The baseball pitching motion requires coordinated, efficient movements of both the lower and upper extremities. Additionally, this highly dynamic movement of pitching requires appropriate timing, mechanics, and strength in an attempt to propel a ball in a controlled manner as fast and as accurately as possible. Consistency of pitching over the course of an inning, and also throughout the course of a game, is the ultimate goal. Threats to consistency, speed, and propensity to injury come from flawed mechanics or fatigue (1).
Overuse injury in youth baseball players has become a major concern. It has been reported that when examining injuries throughout the course of a season, approximately 30% of the young athletes experienced shoulder or elbow pain (24). Additionally, surgery rates of overuse injury have continued to increase, with as many as 13% of all ulnar collateral reconstruction surgeries consisting of high school baseball players (35). With the increase of injury in youth baseball, there has been more focus on the causes of the overuse injury. Of the proposed causes of overuse injury, fatigue or lack of adequate rest has been hypothesized as major precursors (24).
The effects of muscular fatigue in professional and collegiate baseball pitching have been related to altered trunk (38), arm (11), and knee (27) positions. Additionally, muscular fatigue has been investigated by measuring muscular strength after pitching 7 ± 2 innings (26). It was found that only shoulder flexion and internal rotation strength were significantly diminished after pitching a full game. To our knowledge, there have been no attempts to evaluate the effects of extended pitching on muscle activations of the hip and scapula in baseball pitchers younger than high school baseball players. This study was designed to quantify muscle activations during the first and last innings of a simulated game in youth baseball pitchers. It was hypothesized that the hip and scapular muscles would have significant activation in both the first and last innings of pitching and also demonstrate significantly greater activation during the last inning compared with the first inning, which could be an indicator of fatigue, although fatigue will not be directly measured.
Experimental Approach to the Problem
The goal of the experiment was to determine the muscle activations of selected hip and scapula stabilizing muscles in youth baseball pitchers during the first and last innings of a simulated game. The selected muscles for analysis were bilateral gluteus maximus and medius, throwing side latissimus dorsi, lower trapezius, and serratus anterior and upper trapezius. Surface electromyographic (sEMG) data were normalized by percent of the subject's maximum voluntary isometric contraction (%MVIC) (29–32,37). Descriptive statistics were used to investigate muscle activations during the first and last innings of a simulated game. A multivariate analysis of variance (MANOVA) was conducted to examine the differences in muscle activations throughout the pitching motion between the first and last innings.
Twenty-three youth baseball players (11.2 ± 0.8 years; 151.4 ± 8.7 cm; 47.5 ± 10.8 kg), with 2.2 ± 0.6 years of experience, participated. Inclusion criterion was freedom from injury within the past 6 months; however, none of the subjects reported that they had ever suffered an injury that prevented them from throwing. Subjects were tested during the fall baseball season and had not performed any throwing before arrival into the laboratory on the day of testing. The institutional review board of the University approved all testing protocols. Before data collection, all testing procedures were explained to each subject and parent(s)/legal guardian(s), and informed consent and subject assent were obtained.
Subjects reported to the laboratory on a day that they had abstained from physical activity. On arrival, subjects were instructed on the simulated game pitching protocol and outfitted with sEMG electrodes. Muscle belly palpation was performed to determine the location of bilateral gluteus maximus and medius, throwing arm side latissimus dorsi, lower trapezius, and serratus anterior and upper trapezius. Muscle belly locations were shaved, abraded, and cleaned using standard medical alcohol swabs. Once the skin was prepared, single differential electrodes (inter electrode distance: 10 mm) were attached over the muscle bellies and positioned parallel to the muscle fibers using previously published standardized methods (2,5,33). Additionally, a reference electrode was placed on the anterior superior iliac spine. The use of surface electrodes was chosen because they have been deemed to be a noninvasive technique that is able to reliably detect surface muscle activity (2).
Electromyographic data were collected through a Delsys Bagnoli–8-channel EMG system (Delsys, Inc., Natick, MA, USA). Data were sampled at a rate of 1000 Hz, and the signal was full-wave rectified and root-mean squared at 100 milliseconds (29–32,37). After the application of surface electrodes, manual muscle testing (MMT) techniques by Kendal et al. (18) were used to determine steady-state contraction. Three MMTs lasting 5 seconds were performed for each muscle, with the first and last seconds of each test removed to obtain steady-state results (33). The MMT provided baseline MVIC data to which all sEMG were normalized (28–31,37). Electromyographic data were collected through The MotionMonitor (Innovative Sports Training, Chicago, IL, USA) synched with an electromagnetic tracking system (Flock of Birds Ascension Technologies, Inc., Burlington, VT, USA). All postprocessing analyses were performed through MATLAB (The MathWorks, Inc., version 8.2.0, Natick, MA, USA).
After sEMG preparation, subjects were given an unlimited time to perform their own specified precompetition warm-up. Average warm-up time was 10 minutes. The testing protocol was designed to best simulate a competitive game. Subjects were instructed to throw appropriate pitches (fastballs or changeups), from the windup position, based on randomly provided game situations. An expert with 7 years of youth, high school, and collegiate coaching experience developed the game situation protocol. Verbal feedback was given by the investigator based on batter count (balls and strikes), simulated at-bat outcomes (base hit, base on balls, hit-by-pitch, ground-outs, and fly-outs), and simulated runners to the subjects throughout the protocol. Based on the simulation, the subject was allowed to throw the pitch that would normally be thrown in the proposed situation during competition. Subjects were required to produce 3 outs an inning as per the standard rules of baseball. After 3 outs were produced, a rest period was provided to simulate the second half of the inning. Rest periods were randomly altered in length to mimic typical offensive innings in little league baseball. Pitch count was limited to 75 pitches as per age restriction for 10 year olds.
Subjects were instructed to throw maximal effort for strikes over a regulation distance (46 feet; 14.02 meters) to a catcher. A JUGS radar gun (OpticsPlanet, Inc., Northbrook, IL, USA) positioned in the direction of the throw determined ball speed. Subjects threw a variety of fastball and changeup pitches throughout the simulated game. However, for the purpose of this study, only 4 seam fastball strikes were chosen for analysis. Specifically, the fastest 3 fastballs for strikes thrown in the first and last innings were selected for analysis. The pitching motion was divided into 3 phases: (a) from foot contact (FC) to maximum shoulder external rotation (MER), (b) from MER to ball release (BR), and (c) from BR to maximum shoulder internal rotation (MIR).
Muscle Activation Criteria
Muscle activation analysis through sEMG is often debated on what percent of MVIC is clinically significant in the terms of rehabilitation or performance as it is subjective. DiGiovine et al. (8) described muscle activation of the upper extremity during pitching and determined that muscle activation below 20% MVIC is considered clinically low, and 20–40% MVIC is considered clinically moderate. Whereas others have defined muscle activations of 0–20% MVIC as minimal, 20–35% MVIC as moderate, 35–50% MVIC as moderately strong, and ≥50% MVIC as significantly high (3,39). For the purpose of this study, we have adopted the latter of the 2 criteria.
All statistical analyses were conducted using IBM SPSS Statistics 22. Data from each muscle were first normalized and expressed as a percent contribution of the MVIC. Descriptive statistics were expressed by mean and standard error mean. A MANOVA was conducted to examine the differences between muscle activations of bilateral gluteus medius and gluteus maximus, and also throwing side latissimus dorsi, lower trapezius, and serratus anterior and upper trapezius during all 3 phases of the throwing motion at both presimulated and postsimulated game (p = 0.05) time points.
Results displayed no statistically significant differences in muscle activity between first and last innings of the simulated game with an observed power of 0.274 (phase 1), 0.297 (phase 2), and 0.226 (phase 3). Intraclass correlation coefficients (ICC) were calculated to assess reliability of the dependent variables between trials. Median ICC across all conditions was moderate (r = 0.65), with a weak positive minimum (r = 0.33) and strong positive maximum (r = 0.96). Normalized muscle activations for all 3 phases are summarized in Figures 1–6. Data revealed moderate to moderately strong activation of both stride and stance leg gluteus maximus and gluteus medius during all phases in the first and last innings of the simulated game. The latissimus dorsi exhibited minimal activation during phase 1 for the first inning and during phase 2 of the first and last innings. Moderate activation of latissimus dorsi was present during phase 1 in the last inning and during Phase 3 in both the first and last innings. Weak activations were present in all 3 phases during first and last innings for the lower trapezius, with decreases in muscle activity from first to last innings in phases 1 and 2 and an increase from first to last inning in phase 3. Serratus anterior activation increased from weak (first inning) to moderate (last inning) in phases 1 and 2. Phase 3 displayed moderate activation of the serratus anterior, which decreased from the first to the last innings. Upper trapezius data revealed weak activation in phase 1 with an increase in the last inning, whereas in phase 2, also weak activation was noted and decreased from the first to last inning. During phase 3, the upper trapezius displayed moderate activations in both first and last innings, with a slight decrease in the last inning.
The goal of baseball pitching is to consistently deliver the ball with velocity and accuracy. To achieve this, the pitcher must use both their lower and upper extremities effectively. As baseball pitching is a dynamic total body movement, previous studies have examined EMG activity of both the lower (3,15,28,30,41) and upper extremities (13,16,17) in an attempt to understand their contributions to the pitching motion. Particular focus has been on the pelvic stabilizers, particularly the gluteal muscle group (3, 28, 30, 31, 36) and also the scapular stabilizers (13,16). It has been reported that gluteal activation is moderate to significantly high throughout the pitching motion (3, 30) because of the need for not only pelvic stabilization but also hip mobility. Additionally, it has been found that the serratus anterior is moderately active throughout the pitching motion as it acts to stabilize the glenohumeral joint (15). The aforementioned studies are in agreement with the muscle activation findings in this study for the first inning of the simulated game. However, this study also presented new information regarding muscle activation as a pitcher approaches age restricted pitch count (75 for 10 year olds) limit during the last inning of a simulated game.
Youth baseball pitchers are vulnerable to injuries because of the repetitive nature of the sport. It has been reported that fatigue is a major factor affecting pitching mechanics (14). In fact, it has been documented that overuse is a primary risk factor of injury in youth baseball pitchers (12,23,24,34). Additionally, flawed mechanics have also been shown to put high stresses on the upper extremity (6). Thus, it is believed that focus should be directed to the mechanical nature of fatigue. Therefore, this study aimed to examine mechanical properties of pitching through muscle activations during the first and last innings of a simulated game. It was hypothesized that there would be significant differences in muscle activations in the last inning of pitching. Although this study did not assess fatigue, the last inning represented the maximum number of pitches for that subject, per the age restricted pitch count.
Pelvis stability and mobility are needed during the pitching motion to allow for adequate transfer of energy to the upper extremity. During the wind-up and stride phase to FC, the trunk moves around the stance leg, resulting in hip internal rotation (22,25). Proper placement of the stride leg at FC requires hip external rotation, and then once the ball is released, the stance hip requires adequate internal rotation (22,25). This study revealed that during phase 1, FC to MER, the entire gluteal muscle group exhibited moderately strong to significantly high activations both during the first and last innings of pitching. It is the action of the gluteus maximus to extend and externally rotate the hip, whereas the gluteus medius internally rotates and abducts the hip, and both gluteus maximus and medius work to stabilize the pelvis. During phase 2, MER to BR, the pitcher attempts to drive off the stance leg, thus requiring hip external rotation and extension, whereas during phase 3, BR to MIR requires stride leg hip internal rotation in an attempt to keep the stride foot in line with home plate (9,21). This study revealed moderately strong to significantly high activation for 3 of the 4 gluteal muscles in the first and last innings, during phases 2 and 3. The stride leg gluteus medius, in the last inning of phase 3, demonstrated moderate activation. Although not significant, this noted decreased activation in the stride leg gluteus medius could be of concern, as once BR has occurred, the body must rotate around the stride hip to slow the arm and body. Internal rotation of the hip is necessary to extend the time over which the slowing occurs, thus decreasing the braking impulse of the pitch. Future research should consider the implications of this decrease in activation.
The scapular muscles examined in this study, specifically the lower trapezius, and serratus anterior and upper trapezius, act as a force couple and allow for dynamic scapular stability and upward rotation (4,7,19,20). In addition to upward rotation and stability, the serratus anterior also allows for scapular protraction.
During phase 1, for the arm to be positioned into MER, the scapula must retract, upwardly rotate and elevate (20). This study revealed minimal activation of the examined upper extremity muscles. Because the selected upper extremity muscles in this study function to dynamically stabilize and also position the scapula into the position of MER, these reported muscle activations were expected. During phase 2 of the pitching motion, when the arm is in MER moving into BR, the scapula must remain upwardly rotated and move from a position of retraction into a more protracted position (19). Then as the arm progresses into MIR during phase 3, the scapula must continue to protract and remain upwardly rotated. The upper extremity muscle activation increased to moderate activation during phase 3. This increase in activation is believed to be a result of their role as a stabilizer of the scapula as the humerus moves into MIR.
The upper extremity muscle activations examined in this study did not differ significantly between the first and last innings; however, there was particular interest in the activations of the latissimus dorsi and serratus anterior during the last inning of the simulated game. Both of these muscles increased in activation as the pitcher approached their pitch count limit. Lack of strength or endurance in the serratus anterior allows the scapula to rest in a downwardly rotated position (40). Additionally, the serratus anterior is a key contributor to both normal and abnormal scapular motion and stability (10). The increase in latissimus dorsi activity could possibly decrease the demand on the other internal rotators and may play a role at the pelvis as well.
Altered muscle activation of the scapular stabilizing muscles is most frequently observed in the serratus anterior and lower trapezius muscles (19). Inhibition of these muscles decreases their ability to exert force and stabilize the scapula during dynamic movements. If these muscles are inhibited and the pelvic stabilizer is ineffective in controlling the pelvis during pitching, then the ability to transfer energy to the upper extremity may be further compromised.
The results of this study have implications for the coach, sports medicine specialist, strength and conditioning professional, and biomechanist. Examining muscle activations as a pitcher nears their age restricted pitch count limit provides information as to how long a pitcher can perform before mechanical alterations occur. As pitch counts are enforced to prevent youth from throwing with fatigue, this study provides evidence that the restrictions in place currently do not allow statistically significant changes in muscle activation. In fact, diminished activations were noted. Furthermore, the muscles in which these decreases occurred have direct implications for speed, accuracy, and also for altered mechanics. Specifically, degradations in the contribution of these muscles put the thrower at risk of poor performance and injury. Therefore, care should be taken to increase the endurance of these muscles with exercises that work these muscles in the primary directions of the throwing motion. Seventy-five pitches is the age restricted limit for the participants in this project; however, a pitcher needs to be able to reach this number without fatiguing, and there is a possibility that not every 10 to 12 year olds should throw up to the limit of 75 pitches. Furthermore, fatigue is an individualized variable, and those pitchers with “better” mechanics may not encounter fatigue as early as a pitcher with a less efficient pattern. Finally, more research is needed to identify the causes of fatigue, conditioning to prolong the development of fatigue, and also efficient and effective throwing patterns to avoid the cascade effect of compensatory injuries as throwing athletes age.
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