Repetitive overhead throwing may result in overuse injuries such as ulnar collateral ligament injuries of the elbow, rotator cuff injuries, and superior labrum anterior-to-posterior lesions of the shoulder (3). Sparto et al. (19) showed that fatigue may alter the multijoint kinematics and postural stability of individuals engaged in repetitive lifting tasks. Furthermore, Lyman et al. (10–12) reported that muscular fatigue arising from overuse may alter the pitching mechanics of a baseball pitcher. Specifically, a compensatory motion may occur in the torso, shoulder, or elbow as a result of muscular fatigue of the pitcher's trunk or legs. If pitching is continued beyond the point of muscular fatigue, the risk for physical injury is significantly increased. Thus, the authors proposed limiting the number of pitches thrown in a game or a season to safeguard the pitcher's physical health.
Murray et al. (15) showed that the peak shoulder external rotation angle reduced significantly after 5 to 6 innings of pitching, whereas the knee flexion angle of the leading leg increased. However, the exact number of pitches actually thrown was not reported. Furthermore, no subjective rating of the pitcher fatigue or tiredness was provided. Thus, it is unclear from the study how tired the pitchers actually were and whether or not they were physically capable of pitching for a longer period.
Escamilla et al. (5) reported a significant reduction in the ball velocity during late-inning pitching in a simulated baseball game. The authors concluded that if a pitcher remains in a fatigued state for a long period, changes in the pitching mechanics may occur, resulting in a greater risk for injury. Mullaney et al. (14) investigated the effects of extended baseball pitching on the upper and lower extremity muscle strengths and the onset of muscular fatigue. Of the 14 upper and lower extremity strength measurements taken before and after the simulated baseball game, respectively, only the shoulder flexion and internal rotation strength were found to be adversely affected. In other words, only minimal fatigue occurred in the shoulder, scapular, and lower extremity musculature. Thus, the authors concluded that not all of the upper and lower extremity muscle groups fatigue at the same rate during pitching. However, as in Murray et al. (15), no subjective rating of the muscular fatigue or pitcher tiredness was reported.
In general, baseball coaches use past experience to assess the onset of fatigue in a pitcher. However, in the absence of subjective ratings provided by the pitchers themselves, it is difficult to accurately determine the true extent of fatigue. For example, in the study by Mullaney et al. (14), some of the pitchers may have fatigued after throwing fewer than 100 pitches, whereas others may have been able to throw a greater number of pitches. Thus, in accurately determining the effects of extended pitching on the pitching dynamics, it is desirable to quantify both the actual number of pitches thrown and the subjective feeling of muscular fatigue experienced by the pitchers themselves.
The objective of this study is to quantify the changes in the upper extremity muscle strength of adolescent pitchers in the late-inning stage of a baseball game and to identify the physical changes in the pitching kinematic parameters that could predispose the pitcher to overuse injury. Accordingly, joint kinematic data are recorded for 16 skilled adolescent pitchers after a pretraining session (PRE) and a bullpen pitching training session (BPT), posttraining session (POST), respectively. The experimental data are then analyzed to detect any changes in the shoulder and elbow joint kinematics after prolonged pitching. Notably, the perceived degree of muscular fatigue experienced by the pitchers at the end of the bullpen pitching session is evaluated using the Borg's Rating of Perceived Exertion (RPE) scale.
Experimental Approach to Problem
Each pitcher participated in 3 pitching sessions, namely a PRE, a BPT, and a POST. In the BPT session, each pitcher performed a total of 100 pitches (7,16). Kinematic data and muscle strength measurements were obtained before and after the bullpen pitching session by means of a motion capture system and Biodex isometric tests, respectively.
Sixteen male pitchers with ages ranging from 16 to 18 years were recruited from a high school team, which won the National High School Baseball Championship in Taiwan in the year 2011. Informed consent documents were signed for all subjects, including parental consent. All of the participants had at least 5 years' fastball pitching experience, a consistent pitching motion, and no injury or arm pain. Each participant took part in the simulated bullpen practice in place of one of their regular pitching outings during preseason training.
The participants had a mean weight, height, and age of 70.54 ± 7.05 kg, 178.54 ± 4.03 cm, and 16.77 ± 0.73 years, respectively. The participants signed assent forms and took part in the study with the informed consent of their parents. Moreover, the study was approved in accordance with the Ethical Guidelines of Good Clinical Practice (GCP) for human investigation by both the Institutional Review Board (IRB) of Kaohsiung Medical University, Taiwan, and the National Science Council of Taiwan.
Procedures (Motion Capture System and Biodex Isometric Tests)
Reflective markers (12-mm diameter) were attached bilaterally at the lateral malleoli, lateral femoral epicondyles, greater femoral trochanters, lateral superior tip of the acromions, and lateral humeral epicondyles. In addition, a reflective marker was positioned on the ulnar styloid process of the nonpitching wrist. Forty reflective markers were attached to the subjects to form body segments. The marker placements enabled the estimation of the joint centers and 3-dimensional body segment locations to evaluate the kinematics during the pitching motion. Muscle strength evaluation was performed using a Biodex isokinetic dynamometer (Biodex Corp., Shirley, NY, USA). Finally, data recording and reduction were performed using computer software.
Each pitcher participated in 3 pitching sessions, namely a PRE, a BPT, and a POST. After stretching and warming up, each pitcher completed the PRE session by throwing 10 maximum-effort fastball pitches from a pitching mound toward a strike zone net located over a home plate in the indoor motion laboratory. The distance between the pitching mound and the home plate was set as 6.5 m. During the pitching task, the reflective markers were tracked by a motion capture system (Eagle System; Motion Analysis Corporation, Santa Rosa, CA, USA) comprising 8 charge-coupled device (CCD) cameras with a sampling frequency of 200 Hz. In addition, the ball velocity in each pitch was measured using a Stalker Sport radar gun (Stalker Radar, Plano, TX, USA). On completion of the PRE session, the shoulder and elbow strengths of each pitcher were evaluated using a Biodex dynamometer. The Biodex isometric tests included shoulder extension, shoulder flexion, shoulder abduction, shoulder adduction, shoulder internal rotation, shoulder external rotation, shoulder empty can, elbow flexion, elbow extension, forearm pronation, and forearm supination.
After the Biodex isometric tests, each pitcher proceeded to an outdoor bullpen to perform the BPT session. In performing the pitching tasks, the home plate was positioned at the standard distance of 18.4 m (60.5 ft) from the pitching mound. The BPT session was terminated as soon as any one of the following 3 conditions occurred: (a) the ball velocity reduced by 5%, (b) the participant became fatigued and was unable to throw any further pitches, and (c) a total of 100 pitches were thrown (7,16). At the end of the BPT session, each pitcher reported their perceived feeling of muscular fatigue using the Borg's RPE scale, in which a score of 6 indicates no exertion at all, whereas a score of 20 indicates an extreme state of muscular fatigue (tiredness) (18).
After the BPT session, the pitchers returned to the indoor motion laboratory to complete the POST session, in which 10 further fastball pitches were thrown. Having completed the 10 pitches, the muscle strengths of the shoulder and elbow were again evaluated using the Biodex dynamometer. The pitchers returned to the laboratory on both the next day (“DAY1”) and the following day (“DAY2”) to measure the shoulder and elbow muscle strengths once again.
As shown in Figure 1, the pitching motion was divided into 6 phases, namely (a) windup, that is, from the beginning of motion until a balanced position, (b) stride, that is, from a balanced position until lead foot contact (FC), (c) arm cocking, that is, from lead FC to the instant of maximum shoulder external rotation (MER), (d) arm acceleration, that is, from the instant of MER to the instant of ball release (BR), (e) arm deceleration, that is, from the instant of BR until the point of maximum shoulder internal rotation, and (f) follow-through, that is, from the point of maximum shoulder internal rotation until the end of motion. The joint angles were calculated using laboratory-developed kinematics software (8,20) in accordance with Euler's method with a y-x-z rotational sequence (1,9). The angular conventions used in evaluating the pitching kinematic parameters are shown in Figure 2.
Repeated-measure analysis of variance tests were performed to identify any significant differences in the PRE, POST, DAY1, and DAY2 measurements of the muscle strengths and kinematics parameters, respectively. The tests were performed using commercial SPSS statistical analysis software (SPSS Inc., Chicago, IL, USA) with a significance level of p < 0.01 for both the muscle strength parameters and the kinematic parameters.
As shown in Table 1, the key kinematic parameter measurements obtained in this study are consistent with those obtained in previous studies by Dun et al. (4) and Escamilla et al. (5), respectively. Table 2 shows the isometric muscle strength measurements obtained in this study. Table 3 indicates the mean kinematic data measurements obtained in the PRE and POST sessions, respectively. It is seen in Table 2 that 6 of the 11 isometric muscle strength measurements (i.e., shoulder flexion, shoulder extension, shoulder abduction, shoulder adduction, shoulder internal rotation, and shoulder external rotation) are significantly lower in the POST session than in the PRE session. In addition, on DAY1, 4 of the 11 isometric muscle strengths (i.e., shoulder flexion, shoulder abduction, shoulder internal rotation, and shoulder external rotation) are significantly lower than before the pitching session. Moreover, on DAY2, 2 of the 11 isometric muscle strengths (i.e., shoulder internal rotation and shoulder external rotation) are still significantly reduced. As shown in Table 3, the mean ball velocity was significantly reduced in the POST session (i.e., from 117.5 ± 7.27 km·h−1 to 112.81 ± 6.7 km·h−1). Moreover, a significant difference occurred between the 2 sessions in 13 of the 26 kinematic parameters. Finally, the mean Borg's RPE score after the bullpen pitching session was found to be 14.14, indicating a moderate-to-large degree of tiredness (Note that each of the pitchers successfully completed the assigned task of 100 consecutive pitches) (Figures 3 and 4; Table 1).
USA Baseball Medical & Safety Advisory Committee published pitching safety guidelines in 2006. Moreover, Little League Baseball enacted pitch limit regulations in 2008 in which the guidelines are stratified for different age groups and stipulate a set amount of rest based on the number of pitches thrown (7). Fleisig et al. (6) proposed several safety recommendations for adolescent baseball pitchers, including a maximum of 80 pitches per game. Consequently, to obtain representative experimental results, while still satisfying safety guidelines, this study adopted 100 pitches as the target for the BPT session.
As shown in Table 2, the shoulder strengths in flexion/extension, abduction/adduction, and internal rotation/external rotation exhibit a greater reduction than the elbow strengths after the bullpen pitching session. This finding is reasonable as the shoulder joint performs acceleration and deceleration tasks during the pitching motion, whereas the elbow joint simply transmits the force passively. Dale et al. (2) also found that the shoulder internal rotation muscle group was particularly prone to throwing-related fatigue. Of the 7 shoulder isometric measurements in Table 2, the strengths of the shoulder external rotator (supraspinatus, infraspinatus, posterior deltoid, and teres major), shoulder internal rotator (anterior deltoid, pectoralis major, teres major, and subscapularis), shoulder abductor (supraspinatus, biceps brachii long head, and deltoid), shoulder adductor (teres major, triceps brachii long head, pectoralis major, and latissimus dorsi), shoulder extensor (posterior deltoid, teres major, and triceps), and shoulder flexor (anterior deltoid, biceps, and pectoralis major), respectively, all show a significant reduction of more than 6%. This finding suggests the onset of muscular fatigue and is consistent with the reported average Borg's RPE score of 14.14, which indicates a moderate-to-large degree of tiredness.
As pitching exercises involve a coupling effect of the agonist and antagonist, Table 2 shows a significant difference between the PRE and POST session results for shoulder extension vs. flexion, shoulder internal rotation vs. external rotation, and elbow flexion vs. extension but a nonsignificant difference for shoulder abduction vs. adduction and forearm pronation vs. supination. This finding implies that although prolonged pitching has a measurable effect on the pitching mechanics, this effect is also distributed over the upper extremity muscle groups.
Prolonged pitching results in increased stress on the shoulder muscle group, thereby reducing the shoulder muscle strength and constraining the horizontal abduction motion (Table 1 and Figure 3). Table 2 shows that most of the muscles regain their strength on DAY2. In other words, a 2-day rest period is sufficient for most of the muscles to recover. However, it is noted that the shoulder external and internal rotation muscle strengths are still reduced on DAY2. This suggests that the supraspinatus, infraspinatus, teres major, deltoid, subscapularis, and pectoralis major upper muscle groups are subject to particular effort in extended pitching. In general, the results confirm the importance of the shoulder rotators in the performance of baseball pitching tasks and indicate the need for strengthening programs targeting these particular muscle groups to prevent overuse injuries.
As shown in Table 3, the shoulder horizontal abduction angles at the instant of FC, during the arm cocking phase, at the instant of MER, and at the instant of BR, respectively, are all significantly different in the PRE and POST sessions. It is difficult to define a “proper pitching motion” without a prior quantified motion analysis (16). However, this finding suggests that prolonged pitching results in a reduction of the shoulder muscle strength, which leads in turn to a change in the pitching mechanics.
As shown in Figure 3, the variation of the shoulder external rotation angle, shoulder horizontal abduction angle, and knee flexion angle over the course of the pitching motion are very similar in the PRE and POST sessions. However, the shoulder horizontal abduction angle in the POST session is consistently lower than that in the PRE session. As mentioned above, the reduction in the horizontal abduction angle is most likely the result of a fatigue-induced contraction of the shoulder muscle group. In the fatigued condition, the shoulder joint is unable to complete the horizontal abduction motion at the beginning of the arm cocking phase, and thus the abduction angle in the POST session is less than that in the PRE session. It is noted that these findings are consistent with those presented in previous studies. For example, Murray et al. (15) stated that in the fatigued condition that the pitcher is unable to preserve the original or ideal timing patterns of the pitching motion. The authors further suggested that the resulting reduction in the kinematic variables at the shoulder joint leads to a reduction in the ball velocity. The results presented in this study support this contention (Table 3).
The significant difference between the PRE and POST session measurements of the shoulder horizontal abduction angle at the instant of FC, at the moment of MER and BR, and during the arm cocking phase indicates that fatigue has a significant effect on the performance of the shoulder joint throughout the pitching cycle (Note that this assertion is supported by the data presented in Table 2, which shows a reduction in the POST session shoulder strength).
It is difficult to evaluate the exact number of pitches a pitcher can throw before muscular fatigue occurs, and the risk for physical injury increases (7,16). Lyman et al. (10–12) proposed that the onset of muscular fatigue is very individualized and depends on many factors, including the genetic disposition of the pitcher, the overall conditioning and specificity of training, the rest duration between pitching outings, the cumulative stress imposed on the musculoskeletal system over the course of the baseball season, and the number (and type) of pitches thrown. Escamilla et al. (5) further stated that the risk for injury does not necessarily increase with an increasing number of pitches thrown in a game. However, the authors did not actually report the number of pitches thrown by each pitcher during the simulated ballgame, nor did they consider the subjective level of muscular fatigue experienced by each pitcher. As a result, it is difficult to quantify the fatigue experienced by the pitchers in the final pitching innings. In this study, it is more useful to know the number of pitches thrown than the number of innings pitched, as the number of pitches thrown has a more direct effect on the degree of tiredness. Thus, in investigating the effects of muscular fatigue, the performance target was specifically set in terms of the number of pitches thrown (100).
As shown in Table 3, the upper torso forward tilt angle at the instant of MER and at the moment of BR was significantly higher in the POST session than in the PRE session (p < 0.01). Escamilla et al. (5) stated that forward trunk tilt may be related to a transfer of energy to the arm and plays an important role in facilitating energy dissipation during the arm deceleration phase. The finding in this study of an increased forward tilt of the upper torso may indicate fatigue or tiredness of the trunk. Accordingly, such a postural change should be carefully monitored during the late-innings stage of a baseball game to minimize the risk for overuse injury.
The present results have several important kinematic implications. First, the reduced horizontal abduction in all phases of the pitching motion indicates that fatigue has a significant effect on the pitching motion and pitching mechanics. Second, muscle fatigue caused by prolonged pitching has a particularly significant effect on the shoulder external rotation strength (∼13% reduction; Table 2). Hence, the risk for physical injury may increase if pitching is continued beyond the point of fatigue. It is noted that this assertion is consistent with that of Meister (13), who stated that overuse injuries may result in tendinitis of the rotator cuff and biceps of the shoulder.
As shown in Table 3, there is no significant difference in the MER, elbow flexion at the instant of FC, and elbow flexion at the instant of BR in the PRE and POST sessions, respectively. In other words, the pitching biomechanics remain remarkably consistent throughout the pitching process. Pappas et al. (17) argued that the ball velocity decreases if the shoulder does not achieve the required degree of external rotation at the end of the cocking phase. The authors also stated that during extreme external rotation, the glenohumeral joint yields a “tight wringing” of the anterior capsule, which subsequently leads to microinjury or macroinjury and capsular irritation. Overall, the results obtained in this study suggest that a reduced ball velocity, reduced horizontal abduction, increased knee flexion, and increased trunk forward tilt at the instant of BR after extended pitching are all possible signs of fatigue or tiredness. These findings are thus of potential benefit to coaches, pitchers, and trainers in detecting the onset of fatigue and taking appropriate steps to minimize the risk for overuse injuries.
Although 100 pitches were thrown from an outdoor pitching mound located at a distance of 18.4 m from the home plate, the pitching distance in the indoor experiments was just 6.5 m. It is possible that the shorter pitching distance may affect the kinematic parameters. Thus, to confirm the validity of the results presented in this study, further outdoor experiments with a realistic pitching distance are required. In addition, this study has focused specifically on skilled adolescent baseball pitchers. To confirm the general applicability of the results, it is necessary to repeat the experimental procedures described in this study with baseball players of all ages and levels.
This study has investigated the effects of late-inning pitching on the muscle strength and pitching kinematics of adolescent pitchers after 100 bullpen pitches. In contrast to previous studies, this study has not only quantified the number of pitches thrown (100) but also the perceived sense of fatigue of the pitchers after task completion. The mean Borg's RPE index was found to have a value of 14.14, indicating a medium-to-large degree of perceived tiredness. The experimental results have shown that muscular fatigue results in a reduced muscle strength of the shoulder external rotator, shoulder internal rotator, shoulder extensor, shoulder flexor, shoulder abductor, and shoulder adductor. The reduced muscle strength leads to both a lower ball velocity and a decreased horizontal abduction angle throughout the full pitching cycle because of a contraction of the shoulder muscle groups. In addition, a forward tilt of the upper torso was observed at the moment of BR as a result of muscular fatigue of the trunk.
Overall, the results presented in this study indicate that late-inning pitching induces a measureable change in the pitching mechanics. A reduced ball velocity, increased forward trunk tilt, increased knee flexion angle at BR, and increased horizontal adduction angle at BR should all be carefully observed by the coach and catcher as possible indicators of “tiredness” or “fatigue” in late-inning pitching. Preventing the pitcher from continuation of pitching after these signs occur may reduce the risk for fatigue-induced overuse injury. The present results have shown that at least 3 days of rest are required for the recovery of the shoulder internal rotator and shoulder external rotator muscles after 100 pitches. Finally, the results suggest that specific training programs aimed at strengthening the shoulder external rotator, shoulder internal rotator, shoulder extensor, shoulder flexor, and shoulder adductor muscles are required to minimize the risk for fatigue-induced overuse injuries.
The authors gratefully acknowledge the financial support provided to this study by the National Science Council of Taiwan, R.O.C. We certify that no party having a direct interest in the results of the research has or will confer a benefit on us or on any organization with which we are associated, and if applicable, we certify that all financial and material support for this research are clearly identified in the title page of the manuscript. The study was approved by both the Institutional Review Board (IRB) of Kaohsiung Medical University, Taiwan, and the National Science Council of Taiwan.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
baseball pitching; kinematics; shoulder; overuse injuries; muscle strength