Muscle size is a major determinant of muscle strength (1,5,12). Muscle strength exerted during single-joint movements, and related agonist muscle size were previously used to examine this relationship (1,5,9,12), but little attention has been paid to the relationship between muscle size and muscle strength exerted during multi-joint movements. In the assessment of various sports performances, power output determined by muscle strength and joint velocity is more important than muscle strength alone (15). In addition, power evaluation is useful for determining the optimal load for resistance training (6) and assessing the effects of power training (3,7,11,13). To date, the dependence of power output on muscle size has not been fully investigated (8,17,23). O'Brien et al. (17) suggested that muscle volume (MV) should be adopted as an index of muscle size to investigate the relationship between muscle size and power output determined by muscle strength and joint velocity. Two studies (17,23) found a significant correlation between MV determined by magnetic resonance imaging (MRI) and joint power exerted during single-joint movements, but it remains unclear whether power output during multi-joint movements depends on MV. Another study (8) found the same dependence using arm cranking, but MV was estimated using anthropometric values rather than directly measured. Thus, the relationship between muscle size and power output during multi-joint movements has not been precisely examined to date. Considering that greater muscle size leads to heavier body weight, greater muscle size does not always have a positive impact on performance in certain sports such as endurance sports, long jump, high jump, and many other track and field events. Hence, it is essential to clarify the relationship of muscle size with muscle strength and power during multi-joint movements to develop training programs aimed at improving sports performance.
Many sports activities require powerful movements of the upper body. Bench press and bench throw movements are multi-joint exercises that are commonly used for improving upper body performance. Electromyographic studies (19,22) found that the pectoralis major muscle contributes to bench press and throw movements as with the deltoid and triceps brachii. Furthermore, bench press training (training intensity: 75% of one repetition maximum bench press strength [1RMBP]; training volume: 30 repetitions [3 sets of 10 repetitions], with 2–3 minutes of rest between sets, 3 days per week for 6 weeks) increased the size of the pectoralis major muscle significantly in other previous studies (18,24). These suggest that the pectoralis major muscle is the agonist muscle for bench press and bench throw movements. This study sought to examine the relationships of pectoralis major muscle size with 1RMBP and peak power output during the bench throw exercise to determine the significance of the effects of muscle size on multi-joint movements. We hypothesized that both of the 1RMBP and peak power output during the bench throw exercise depend on the pectoralis major muscle size.
Experimental Approach to the Problem
Considering the unit of muscle strength per size (N·cm−2), the muscle cross-sectional area (MCSA) of the pectoralis major muscle was used to examine the association with 1RMBP. Meanwhile, a previous study (17) suggested that MV should be adopted as an index of muscle size to examine the relationship between muscle size and power output determined by muscle strength and joint velocity. Hence, the relationship between MV of the pectoralis major muscle and bench throw peak power (PPBT) was investigated in this study. Given that the relative intensity at which PPBT is observed is a very individual response (6) and covers a wide range (30–60% of 1RMBP) (4,6,10,16), several different loads were used in the PPBT measurement to avoid affecting evaluation of the PPBT and MV-PPBT relationship.
A total of 18 male collegiate athletes, including 10 American football players (age, 22.1 ± 1.3 years [20–24 years]; body height, 176.8 ± 6.1 cm [169.0–189.5 cm]; body mass, 81.3 ± 10.0 kg [66.2–96.6 kg]; mean ± SD [range]), 4 soccer players (age, 19.8 ± 0.8 years [19–20 years]; body height, 165.7 ± 2.1 cm [163.1–168.1 cm]; body mass, 61.7 ± 1.8 kg [60.1–63.8 kg]; mean ± SD [range]) and 4 kayakers (age, 20.0 ± 0.5 years [19–20 years]; body height, 172.6 ± 3.6 cm [167.9–176.1 cm]; body mass, 65.8 ± 3.5 kg [60.7–68.7 kg]; mean ± SD [range]) volunteered as subjects. All subjects had previously performed systematic resistance training for more than 1 year, and their daily training programs included barbell bench press exercises. They had no orthopedic abnormalities in their upper limbs or trunk. The study protocol was approved by the Ethics Committee of the Japan Institute of Sports Sciences, and all experimental procedures were performed in accordance with the Declaration of Helsinki. Each subject was informed in advance of the purposes and procedures of this study, as well as the possible risks of measurement. Written informed consent was obtained from each subject before enrollment.
The MCSA and MV of the pectoralis major muscle were determined using a 1.5-T whole body MRI scanner (Magnetom Symphony; Siemens Medical Systems, Munich, Germany) with a 6-channel body coil. Transverse scans were performed with a conventional T1-weighted Spin-echo sequence (repetition time, 878 ms; echo time, 16 ms; slice thickness, 10 mm; interspaced distance, 0 mm; scan time, 28 seconds). Imaging was performed in a 480–500 mm field of view with a 256 × 256 matrix size (interpolated to a 512 × 512 matrix size). Within the device, subjects rested quietly in the supine position with their arms extended. A series of transverse images of the upper trunk were scanned. As the subject's breathing caused an increase in signal noise in the obtained images, the subject was instructed to refrain from breathing at the moment that each scan was undertaken. From the scanned images, the outlines of the pectoralis major muscles were digitized, and each MCSA was measured using a personal computer with an image analysis software package (SliceOmatic version 4.3; TomoVision, Magog, Quebec, Canada). Noncontractile tissue, which was imaged in different tones compared with contractile tissue, was excluded.
Measurements were performed in a single session by a highly trained analyst. The maximal value of MCSA (MCSAMAx) of the pectoralis major muscle was adopted to investigate the association with 1RMBP. Muscle volume of the pectoralis major muscle was calculated by multiplying the sum of MCSA of the pectoralis major muscle along its length by an interval of 10 mm. To verify the accuracy of the analysis, MCSAs of 3 continuous slices from the muscle belly were measured 2 times. The coefficients of variance (CVs) of the 2 values were 0.9 ± 0.6%. The day-to-day reproducibility of the MCSAMAx and MV measurements was tested for 1 subject. The CVs of the 2 measured values were 0.3% for MCSAMAx and 0.5% for MV.
The 1RMBP was measured to the nearest successful 2.5-kg value using a Smith machine (EPIC Smith Machine F211; FreeMotion Fitness, Logan, UT, USA). Each subject was instructed to lie in a supine position on a bench with his feet placed on the floor and to perform a specific warm-up of 3–5 repetitions at 50% of self-reported 1RMBP and 1–2 repetitions at 80% 1RMBP. After performing the warm-up exercises, 1RMBP measurements were initiated. The subjects gripped the barbell slightly wider than shoulder width, lowered the barbell with control until it lightly touched the chest, and then lifted the barbell back to a straight arm position while keeping their feet and hips in contact with the floor and bench, respectively. An attempt was considered successful when the movement was completed through a full range of motion without deviating from proper technique and form. The initial load was selected by each subject, and the load was increased until the subjects could not lift the weight through the full range of motion. To avoid the effect of fatigue on the final result, the rest period used between each attempt was more than 5 minutes (21), and 1RMBP measurements were required to be completed within 3–5 attempts after the warm-up.
At least 2 days after the 1RMBP measurement, PPBT was determined using the above mentioned Smith machine (EPIC Smith Machine F211; FreeMotion Fitness) and a linear position transducer system with specialized software (Ballistic Measurement System; Innervations Inc., Muncie, IN, USA). Many previous studies (2,7,14,20) have used this linear position transducer system for determining power output during ballistic movements such as bench press, bench throw, squat jumps, and hang power clean. When performing bench throw, subjects lay supine on a bench with their feet flat on the floor. The starting position of the bar was on the safety stopper nearest to the chest. Subjects explosively pushed the bar upwards from the starting position and threw it to the maximal height with their elbows fully extended. Thus, only concentric movement was involved in the bench throw exercise. The subjects were not allowed to raise their head and trunk off the bench. All subjects used an initial weight of 30 kg, because the weight of the bar without plates and counterweights in the Smith machine was 30 kg. To approximate the total number of trials among the subjects to some extent, the load was increased to 90% of 1RMBP in increments of 5 kg for all subjects except those with 1RMBP values more than 75 kg, for whom the load was increased in increments of 10 kg. Three trials were performed for each load. The rest period used between each trial was more than 3 minutes (15,16,21), and we confirmed that the subjects could perform each trial without feeling fatigue or pain. The order of loading was not randomized to avoid the risk of injury associated with the immediate loading of a heavy weight.
As described above, a linear position transducer system with specialized software (Ballistic Measurement System; Innervations Inc.) was used to measure the position of the bar of the Smith machine during the tests. The position data were transferred to a personal computer with a sampling frequency of 500 Hz. The displacement-time data were filtered using a fourth-order Butterworth digital filter with a cut-off frequency of 14 Hz (15). Power was calculated as follows:
where P is the power (W), M is the mass of the weight used (the bar with plates) (kg), g is the acceleration due to gravity (m·s−2), d2y/dt2 is the second-order differentiation of displacement with respect to time (i.e., the acceleration of the bar with plates) (m·s−2), and dy/dt is the first-order differentiation of displacement with respect to time (i.e., the velocity of the bar with plates) (m·s−2). In each trial, the peak power was measured. The maximal peak power of all the trials was defined as PPBT. The CVs of the 3 peak power values for each load were 3.3 ± 4.7% with intraclass correlation coefficients (1, 1) of 0.973.
Descriptive data are presented as mean ± SD. To investigate the MCSAMAx-1RMBP and the MV-PPBT relationships, a simple regression analysis was performed to calculate the Pearson's product-moment correlation coefficient between MCSAMAx and 1RMBP and that between MV and PPBT, and to examine whether the y-intercept of each regression line differed from 0. Statistical significance was set at P ≤ 0.05. When the result of the correlation coefficient is presented, statistical power is shown with the p value.
The 1RMBP was 91.9 ± 16.9 kg and MCSAMAx was 94.4 ± 15.2 cm2. The 1RMBP was significantly correlated with MCSAMAx (r = 0.866, P < 0.001, statistical power = 0.999; Figure 1). The y-intercept of the regression line between MCSAMAx and 1RMBP was not significantly different from 0 (Figure 1).
The PPBT and MV were 611.5 ± 111.2 W and 923.8 ± 214.7 cm3, respectively. The relationship between MV and PPBT is shown in Figure 2. There was a significant correlation between the 2 parameters (r = 0.821, P < 0.001, statistical power = 0.995), and the y-intercept of the regression line was significantly greater than 0. The relative intensity where PPBT was found was 43.9 ± 8.0% of 1RMBP.
There was a significant correlation between MCSAMAx and 1RMBP (Figure 1) and between MV and PPBT (Figure 2), indicating that pectoralis major muscle size is a major determinant of bench press and throw performances. Thus, similar to muscle strength and joint power exerted during single-joint movements (1,5,17), those exerted during multi-joint movements also depend on related agonist muscle size. In addition, the y-intercept of the MCSAMAx-1RMBP regression line was not significantly different from 0 (Figure 1). As described previously (9), the regression line between MCSA and muscle force cannot have a true intercept because if MCSA equals zero, there must be no force. Hence, the result of the y-intercept of the MCSAMAx-1RMBP regression line strongly emphasizes the dependence of bench press strength on pectoralis major muscle size. The y-intercept of the MV-PPBT regression line, however, was significantly greater than 0 (Figure 2) unlike the MCSAMAx-1RMBP regression line. Power is determined not only by force but also by velocity. Therefore, factors other than muscle size, such as muscle fiber type composition are expected to more strongly affect bench throw power than bench press strength. Moreover, considering that the pectoralis major muscle and other muscles such as the deltoid and triceps brachii contribute to bench press and/or throw movements (19,22), the degree of contribution of each muscle to PPBT may also differ from that to 1RMBP. These factors are likely to influence the present results of the regression line y-intercepts.
We would like to discuss 2 limitations of this study. One limitation is that the intensity where PPBT was found varied between individuals. Considering that the relative intensity at which PPBT is observed is an individual response (7) and covers a wide range (30–60% of 1RMBP) (4,6,10,16), several different loads were used in the PPBT measurement. The relative intensity, which produced PPBT was 43.9 ± 8.0% of 1RMBP in this study, and was within the previously reported values (4,6,10,16). Therefore, it is likely that the present findings were affected little by the methodology used in this study. Another limitation is that, although maximum strength was evaluated using the bench press, peak power was assessed by the bench throw rather than the bench press. Consequently, there was a difference in the selected movements between the strength and power measurements, which may influence the present results. However, bench press peak power is lower than PPBT (10,15), because a braking phase appears during the bench press in contrast to the bench throw (21). Therefore, we consider the use of bench throw rather than bench press to be appropriate to precisely examine the relationship between muscle size and power output during upper body multi-joint movements.
In summary, both 1RMBP and PPBT were related to pectoralis major muscle size, supporting the hypothesis presented in this study. However, the dependence on the pectoralis major muscle size was different between 1RMBP and PPBT. It seems that this discrepancy is attributable to the differences in (a) the effects of factors other than muscle size and (b) the degree of contribution of each muscle (pectoralis major muscle, the deltoid, and triceps brachii) between them.
Greater muscle size leads to heavier body weight. Heavier body weight can be a negative factor for certain sports such as endurance sports, long jump, high jump, and many other track and field events. Correspondingly, greater muscle size does not always positively affect sports performance. However, the results of this study suggest that the contribution of muscle size to the power output developed by multi-joint movements is large. This power output has a significant positive impact on sports performance, and we therefore recommend that athletes and their coaches design training programs for improving sports performance by balancing the advantage of increased muscle size against the potential disadvantage of increased body weight.
This study was supported by a Grant-in-Aid for Scientific Research from the Japan Institute of Sports Sciences. The authors thank the members of the muscle strength and power research project at the Japan Institute of Sports Sciences for their critical comments and excellent technical support. The authors also thank Enago (www.enago.jp) for the English language review. The results of this study do not constitute endorsement of the product by the authors or the NSCA. The authors declare that they have no conflicts of interest.
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