0.01) linear relationships were indicated between lower body muscular strength, PP, and all explosive performance tests. Table 2 offers a correlations matrix to delineate the strong significant relationships between each of these measures. In regard to muscular strength, further examination demonstrated that relative muscular strength (adjusted for body mass) was more highly related to most power and performance measures than was absolute muscular strength. Interestingly, VJ was found to be most highly correlated to sprint acceleration and velocity. To assess the effect of muscular strength on the relationships between muscular power and sport performance measures, partial correlations were calculated between

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TABLE 1. Descriptive statistics (mean ⫾ SD) for men, women, and the combined group.* Women (n ⫽ 35) 68.69 85.79 1.27 0.47 3,930.89 1.74 11.48 1.65 6.22

Body mass (kg) 1RM squat (kg) 1RM squat/body mass VJ (m) VJ PP (W) Broad jump (m) Agility T-test (s) Spring acceleration (m·s⫺2) Sprint velocity (m·s⫺1)

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

Men (n ⫽ 19) 84.58 155.77 1.85 0.7 6,020.63 2.34 9.89 2.18 7.4

13.08 16.38 0.22 0.06 642.21 0.59 0.64 0.17 0.33

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

Combined (n ⫽ 54) 74.28 110.42 1.47 0.55 4,666.17 1.95 10.91 1.83 6.62

7.80 23.98 0.29 0.07 508.48 0.59 0.46 0.18 0.28

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

13.75 38.79 0.37 0.13 1,169.22 0.65 0.96 0.31 0.65

* 1RM ⫽ 1 repetition maximum; VJ ⫽ vertical jump; PP ⫽ peak power.

TABLE 2. Correlations matrix for strength, power, and sport performance.* Correlation coefficients

1RM squat (kg) 1RM squat/body mass VJ (m) VJ PP (W) Broad jump (m) Agility T-test (s) Sprint acceleration (m·s⫺2) Sprint velocity (m·s⫺1)

Body mass

1RM squat

1RM squat/body mass

VJ

VJ PP

Broad jump

Agility

Sprint acceleration

0.657† 0.231 0.400† 0.796† 0.275 ⫺0.327‡ 0.268 0.331‡

0.879† 0.859† 0.917† 0.767† ⫺0.784† 0.820† 0.854†

0.852† 0.685† 0.814† ⫺0.805† 0.876† 0.881†

0.873† 0.835† ⫺0.856† 0.889† 0.908†

0.697† ⫺0.739† 0.732† 0.778†

⫺0.900† 0.831† 0.856†

⫺0.854† ⫺0.889†

0.956†

* 1RM ⫽ 1 repetition maximum; VJ ⫽ vertical jump; PP ⫽ peak power. † Correlation is significant at the 0.01 level (2-tailed). ‡ Correlation is significant at the 0.05 level (2-tailed).

TABLE 3. Partial correlations between muscular power (vertical jump) and tests of sport performances with muscular strength controlled (n ⫽ 54).* Correlation coefficients†

VJ

VJ PP

Broad jump

1.00 0.468 ⫺0.540 0.544 0.616

0.756 0.298 ⫺0.376 0.334 0.472

0.468 1.000 ⫺0.707 0.425 0.509

Control variables 1RM squat/body mass

VJ (m) Broad jump (m) Agility T-test (s) Sprint acceleration (m·s⫺2) Sprint velocity (m·s⫺1)

Agility T-test

Sprint acceleration

Sprint velocity

⫺0.540 ⫺0.707 1.000 ⫺0.477 ⫺0.624

0.544 0.425 ⫺0.477 1.000 0.795

0.616 0.509 ⫺0.624 0.795 1.000

* 1RM ⫽ 1 repetition maximum; VJ ⫽ vertical jump; PP ⫽ peak power. † All correlation coefficients are significant at the 0.05 level (2-tailed).

VJ, broad jump, agility, sprint acceleration, and sprint velocity, while controlling for the effect of muscular strength. Essentially partial correlational analyses allow for the correlation between 2 variables while holding constant the external influences of a third. Table 3 shows the significant, yet greatly diminished correlations when the extraneous effect of muscular strength is controlled. For example, when normal bivariate correlations were examined between VJ and both sprint velocity and sprint acceleration, a demonstrated 80 and 83% covariation (percent covariation, or coefficient of determination ⫽ [Pearson r]2) was exhibited, respectively. However, with partial correlation controlling for the third variable-muscular strength capacity, a more scrupulous assessment of 30 and 38% covariation was established, respectively.

DISCUSSION For populations of very high levels of training, minute changes in muscular fitness may have profound performance ramifications. Further, it is at these levels of muscular development that highly specific, exclusive regimens are often implemented. Many researchers have investigated how strength training affects the force-velocity relationship. Current data were consistent with previous studies that have examined a positive interaction between lower body muscular strength and various performance measurements related to lower body muscular power, including VJ height (2, 6, 8, 15, 40), broad jump distance (6, 20, 32), and sprinting performance (15, 39, 40). Subsequently, current data confirm that relative

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TABLE 4. Correlations matrix for females and males.* Correlation coefficients 1RM squat/body mass 1RM squat

VJ

VJ PP

Broad jump

Agility

Sprint acceleration

Sprint velocity

Females (n ⫽ 35) 1RM squat (kg) 1RM squat/body mass VJ (m) VJ PP (W) Broad jump (m) Agility T-test (s) Sprint acceleration (m·s⫺2) Sprint velocity (m·s⫺1)

1.000 0.521† 0.371‡ 0.719† 0.313 ⫺0.408‡ 0.379‡ 0.402‡

0.521† 1.000 0.552† ⫺0.062 0.638† ⫺0.633† 0.718† 0.712†

0.371‡ 0.552† 1.000 0.420‡ 0.594† ⫺0.713† 0.614† 0.622†

0.719† ⫺0.062 0.420‡ 1.000 0.047 ⫺0.210 0.034 0.072

0.313 0.638† 0.594† 0.047 1.000 ⫺0.788† 0.612† 0.668

0.408‡ ⫺0.633† ⫺0.713† ⫺0.210 ⫺0.788† 1.000 ⫺0.630† ⫺0.693†

0.379‡ 0.718† 0.614† 0.034 0.612† ⫺0.630† 1.000 0.874†

0.402‡ 0.712† 0.622† 0.072 0.668† ⫺0.693† 0.874† 1.000

Males (n ⫽ 19) 1RM squat (kg) 1RM squat/body mass VJ (m) VJ PP (W) Broad jump (m) Agility T-test (s) Sprint acceleration (m·s⫺2) Sprint velocity (m·s⫺1)

1.000 0.843† 0.538‡ 0.663† 0.445 ⫺0.169 0.394 0.432

0.843† 1.000 0.667† 0.390 0.528‡ ⫺0.333 0.651† 0.716

0.538‡ 0.667† 1.000 0.734† 0.510‡ ⫺0.261 0.632† 0.646†

0.663† 0.390 0.734† 1.000 0.337 ⫺0.033 0.201 0.189

0.445 0.528‡ 0.510‡ 0.337 1.000 ⫺0.613† 0.484‡ 0.424

⫺0.169 ⫺0.333 ⫺0.261 ⫺0.033 ⫺0.613† 1.000 ⫺0.491‡ ⫺0.579‡

0.394 0.651† 0.632† 0.201 0.484‡ ⫺0.491‡ 1.000 0.836†

0.432 0.716† 0.646† 0.189 0.424 ⫺0.579‡ 0.836† 1.000

* 1RM ⫽ 1 repetition maximum; VJ ⫽ vertical jump; PP ⫽ peak power. † Correlation is significant at the 0.01 level (2-tailed). ‡ Correlation is significant at the 0.05 level (2-tailed).

muscular strength may actually be more applicable to most explosive performance measures than absolute lower-body muscular strength. Conceptually, this would suggest that in regard to transference of training for young collegiate athletes, neuromuscular adaptations are vital to enhance body mass-adjusted force production ability and should be maximized to complement muscle architectural and hypertrophic responses (5, 25, 28, 34). These findings are supported by previous investigations concerning the relationship between relative lower body strength and explosive movement such as sprinting and jumping (3, 24, 39). Clearly, however, further investigation is warranted to determine the correct scaling model (i.e., mathematical transformation of the function associated with strength and body mass, such as with allometric scaling, the Wilks index, the Sinclair formula, etc.) that could be used to assess normalized muscular strength performance for the free weight squat exercise among young collegiate athletes and offer a more precise interpretation of the resultant relationship with sport specific performance measures. Many professionals consider maximum strength to be the basic quality that ultimately affects muscular power, irrespective of external resistance (36). Current data support this conjecture, as demonstrated by very high significant correlations for the entire group of young athletes between strength and VJ (p ⬍ 0.001, r ⫽ 0.852), broad jump (p ⬍ 0.001, r ⫽ 0.814), agility (p ⬍ 0.001, r ⫽ ⫺0.805), sprint acceleration (p ⬍ 0.001, r ⫽ 0.876), and maximum running speed (p ⬍ 0.001, r ⫽ 0.881). Moreover, as may be seen in Tables 2 and 3, the elevated correlations between each of the powerful performance measures were greatly diminished when partial correlations controlled for the effect of strength. The subsequent decrease in percentage of covariation between these performance measures ranged from approximately 20–60%. Findings thoroughly demonstrate the unequivocal influ-

ence of force production capacity on measures of sport specific performance and muscular power. Moreover, this implies that measurement of muscular strength, to a large extent, may differentiate the variance in speed/acceleration profiles between subjects within this study. Further, when current data were divided to assess performance differences among sexes, the mean relative and absolute performance measures for men were expectedly significantly different (p ⬍ 0.03 and p ⬍ 0.05, respectively) in magnitude for each of the tests. More important, however, when examining the various strengthpower-performance relationships, trends remained relatively unaffected (i.e., muscular strength still shared significant correlations between measures of explosive movement). Intuitively, this may mean that regardless of sex, body mass–adjusted muscular strength, as expressed through the free weight squat exercise, largely influences the performance capacity of lower body powerful activities in first year collegiate athletes. Table 4 offers exact correlation coefficients between measures of strength, power, and performance for both men and women. On the contrary, several recent reports have suggested that maximal strength affects muscular power in a manner with diminishing influence as the load decreases (31). As this contention implies, at some point, another factor, the rate of force production, may actually become a more appropriate training directive than strength development. The concept of shifting the training emphasis away from force production capacity, to speed development, is the underlying notion behind the aforementioned principle of velocity specificity. Many disciplinarians have theorized that the magnitude of maximal force and the maximal potential velocity of force production are 2 independent entities of muscle tissue functionality. Likewise, high concentric force capacities are thought to be necessary for accelerating a body at rest, and high contraction velocities are necessary to maintain a high move-

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ment speed. Hence, unique training strategies must be directly applied to promote enhancement of these distinct muscular fitness components. These theorized interactions of muscular functionality have been supported by research with highly trained, very homogeneous populations and have suggested a general nonsignificant relationship between basic slow-velocity force production and high speed movement-proficiency (3, 10). Seemingly, this decrement in contribution of sheer force production to power manifests as an inability to explain variances in speed/acceleration profiles between individuals of progressive strength capacities, simply by examining strength. Recently, Cronin et al. (10) examined such variables among a select group of elite-level rugby athletes. Various measures of strength and power were assessed in order to extrapolate the relative contribution to PP output, first-step quickness, sprint acceleration, and maximal sprint speed (10). Data analyses demonstrated a nonsignificant relationship between 3 repetition maximum isoinertial squat strength and nearly every power-, explosive-, and sprint-performance measure. The authors concluded that their findings ‘‘supports the contention that strength and power indices are not the same and should be measured separately’’ (10, p. 352). To further rationalize these findings, data from previous studies (3, 9) were cited to support the authors’ claim, as well as the respective results. Interestingly, each of these studies examined the interaction of strength and power correlates among small, select groups of highly trained and/or professional athletes. The Pearson product-moment correlation coefficient was chosen for data analyses because it is an effective way to measure the association between 2 variables on interval or ratio scales, such as the relationship between muscular strength capacity in kilograms and VJ height in inches. Calculation of the correlation coefficient relies on variances, both among individuals within a sample and between the 2 variables being measured for covariation (i.e., the degree to which the 2 variables change together) (16). When 2 variables covary, they may be correlated to one another positively or negatively. Higher magnitudes on one variable occurring with higher magnitudes of another, and lower magnitudes on both, is a demonstration of a positive correlation. The other possibility is that 2 variables may vary inversely or oppositely, such as with a negative correlation (i.e., the higher magnitudes of one variable correspond with the lower magnitudes of the other, and vice versa). An effective way to measure the general relationship between 2 fitness variables is to examine the associations within a semiheterogeneous group, such as was the case in the current investigation. As mentioned, the calculation of the Pearson correlation coefficient relies on some variance in the tested variables in order to even detect covariance. Hence, when there is a decreased, or nonsignificant relationship between 2 variables, 1 of 3 explanations may be considered, including (a) the 2 variables do not share a relationship, (b) the 2 variables share a relationship that is not a linear relationship, or (c) a relationship may exist, but there was not enough variance within the data for 1 or both variables, to accurately detect a correlation between variables. Intuitively, the latter may actually rationalize some of the recent findings (3, 10) for a diminished relationship between strength ca-

pacity and muscular power, 2 fitness components known to be highly interrelated. Nevertheless, for correlation research conducted on fitness variables (i.e., variables known to change with training), this diminished covariance may also signify the movement toward, or beyond a virtual threshold wherein increases in the magnitude of 1 variable will not necessarily elicit a subsequent predictable improvement in another variable. For instance, muscular strength is known to be related to muscular power, but there may be a point at which force capacity improvement will not continue to transfer to muscular power adaptation. By examining a small homogeneous group of subjects, it is impossible to observe/detect such a point of transition, as only a snapshot of the true relationship is being offered. Similarly, as the current investigation examined relationships among tests for a semiheterogeneous sample population (i.e., athletes of similar training experience, yet with different sporting involvement), strong relationships were detectable, but little is still gained about these interactions as athletes progress to higher levels of training. Further research should therefore be conducted to examine the variation in correlation coefficients between lower body muscular strength, lower body power output, and explosive lower body movement among various levels of training/athletic echelons.

PRACTICAL APPLICATIONS The results of the present investigation do not infer cause and effect, as correlational data offer only a glimpse of the relationship between variables. Nevertheless, all of the established relationships for low velocity muscular strength and measures of power, jumping ability, agility, linear sprint acceleration, and sprinting speed were very strong. Data do not refute the principle of velocity specificity, as it applies to exercise prescription, but enough evidence exists to suggest that closed-kinetic-chain lowerbody muscular strength capacity (especially body mass adjusted strength capacity) is very influential in the performance of powerful, speed-related activities, among first-year college athletes. For all athletes, the ability to summon these muscular fitness components to perform skill-related activity is essential for performance and competitive success. Just as inferior skill limits the extent of success in sport performance and competition, for today’s athlete, inferior muscular development will greatly limit the athletic achievement of even the most coordinated, skilled individual. The synergism of combining the appropriate training with the appropriate practiced skill/movement is a key determinant to high-level athletic participation. Many coaches and athletes therefore rely heavily on the principles of specificity to maximize training effectiveness. In combination with quality of movement, optimizing muscular adaptation for PP output resides as a principal developmental effect. This study convincingly sustains the contribution of basic force production as the primary underlying physical element that influences muscular power, as well as movement across various speeds. Certainly, most athletes require a diverse performance enhancement program that addresses numerous fundamental aspects of conditioning and sport-specific movement. Nevertheless, from these findings it is recommended that enhancement of relative muscular strength capacity be the fundamental training objective

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for less experienced individuals. Further, the free weight squat may offer this necessary stimulus to elicit optimal, transferable development across various explosive, biomechanically diverse movements.

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