Movements of the upper limbs are performed by smooth and mutually coordinated movements at the shoulder, elbow, and wrist joints (7,10) when reinforcing both arms, such as when exercising using a barbell. Because the upper limbs are often overused in many competitive sports, joint disorders of the shoulders and elbows are common (11,15,20,22) and may progress to serious problems, thereby influencing the future competitive life of athletes; therefore, they cannot be ignored. Joint disorders of the upper limbs, such as baseball elbow (medial epicondylitis) and rotator cuff tears, are common in baseball players because of overuse and pitching with a bad technique (e.g., incorrectly throwing a breaking ball repeatedly) (8,11,13).
Recently, guidelines for the prevention of joint disorders, particularly in pitchers, have been reported (8,9,13). In addition to mastering the correct pitching technique, the guidelines emphasized the necessity to restrict the number of pitches thrown. However, the influence of favoring one side of the body on the differences in muscle power between the upper limbs has not been examined in detail (13,15). For example, in movements associated with pitching and batting while playing baseball, the upper limbs are not used in a bilateral and symmetrical method, in contrast to swimming; instead, one upper limb is overused. With the exception of players such as switch-hitters, almost all players have a dominant upper limb for batting or throwing. During competition, players repeatedly perform 1-sided movements that can result in unilateral development of the muscles or range of motion of the upper or lower limb (3,17,20). Muscle imbalance in itself can cause joint disorders (22). Indeed, if a player with a muscle imbalance is trained using equal loads on both the left and right sides of the body, it could cause joint disorders because the weaker side may become overloaded. Therefore, training with both arms using a barbell at the same time (e.g., the arm curl) results in equal loading on both arms. However, equal distribution of a load can also cause joint disorders when the difference in muscle power between the upper limbs is very large. In particular, in the case of athletes such as baseball players who tend to use specific parts of their bodies, it is highly possible that the strength difference is also large (21). If the imbalance in muscle power is known beforehand, it may be possible to take suitable measures to prevent subsequent joint disorders. In particular, because pitching and hitting in baseball are asymmetrical motions, there is a high possibility that the strength difference between the upper limbs is large. Therefore, the present study focused on strength differences between the upper limbs and evaluated differences in muscle power of handgrip, elbow flexion, and shoulder internal rotation between the dominant and nondominant arms of baseball players. In addition, these differences were examined by close isotonic strength exertion to mimic an actual exercise state but not conventional isokinetic or isometric strength. This study aimed to examine the differences in muscle power between the dominant and nondominant upper limbs of baseball players, to provide practical applications on how to train the upper limbs more effectively.
Experimental Approach to the Problem
Revealing the differences in muscle power between the upper limbs of baseball players in regard to joints and loads may provide useful knowledge for preventing joint disorders during sports training. In this study, we focused on the upper limbs of baseball players and examined the differences in muscle power between the dominant and nondominant upper limbs and evaluated the correlations between muscle power of both the upper limbs according to the load and type of joint. To conduct this study, we developed a muscle power measurement device (Takei Scientific Instruments Co., Ltd., Tokyo, Japan).
The study was based on the following hypothesis: baseball players have imbalance in the muscle power of both upper limbs and the relationship between them is low.
The subjects comprised 33 healthy, male, university baseball players (mean age, 20.4 ± 1.1 years; height, 173.4 ± 6.2 cm; weight, 68.7 ± 7.6 kg; baseball experience, 11.2 ± 1.3 years), who performed exercise for 4-hour sessions 4 times per week. They have carried out batting and fielding practices for respective about 1 hour, and game practice for about 2 hours a day. In addition to the technical practice, individual strength training was conducted 1–3 times per week. The major weight training exercises for the upper body included bench press, arm curl (with barbell), and arm extension (with dumbbell). All the subjects were injury free at the time of testing and were judged to be right handed on the basis of Demura’s Handedness Inventory criteria (5,18,19). This study was approved by the appropriate institutional review board, and written informed consent was obtained from all the subjects before their participation. All the subjects were fully informed of the possible risks and stresses associated with the study, and they provided consent before participation.
Measurement of Muscle Strength and Power
Muscle power was determined using a muscle power measurement device (Takei Scientific Instruments Co., Ltd.), which was newly developed to measure the muscle power of the upper limbs and the speed at which a subject can pull a movable handgrip attached to a pulley and a suspended weight with the upper limb outstretched (Figure 1). The muscle power of each tested upper limb joint was measured after determining the maximal static strength.
Because this device differs from the conventional isokinetic dynamometer or tensiometer (the device measuring isometric strength) and can measure the muscle power of isotonic muscle action, it is possible to measure muscle power similar to actual exercise and strength training.
During the power measurement of each muscle, the subjects were instructed not to perform compensatory or recoil-operated motions. The measurements were made with the subjects in the following postures: For the measurement of handgrip power, the subjects placed the palm to a fixed bar and grasped a handle with 4 fingers with the thumb hooking the bar and then explosively pulled the handle toward the fixed bar. The subjects sat on an adjustable ergometric chair with the arm supported by an armrest and placed in the sagittal plane and positioned horizontally so that the forearm was perpendicular to the hand in a semiprone position. The grip width could be arbitrarily adjusted for each individual using a dial to achieve up to a 90° angle at the proximal interphalangeal joint (PIP). For the measurement of elbow flexion power, the subjects sat horizontally in an adjustable ergometric chair and placed the axilla on the edge of the table with the forearm in supination. The subjects then placed the palm on the handle that was connected to a rope attached to a constant load mass and pulled the handle by explosive elbow flexion. For the measurement of shoulder internal rotation power, the subjects sat in an adjustable ergometric chair immediately beside the fixed bar and placed the axilla on the edge of a table with the forearm in a semiprone position and elbow joint flexed at a right angle (Figure 2).
The subjects performed the maximal voluntary contraction (MVC) test twice before the power test to determine relative loads, and the highest value was used as MVC. Maximal voluntary contraction was measured using an attached strain gauge meter, and all measurements were made in the same posture.
According to the procedures of previous studies, the load was selected as 40%, 50%, and 60% of the MVC measurement, and peak power (W) was calculated using the following formula (1,2,16,23):
The muscle power of each joint was measured twice after 2 practice trials using the dominant and nondominant upper limbs.
Two-way repeated measure analysis of variance (ANOVA; 2 × 3: dominant, nondominant × loads) was used to determine the mean differences in the average values of muscle power of each joint. When a significant correlation was found, a multiple comparison test was performed using the Tukey’s honestly significant difference method. The mean difference was assessed by effect size (ES). The relationships between the muscle power of each joint in both upper limbs at each load were examined using Pearson’s correlation coefficient. We classified correlations as follows: high (±0.80–1.0), moderately high (±0.60–0.79), moderate (±0.40–0.59), and low (±0.20–0.39). Statistical significance (α) was set at a p ≤ 0.05.
Table 1 includes the means and SDs of the muscle power of each joint in the dominant and nondominant upper limbs at each load, the results of the 2-way ANOVA (dominant, nondominant × loads), and ES values. As shown, there were no significant differences in the main factors between the dominant and nondominant upper limbs and loads for elbow flexion power. However, a significant difference in relative load factors for handgrip power was observed, with 40% MVC being lower than 60% MVC in the dominant hand and lower than 50% and 60% MVC in the nondominant hand (ES = 0.30–0.48). Shoulder internal rotation power showed a significant difference between the dominant and nondominant upper limbs, with power in the dominant upper limb being greater at all the loads (ES = 0.57–0.66).
Table 2 shows the correlation between the power of each joint in each upper limb at each load. Correlations for handgrip and elbow flexion powers were moderately high or high and significant at all the loads (r > 0.77). Correlations for shoulder internal rotation power were significant and moderately high (r = 0.79) at 40% MVC but low or moderate (r = 0.36–0.51) at 50% and 60% MVC.
Significant differences in handgrip power were observed between loads, with power at 40% MVC being lower than that at 50% or 60% MVC. However, the differences were not large (ES = 0.30 and 0.48, respectively). In addition, no significant differences in power were observed between the dominant and nondominant upper limbs for both handgrip and elbow flexion power at all the loads.
The PIP, metacarpophalangeal joint, and elbow joint, unlike the glenohumeral joint, are structured to move in only the direction of flexion; therefore, their motion is relatively simple and stable. Moreover, in addition to the practice of baseball, the nondominant arm is also frequently used to grab and hold heavy things in daily life. Therefore, even in baseball players, who principally use the dominant upper limb, our study results showed no marked bilateral differences in the power measurements of the handgrip and elbow flexion tests. In addition, there were no differences in the power measurements at any load. Bilaterally symmetrical muscle training using elbow flexion (e.g., arm curls) is often performed using both upper limbs simultaneously using a barbell. In the present study, the correlation between the muscle power of both upper limbs was high because no significant differences were found between the upper limbs. Hence, the possibility of a disorder developing in the elbow joints and the hand during strength training using both the upper limbs simultaneously may be lower. However, athletes may be unaware of this possibility.
Throwing and batting are very important skills in the sport of baseball, and the glenohumeral joint of the dominant upper limb is recurrently used in these actions. Various muscle groups, ranging in size from small to large, are attached to the glenohumeral joints (6). In addition, because the glenoid cavity is shallow with a spherical humeral head, the glenohumeral joint is movable in various directions. For these movements, rotator cuff function is particularly important, and it is possible to elevate and rotate the upper limbs smoothly because of the well-balanced functionality (muscle action) of the rotator cuff muscles (20). The internal rotation power exertion of the glenohumeral joint of the dominant upper limb is important for batting during the period between the end cocking stage and the accelerated stage of windup motion (6–8). In addition, the muscle strength for these movements is related to pitching speed (4,12). Conversely, the nondominant shoulder seldom performs glenohumeral joint rotation internally during catching and batting movements and does not exert large muscle power during the pitching motion. In short, our results showed that unilateral dominance of an upper limb in pitching affects the laterality of shoulder internal rotation power. When these changes accumulate over a period, smooth movement of the glenohumeral joint becomes limited, potentially leading to future joint disorders. Such an imbalance in strength between arms is characteristic of unilateral dominant athletes. However, maintenance of imbalanced strength and muscle tone between the upper limbs has been reportedly correlated to glenohumeral joint disorders (14,22). Therefore, appropriate strength training is important to ameliorate strength differences between opposing limbs.
In the present study, we showed a moderately high relationship between the shoulder internal rotation powers of the bilateral shoulders at small loads (40% MVC) (r = 0.79). Similar results were observed for handgrip and elbow flexion powers, although the relationship became less pronounced with increasing load. In short, performance training to enhance shoulder internal rotation power should avoid exercises that forcibly impose the same loads on the bilateral arms with a large strength difference. In particular, while training with loads more than 50% MVC, when setting a load based on the stronger arm (the shoulder of the dominant side), the arm would be under an excessive load imposed by the opposite arm; therefore, injury may occur.
The imbalance in shoulder internal rotation power between the upper limbs in baseball players, unlike the muscle power of the handgrip and elbow flexion tests, was marked, with the shoulder internal rotation power of the dominant upper limb being greater. In addition, the relationship between the muscle power of the upper limbs tended to differ with load. In short, even in the same upper limb, the muscle power varies across joints. For baseball players with large differences in muscle development between the upper limbs, muscle development training for shoulder internal rotation power with large loads or physical training may increase the risk of glenohumeral joint disorders. Therefore, baseball players and strength/conditioning coaches should consider the above findings when preparing a training plan to prevent glenohumeral joint disorders. In conclusion, it would be desirable for players with a large imbalance in shoulder internal rotation power between arms to adopt a training program to reverse the capability differences. For instance, dumbbell or tube training with light loads could be used to increase the strength of the weaker side subscapular muscles and would help prevent joint damage.
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