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Isokinetic Leg Strength Profile of Elite Male Basketball Players

Bradic, Asim1; Bradic, Josipa2; Pasalic, Emir1; Markovic, Goran2

Journal of Strength and Conditioning Research: July 2009 - Volume 23 - Issue 4 - p 1332-1337
doi: 10.1519/JSC.0b013e3181a0227e
Research Note
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Bradic, A, Bradic, J, Pasalic, E, and Markovic, G. Isokinetic leg strength profile of elite male basketball players. J Strength Cond Res 23(4): 1332-1337, 2009-In this study, we described the leg strength profile of elite male basketball players and evaluated the positional differences in absolute and relative leg strength. Forty-three elite male basketball players (15 guards, 14 forwards, and 14 centers) performed maximal isokinetic concentric knee extension and flexion efforts (60·s−1 and 180°·s−1) and ankle plantar flexion and dorsiflexion efforts (30°·s−1 and 60°·s−1). Significant (p < 0.01) positional differences in absolute leg muscle strength were observed, with centers having significantly (p < 0.05) greater peak torques compared with guards in all of the tested muscle groups and at all angular velocities. Moreover, centers also possessed significantly (p < 0.05) stronger plantar flexors at 30°·s−1 and dorsiflexors at 60°·s−1 compared with forwards, and forwards possessed significantly (p < 0.05) stronger knee extensors, plantar flexors, and dorsiflexors compared with guards. Normalization of strength values with body mass (N·m·(kg−1)) canceled out the positional differences in knee extensor and flexors strength, but the groups still differed significantly (p < 0.01) in relative plantar flexion and dorsiflexion strength. Specifically, centers had significantly (p < 0.05) stronger plantar flexors and dorsiflexors at both angular velocities compared with guards, as well as significantly (p < 0.05) stronger plantar flexors at 30°·s−1 and dorsiflexors at 60°·s−1 compared with forwards. Our results suggest that the positional differences in quadriceps and hamstring muscle strength of elite male basketball players are the result of respective differences in body size, whereas factors other than body size are responsible for the positional differences in ankle plantar flexor and dorsiflexor strength. These results together with the presented normative isokinetic leg strength data can be useful for coaches and therapists when designing and evaluating position-specific strength training and rehabilitation programs of elite male basketball players.

1Faculty of Sport and Physical Education, University of Sarajevo, Sarajevo, Bosnia and Herzegovina; and 2School of Kinesiology, University of Zagreb, Zagreb Croatia

Address correspondence to Goran Markovic, gmarkov@kif.hr.

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Introduction

Basketball performance depends on myriad of factors related to physical fitness (6,21). Investigators who studied the fitness profile of basketball players reported anaerobic leg power and rapid movement performance (sprinting, jumping, change-of-direction ability) to be determinants of successful basketball performance (7,9,18). Because both anaerobic power and rapid movement performance depend on muscular strength of relevant muscle groups (12,16), it can be expected that leg muscle strength also represents an important fitness component for successful basketball performance (9).

Several studies evaluated muscular strength of the lower extremities in basketball players (2,5,8,9,19,20,23,24). However, most of these studies were performed in either young (2,5,23) or nonelite adult basketball players (8,9,19,20). Moreover, only one of these studies included the examination of lower leg muscles in nonelite basketball players (19), although it is well known that these muscles contribute significantly to both jump and sprint performance (10). Therefore, we are currently deprived of knowledge about the leg strength profile of elite male basketball players.

An important issue in muscle strength assessment that has been consistently neglected in literature is the effect of body size (11). Specifically, most previous studies have presented strength data either non-normalized for body size or normalized using inappropriate methods (11). Correspondingly, none of the above-cited studies considered the effect of body size when describing muscular strength of basketball players. This is surprising, given that there exist significant differences in body size among playing positions in basketball (17,18) and that body size has a significant positive relationship with muscular strength (11,12,16).

In the present study, we evaluated isokinetic strength of both knee and ankle extensors and flexors of elite male basketball players. We also analyzed the positional differences in muscular strength among basketball players using both absolute and relative (i.e., mass specific) measures. In line with both theoretical predictions and experimental evidence (11,12), we hypothesized that there exist significant positional differences in leg strength of elite basketball players, and that these differences are the result of positional differences in body mass.

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Methods

Experimental Approach to the Problem

In this cross-sectional study, we tested a group of elite male basketball players playing at 3 standard playing positions. We used an isokinetic apparatus for leg strength evaluation, primarily because of its capability to measure strength of specific muscle groups in standardized conditions and at different contraction speeds. The experimental data were obtained within one testing session at the beginning of the preparatory period for the new basketball season (i.e., after 2 weeks of detraining). None of the tested basketball players experienced any major lower-body musculoskeletal injury during the previous season. In the 24 hours before the experiment, the subjects did not participate in any strenuous exercise.

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Subjects

Altogether, 43 elite professional male basketball players (age: 24 ± 3.8 years, training experience: 13.6 ± 2.3 years; mean ± SD) volunteered for this study. They were either members of Bosnia and Herzegovina's top basketball team (n = 18) or members of one of the top teams playing in professional leagues in several European countries (n = 25). Eighteen players were also members of the Bosnia and Herzegovina's national basketball selection. Fifteen of them were guards, 14 were forwards, and 14 were centers. The measurement procedures and potential risks were verbally explained to each participant before obtaining a written informed consent according to the Helsinki Declaration. The institutional review board approved this study design.

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Testing Procedure

Body mass was obtained to the nearest 0.1 kg using a balance beam scale (Model Seca 700; Seca, Hamburg, Germany), whereas stature was measured using a stadiometer (Model Seca 216; Seca) to the nearest 0.5 cm. Percent body fat was estimated with skinfold measurements taken from 4 sites (Harpenden Skinfold Caliper), using the technique described by Durnin and Womersley (4).

Isokinetic concentric strength of knee extensors, knee flexors, ankle plantar flexors, and ankle dorsiflexors was measured with a Biodex System 3 isokinetic dynamometer (Biodex Corporation, Shirley, NY) according to standard procedures (14). A standardized warm-up of cycling, dynamic stretching, and 10 submaximal concentric contractions of the tested muscle groups preceded the formal testing. All measurements were performed from a seated position with the hip flexed at approximately 85°. Stabilization straps were applied across the trunk, waist, and distal femur of the tested leg.

Plantar flexors and dorsiflexors of each ankle were concentrically measured at 30°·s1 and 60°·s1 (5 repetitions each). The subject was positioned in the chair with the knee fully extended. The foot was placed on a footplate and secured with 2 tight straps. The ankle joint of the subject was aligned with the axis of the dynamometer. The reference angle corresponded to the ankle's neutral position (90°). The movement range covered the entire comfortable active range of motion of the subject's ankle joint.

The knee extensor and flexor muscles of each leg were concentrically measured at 60°·s1 and 180°·s1 (5 repetitions each). The subject was strapped into the chair, using the lateral femoral condyle as an anatomical reference for the axis of rotation. The length of the lever arm was individually determined, and the resistance pad was placed proximal to the medial malleolus. Gravity correction was applied after direct measurements of the mass of the lower limb lever arm system at 30° knee extension. Range of motion varied from 90° knee flexion to 10° extension (0° equals full extension). The values of the peak torques over 5 consecutive contractions for each tested muscle group were used for the data analysis. One minute of rest was allowed between assessments at different angular velocities. In line with the results of previous studies (19,20), we also observed no significant interlimb differences in peak torques of the tested muscle groups. Therefore, only the results of dominant (kicking) leg were reported. Test-retest reliability of all the applied isokinetic measurements in our laboratory proved to be very high (intraclass correlation coefficients ≥ 0.9).

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Statistical Analyses

Descriptive statistics (means and SD) were calculated. One-way analysis of variance (ANOVA) and Bonferroni corrections for multiple comparisons were used for analyzing the positional differences in age, body mass, and stature. Positional differences in absolute and relative (i.e., per kg of body mass; (12)) leg muscle strength of basketball players were determined with 2 separate multivariate analyses of variance (MANOVAs). In case an overall F test has shown significance, a series of ANOVAs and Bonferroni multiple comparisons tests were performed to evaluate differences in each measured leg strength variable among the 3 groups. The level of statistical significance was set at p ≤ 0.05.

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Results

The physical characteristics of basketball players are presented in Table 1. The 3 groups of basketball players differed significantly (p < 0.01) in both body mass, stature, and percent body fat (Table 1). Specifically, centers were significantly (p < 0.05) taller and heavier compared with forwards and guards, and forwards were significantly (p < 0.05) taller and heavier compared with guards. Finally, centers had a significantly higher body fat percentage (p < 0.05) compared with both forwards and guards.

Table 1

Table 1

Isokinetic results related to absolute and relative peak torques for the forwards, guards, and centers are presented in Tables 2 and 3. The first MANOVA revealed significant differences (Wilks' lambda = 0.11, F16,66 = 8.3, p < 0.001) in absolute peak torques among the 3 groups of basketball players. Follow-up ANOVAs showed that the 3 groups differ significantly (p < 0.05-0.001; Table 2) in all the measured isokinetic leg strength variables. Further post hoc multiple comparison tests revealed centers as having significantly higher (p < 0.05) strength compared with guards in all the tested muscle groups and at all angular velocities. Moreover, centers also possessed significantly (p < 0.05) stronger plantar flexors at 30°·s1 and dorsiflexors at 60°·s1 compared with forwards. Finally, forwards possessed significantly (p < 0.05) stronger knee extensors, plantar flexors, and dorsiflexors compared with guards.

Table 2

Table 2

Table 3

Table 3

After normalization of isokinetic peak torques with body mass, MANOVA also revealed significant differences (Wilks' lambda = 0.17, F16,66 = 5.9, p < 0.001) in leg muscle strength among the 3 groups of basketball players. Follow-up ANOVAs showed that the 3 groups differ significantly (p < 0.05; Table 3) in relative plantar flexors and dorsiflexors strength. Further post hoc multiple comparison tests revealed centers as having significantly stronger (p < 0.05) plantar flexors and dorsiflexors at both angular velocities compared with guards. In addition, centers possessed significantly stronger (p < 0.05) plantar flexors at 30°·s1 and dorsiflexors at 60°·s1 compared with forwards.

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Discussion

The tested group of basketball players had similar body mass, stature, and percent body fat as Greek and Serbian elite basketball players (17,18). Moreover, similar to our results, the authors of these 2 studies also found significant positional differences in physical characteristics of elite basketball players (17,18). Absolute and relative concentric knee extensor and flexor peak torques observed in our study are somewhat lower compared with the values observed in the Greek national basketball team (24). These differences could be the result of different testing schedule. Specifically, Greek national basketball team was tested during the competition period (24), whereas our testing was performed at the beginning of the preparation period. Regarding the absolute and relative ankle plantar flexor and dorsiflexor strength, our results compare well with the results observed in second-division professional basketball players (19) and in power-trained athletes (i.e., rugby players, soccer players, and sprinters) (3).

The main aim of this study was to test the hypothesis that there exist significant positional differences in leg muscle strength of elite basketball players and that these differences are the result of positional differences in body size. So far, only 2 studies compared the physical and physiological characteristics of centers, guards, and forwards in basketball, but they focused on body size, body composition, and aerobic and anaerobic performance (17,18). Our results revealed significant positional differences in absolute isokinetic strength of the tested muscle groups, thereby supporting the first part of our research hypothesis. Specifically, the greatest absolute concentric peak torques were produced by centers, followed by forwards and guards (Table 2). Given that 3 groups of basketball players differed significantly in body mass and that muscle strength is highly related to both muscle and body mass, these results were expected. Specifically, both the theory of geometric similarity and experimental data (11,12) suggest that muscle torque should be proportional to body mass. Consequently, for comparison of muscle strength of athletes of different size, strength values should be appropriately normalized for body size (in our case, per kilogram of body mass).

However, contrary to the second part of our research hypothesis, normalization of isokinetic strength results with body mass did not completely cancel out the positional differences among basketball players. Specifically, positional differences in relative isokinetic strength of lower leg muscles were still present, with centers having significantly stronger plantar flexors and dorsiflexors compared with both guards (at both angular velocities) and forwards (plantar flexors at 30°·s1 and dorsiflexors at 60°·s1). These results suggest that factors other than body size are responsible for the positional differences in lower-leg muscle strength among the tested group of elite basketball players. Body composition could be the first possible explanation. Indeed, we and several other researchers (17,18) observed significant positional differences in percent body fat among professional basketball players. However, in all cases, centers had the highest (not the lowest) percent body fat. Moreover, normalization of isokinetic strength with lean body mass (data not shown) did not change our findings. It is therefore unlikely that body composition could explain the observed phenomenon.

It is more likely that the observed positional differences in relative plantar flexor and dorsiflexor strength among the tested basketball players have muscular and/or neural origin. Because we did not measure the neural activation and/or cross-sectional area of lower leg muscles, this question remains to be resolved in future studies. However, regardless if the observed phenomenon has muscular and/or neural origin, its presence is likely to be the result of position-specific training and game activity in basketball. In particular, it is well known that the main role of centers in basketball is ball acquisition through offensive and defensive rebounding. Most of these rebounds include small-amplitude 1- and 2-leg concentric and stretch-shortening cycle vertical jumps (21) that particularly stress ankle plantar flexors (13). In addition, each vertical jump ends with the landing phase, in which a strong activation of ankle dorsiflexors is observed (1). Therefore, much of the training and game activities of centers involve brief explosive actions of ankle plantar flexors and dorsiflexors. Compared with centers, forwards and guards perform considerably less vertical jumps during the game (22). These results together with observations that explosive concentric and stretch-shortening cycle muscle actions increase muscle strength and neural activation of muscles with moderate muscle fiber hypertrophy (15) support our conjecture that the positional differences in relative isokinetic plantar flexor and dorsiflexor strength among elite basketball players could be the result of position-specific training and game activity.

To conclude, this is the first study that evaluated isokinetic knee and ankle strength profile of elite male basketball players and analyzed the position differences in absolute and relative leg muscle strength. Our results show that the significant positional differences in knee extensor and flexor strength of basketball players are the result of positional differences in body size. In contrast, factors other than body size are responsible for the positional differences in ankle plantar flexor and dorsiflexor strength in favor of larger basketball players. Further studies are needed to explain the observed phenomenon and to test the validity of our findings in other groups of basketball players.

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Practical Applications

Most professional basketball teams assess muscle strength primarily using free weights. However, this study showed that isokinetics could provide useful information regarding position-specific strength of particular muscle groups in basketball. We, therefore, recommend the inclusion of isokinetic dynamometry as a supplemental strength diagnostics tool for elite basketball players. In that regard, the results of this study could be of practical importance for coaches. Specifically, this study provides position-specific normative data for the isokinetic leg strength of elite male basketball players. These data should be useful for strength and conditioning professionals and physical therapists when (a) interpreting the results of isokinetic leg strength testing and (b) when designing and evaluating training and rehabilitation programs for elite basketball players. Another practical application of our results is related to positional differences in relative lower-leg strength in favor of larger basketball players. In particular, our results suggest that the largest basketball players (i.e., centers) should possess the greatest relative lower-leg strength. We, therefore, recommend that coaches design position-specific strength training programs for elite male basketball players, particularly when strengthening the ankle joint muscles.

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Acknowledgments

We thank Dr. Chris Knight for his assistance in preparation of this article. This study was supported in part by the Croatian Ministry of Science, Education and Sport (034-0342607-2623) to G.M.

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Keywords:

body size; muscle strength;

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