COMPARING TRAINING VOLUMES BETWEEN SOFTBALL PITCHERS WITH AND WITHOUT UPPER EXTREMITY PAIN

Peyton N. Gober, Nicole Bordelon, Katherine Everhart, Gretchen Oliver, FACSM. Auburn University, Auburn, AL. BACKGROUND: Youth baseball pitchers with upper extremity (UE) pain have higher training volumes and play more months per year than pitchers without pain. Similar research has not been done in softball despite comparable UE injury rates. The purpose of this study was to investigate the differences between training volumes and months per year playing between softball pitchers with and without UE pain. METHODS: 36 youth and high school softball pitchers (14.0 ± 2.0yrs, 58.8 ± 28.5cm, 62.0 ± 19.9kg) active on a team roster within the past 6 months completed an online survey via Qualtrics. Participants were asked Do you currently experience any pain/discomfort in the upper extremity?. Based on response, they were placed in UE pain (n=11;15.2 ± 1.9yrs; 164.9 ± 6.7cm; 72.0 ± 22.4kg) or no UE pain (n=25; 13.5 ± 1.8yrs; 156.2 ± 33.8cm; 57.6 ± 17.4kg) groups. Participants were asked to indicate the number of in and off-season hours per week spent practicing softball, practicing other sports, and strength and conditioning training. Lastly, participants were asked about their months per year spent training for softball. A Mann-Whitney U test was performed to examine the differences between training volumes between pitchers with and without UE pain. Median and interquartile data were reported for each variable. RESULTS: The Mann-Whitney U test indicated there were no significant differences between hours per week of in [pain:14(10,20); no pain: 12(8,15)] and off-season [pain: 8(5,12); no pain: 6(4,12)] practicing softball, in [pain: 2(0,7); no pain: 0(0,6)] and off-season [pain: 3(0,8); no pain: 0(0,3)] practicing other sports, and in [pain: 5(2,8); no pain: 4(2,8)] and off-season [pain: 7(2,14); no pain: 5(3,7)] strength and conditioning training. There were also no differences between months per year [pain: 12(11,12);no pain: 12(11,12)] of playing softball between softball pitchers with and without UE pain (all p-values \u3e 0.132). CONCLUSION: The findings show other modifiable risk factors should be examined to identify the susceptibility of UE pain in youth and high school softball pitchers. However, results should be interpreted with caution considering the relatively small sample size (n=36). Future studies should compare other modifiable risk factors such as throwing volumes, years spent playing competitive softball, and degree of sport specialization between softball pitchers with and without UE pain

PELVIS AND TRUNK KINEMATIC DIFFERENCES EXIST BETWEEN BASEBALL TEE AND FRONT TOSS HITTING

Mary Beth McCullough1, Nicole Bordelon1, Kenzie Friesen2, Anthony Fava1, Katherine Everhart1, Gretchen Oliver, FACSM1. 1Auburn University, Auburn, AL. 2University of Saskatchewan, Saskatoon, SK. BACKGROUND: Kinematic differences exist between baseball live pitching and machine hitting; however, the differences between common hitting practices are unknown. The purpose of this study was to compare kinematics between tee and front toss hitting. METHODS: 21 baseball players (13.9±3.1yrs; 167.1±17.4cm; 64.7±20.1kg) performed three maximal effort swings using front toss and a stationary tee. The tee was adjusted to the middle of the strike zone, and front toss was pitched underhand (9.14m away). Kinematic data (pelvis and trunk rotation, trunk flexion, and lateral flexion) were averaged across three trials and collected using an electromagnetic tracking system. Front and back-side were defined as the participant’s left and right sides, respectively. Hitting variables were analyzed at foot contact (FC), ball-contact (BC), and follow-through (FT). A 2(condition)x3(event) within-subjects RM·MANOVA was used to determine kinematic differences between tee and front toss. RESULTS:The RM·MANOVA was significant for condition (Wilks’ Λ=.248; p\u3c.001), event (Wilks’ Λ=.003; p\u3c.001), and interaction between condition and event (Wilks’ Λ=.215;p\u3c.001). Univariate ANOVAs showed a significant condition effect for pelvis [F(1, 20)=12.3; p=.002] and trunk rotation [F(1, 20)=52.0; p\u3c.001]. Event effect was significant for pelvis rotation [F(1.6, 32.2)=841.5; p\u3c.001], trunk rotation [F(2.0, 39.3)=1439.7; p\u3c.001], trunk lateral flexion [F(1.9, 38.7)=19.9; p\u3c.001], and trunk flexion [F(1.4, 29.0)=12.8; p\u3c.001]. There was a significant interaction for pelvis rotation [F(1.6, 31.3)=11.9; p\u3c.001], trunk rotation [F(1.6, 31.4)=24.2; p \u3c.001], trunk lateral flexion [F(1.3, 25.5)=14.1; p\u3c.001], and trunk flexion [F(1.7, 33.9)=7.2; p=004]. Simple effects showed significant differences between conditions for pelvis rotation at FC (p\u3c.001), trunk rotation at all events (all p-values \u3c0.014), and trunk lateral flexion at FC (p=.026) and BC (p=.008). Specifically, there was greater pelvis rotation toward the front-side at FC, greater trunk rotation toward the front-side at all events, and greater trunk lateral flexion toward the back-side at BC during front toss, and greater trunk lateral flexion toward the back-side at FC during tee hitting. CONCLUSIONS: Kinematic differences exist between baseball tee and front toss hitting. Hitters display a more closed pelvis and trunk position when hitting off a tee compared to front toss

INFLUENCE OF IN-GAME SITUATION ON BALL RELEASE PARAMETERS IN DIVISION 1 COLLEGE BASEBALL PITCHERS

BACKGROUND: Success at high competition levels often hinges on performing well during high pressure (leverage) situations. Advanced baseball analytics enable teams to quantify the pressure of in-game situations, and markerless motion capture makes it possible to capture in-game biomechanical data. Research suggests consistent pitch release parameters improve performance across a season. Therefore, this study aimed to compare fastball release parameters and pitch velocity in college baseball pitchers between in-game high and low Leverage Index (LI) situations. METHODS: In-game markerless motion capture data (300Hz) for fifty-six NCAA Division 1 baseball pitchers (1.89±0.1m; 92.7±8.9kg) were analyzed. Pitch velocity and hand position (relative to the middle of the pitching rubber) in the anterior/posterior, superior/inferior, and medial/lateral directions at ball release were analyzed. Play-by-play data were scraped for LI components (score, innings, outs, and baserunners) and matched to the appropriate pitch. LI is an estimate of in-game pressure that measures the potential change in win expectancy and is calculated as the sum of the weighted change in win probability divided by the average win probability added per swing. Each pitcher’s highest and lowest LI fastball pitches were used for within-pitcher comparison. A one-way repeated measures multivariate analysis of variance (RM·MANOVA) determined if release parameters and pitch velocity differed between high and low LI situations (α = .05). RESULTS: Descriptive statistics (high LI mean ± SD vs. low LI mean ± SD) for the parameters analyzed are as follows: LI (2.8±1.6 vs. 0.4±0.5), pitch velocity (40.45±3.2 vs. 40.54±1.4 m/s), anterior/posterior hand position (1.72±0.1 vs. 1.73±0.1 m), medial/lateral hand position (0.52±0.2 vs. 0.47±0.2 m), and superior/inferior hand position (1.79±0.2 vs. 1.82±0.2 m). The RM·MANOVA revealed no significant differences in release parameters between high and low LI pitches (F4,52 =.322, p=.862). CONCLUSION: While prior research identified a relationship between consistency in the release position and season long performance, these release parameters are not influenced by high or low pressure (leverage) situations. Additional comparisons of high and low leverage situations, as determined by LI, should include analysis of additional kinematics, ball flight metrics, and ball/strike outcomes

COMPARING KINETICS OF PITCH TYPE BETWEEN YOUTH BASEBALL PITCHERS WITH AND WITHOUT UPPER EXTREMITY PAIN

Chris Novellino, Anthony Fava, Nicole Bordelon, Adam Nguyen, Katherine Everhart, Gretchen Oliver, FACSM. Auburn University, Auburn, AL. BACKGROUND: Throwing curveballs (CV) has not been recommended for youth pitchers. Though, it has been shown in healthy pitchers that the CV produces less shoulder and elbow forces than fastballs (FB). It is unknown if these lower force patterns are exhibited among pitchers reporting pain. The purpose of this study was to examine shoulder and elbow forces between the CV and FB in youth pitchers with (W) and without (WO) upper extremity (UE) pain. METHODS: Eighteen youth baseball pitchers (12.9±1.4 y, 164.7±12.6 cm, 54.8±13.9 kg) performed three maximal effort CV and FB pitches for a strike at regulation distance (18.4 m). The fastest pitch was analyzed from stride foot contact to ball release. Participants were placed in the pain group if UE pain was reported on a health history questionnaire (n=9, 13.0±1.5 y, 165.1±11.5 cm, 55.1±13.8 kg), and were matched with pitchers who did not report pain (n=9, 12.8 ± 1.2 y, 164.4±13.6 cm, 54.6±14.1 kg). An electromagnetic tracking system was used to obtain kinematic and kinetic data. A 2x2 (pitch type) mixed design MANOVA was used to determine if there was a difference in CV and FB UE kinetics between pitchers W and WO UE pain. The between-subjects factor was pain (pain/no pain) and the within-subjects factor was pitch type (FB/CV). RESULTS: The mixed design MANOVA was not significant for pain (Wilks’ Λ=.934; p=.804) and the interaction between pain and pitch type (Wilks’ Λ=.893; p=.649). However, it was significant for pitch type (Wilks’ Λ=.543; p=.032). Univariate ANOVAs for pitch type were significant for elbow valgus [F(1, 16)=10.557; p=.005] and shoulder net force [F(1, 16)=6.824; p=.019]. It was not significant for elbow net force [F(1, 16)=4.141; p=.059]. Simple effects analyses showed elbow valgus moment was significantly greater in the FB (M=42.3 N·m, SD=5.4) compared to CV (M=32.3 N·m, SD=4.4). Shoulder net force was significantly higher in the CV (M=682 N, SD=69.2) compared to the FB (M=617 N, SD=71.2). CONCLUSION: There was no difference in FB and CV upper extremity kinetics between youth pitchers W and WO pain. Youth pitchers have traditionally been cautioned against throwing the CV. The current study revealed higher shoulder net forces during the CV. However, the elbow valgus moment was significantly less for the CV than the FB. Further research is needed to understand the relationship between breaking pitches and risk of injury

KINEMATIC PREDICTORS OF INGAME FASTBALL VELOCITY IN NCAA DIVISION I BASEBALL PITCHERS

BACKGROUND: Pitch velocity is a key measure of fastball performance in baseball pitchers. The kinematics related to ball velocity are traditionally examined in laboratory settings with marker-based motion capture systems. However, limited research exists investigating their influence on in-game performance. The purpose of this study was to identify the kinematic predictors of in-game fastball velocity in collegiate baseball pitchers. METHODS: Kinematic data from 57 NCAA Division I pitchers (height: 1.89 ± 0.06m, mass: 93.1 ± 1.18kg) were collected at 300hz using an eight-camera markerless motion capture system (KinaTrax, Boca Raton, FL). Data were processed and filtered using propriety KinaTrax software. Kinematics were measured at foot contact (FC) (stride length, shoulder rotation, shoulder horizontal abduction, and shoulder abduction), maximum shoulder external rotation (MER) (shoulder rotation and elbow flexion), ball release (BR) (elbow flexion, shoulder abduction, trunk flexion, trunk lateral flexion, and lead knee flexion). Maximum knee extension velocity as well as trunk and pelvis rotation velocity were also included for analysis. Fastball velocity was recorded using a TrackMan (Scottsdale, AZ) V3 Game Tracking unit. The average of each subject’s first five fastballs thrown in a game was used for analysis. A backward multiple linear regression (α = .05) was used to identify the kinematic predictors of in-game fastball velocity. RESULTS: The average fastball velocity from the cohort was 40.9 ± 1.22m/s (91.4 ± 2.73mph). After the backward elimination (cutoff of p ≥ 0.05), three predictors explained 17.7% of the variance in fastball velocity (F(3,54)=3.81, R2 =.177, p=.014). As fastball velocity increases by 1 m/s (2.2 mph), max lead knee angle velocity increases by 0.003°/s (β=0.003, t=2.46, p=.017), max trunk rotational velocity increases by 0.005°/s (β=0.005, t=2.40, p=.020), or lead knee flexion at BR decreases by 0.027° (β=-0.027, t=-2.33, p=.024), while all other variables are held constant. CONCLUSION: The findings show greater stride knee extension at BR, maximum stride knee extension velocity, and maximum trunk rotational velocity are predictors of in-game fastball velocity. Investigating in-game data provided similar results to those of prior laboratory studies, while a higher pitch velocity, fewer significant variables, and lower coefficient of determination were noted

THE RELATIONSHIP BETWEEN STRIDE MECHANICS AT FOOT CONTACT AND HITTING PERFORMANCE IN COLLEGIATE SOFTBALL ATHLETES

Maia Engelkes, Nicole Bordelon, Jessica Downs-Talmage, Anthony Fava, Billy Lozowski, Adam Nebel, Gretchen Oliver, FACSM. Auburn University, Auburn, AL. BACKGROUND: A relationship between exit velocity and stride mechanics in slow-pitch softball and amateur baseball athletes has been established. However, a relationship has yet to be investigated in an elite fast-pitch softball population. The purpose of the study was to determine if there is a relationship between stride mechanics and hitting performance (exit velocity) in collegiate softball athletes. METHODS: Sixteen National Collegiate Athletic Association Division I collegiate softball athletes [right-handed (n = 11); 19.4 ± 0.9yrs; 171.0 ± 7.0cm; 72.2 ± 9.4kg] who were active on a team roster and injury free for the past six months participated. Participants performed three maximal effort swings off a stationary tee positioned in the middle of the strike zone. Kinematic data at stride foot contact [stride length (% body height), stride foot angle (°), stride foot position in the z-direction (°), stride knee flexion (°), stride hip flexion (°), pelvis rotation (°), and percent center-of-mass (%COM)] were collected at 240 Hz using an electromagnetic tracking system. 0% indicated the COM was directly over the back leg, whereas 100% indicated the COM was directly over the stride leg. A force plate with a sampling frequency of 1200 Hz was also used to determine stride foot contact. The participant’s stride leg was defined as their lead leg. Exit velocity (mph) was measured with a Rapsodo® 2.0 hitting unit positioned at 4.3 m from the front of home plate, with the highest value for each participant used for the analysis. Simple linear regression analyses were performed to determine if there was a relationship between kinematics at stride foot contact and hitting performance in collegiate softball athletes. RESULTS: The mean exit velocity was 72.4 ± 5.4mph. Regression analyses indicated stride length (R2 = .003), stride foot angle (R2 = .093), stride foot position in the z-direction (R2 = .013), knee flexion (R2 = .002), hip flexion (R2 = .034), pelvis rotation (R2 = .045), and %COM (R2 = .080) were not related to hitting performance in this population of collegiate softball athletes (all model p-values \u3e .288). CONCLUSIONS: A relationship between stride mechanics at foot contact and exit velocity did not exist in this population of collegiate softball athletes; however, measuring kinematic values at stride foot contact is only one way to examine a hitter’s stride. Future softball hitting research should examine other stride parameters such as timing and joint angular velocities

COMPARISON OF IN-GAME TRUNK AND UPPER EXTREMITY KINEMATICS BETWEEN PITCH TYPES IN COLLEGIATE BASEBALL PITCHERS

BACKGROUND: Baseball pitchers commonly experience shoulder and elbow injuries. Pitch type has been identified as a possible risk factor, but previous biomechanical studies were limited to laboratory analysis. The purpose of this study was to compare in-game trunk and upper extremity kinematics between fastballs, breaking balls, and changeups in collegiate baseball pitchers. METHODS: Twenty-four NCAA Division I collegiate baseball pitchers (1.9±0.1m; 94.5±10.5kg) who threw at least five in-game fastballs, breaking balls, and changeups were included in this study. Trunk and throwing-arm kinematics were recorded during games using a markerless motion capture system (300Hz). A TrackMan V3 Game Tracking unit defined pitch type. Kinematics of each pitcher’s first five pitches of each pitch type were averaged for analysis. A repeated measures multivariate analysis of variance (MANOVA) was used to assess within-subject differences between each pitch type (α = .05). The variables of interest were shoulder abduction at foot contact (FC), shoulder horizontal abduction at FC, shoulder external rotation at FC, shoulder maximum external rotation (MER), elbow flexion at MER, maximum shoulder rotational velocity, lateral trunk tilt at ball release (BR), shoulder abduction at BR, and elbow flexion at BR. RESULTS: The MANOVA revealed differences in kinematics between pitch types (F18,78 = 3.49, p\u3c.001). Follow-up univariate testing revealed significant differences across pitch types for MER (F2 = 3.77, p=.03) along with shoulder abduction (F2 = 6.27, p=.004), lateral trunk tilt (F2 = 11.63, p\u3c.001), and elbow flexion (F2 = 5.06, p =.01), all at BR. Pairwise comparisons revealed that fastballs had greater MER than breaking balls (mean within-subjects difference: 1.5±2.6°, p=.03). At BR, changeups resulted in greater shoulder abduction than fastballs (1.2±2.0°, p=.03) and breaking balls (1.5±2.3°, p=.01). Changeups produced less lateral trunk tilt at BR than fastballs (-3.6±4.8°, p=.003) and breaking balls (-2.9±4.2°, p=.008) and greater elbow flexion at BR than breaking balls (1.2±1.7°, p=.006). All other variables were not significantly different (p \u3e.10). CONCLUSION: Collegiate pitchers had in-game kinematic differences between pitch types, especially at BR. It is unknown if these differences alter injury risk, but they may impact performance by showing the batter which pitch type will be thrown

LOWER BODY POWER IS RELATED TO HITTING PERFORMANCE IN YOUTH BASEBALL ATHLETES

Trent Agee, Nicole Bordelon, Katherine Everhart, Anthony Fava, Billy Lozowski, Yuki Yanagita, Adam Nebel, Gretchen Oliver, FACSM. Auburn University, Auburn, AL. BACKGROUND: Baseball hitting requires force generated at the lower extremities to be transferred through the trunk, upper extremities, and finally the bat to maximize performance. Therefore, lower body power may be related to hitting performance. The purpose of this study was to determine the relationship between lower body power and baseball hitting performance. METHODS: Fifty-one youth (9-17 yrs) baseball athletes [right-handed (n = 48); 11.5 ± 1.7 yrs, 152.4 ± 13.2 cm, 50.5 ± 15.5 kg] who were active on a team roster and injury free for the past six months participated. Athletes performed two trials each of a maximal effort standing broad jump (SBJ), triple broad jump (TBJ), and single leg lateral rotational jump (LRJ) (bilaterally). Athletes were then instructed to perform three maximal effort swings off a stationary tee positioned in the center of the strike zone. Hitting performance (exit velocity) was measured using a Rapsodo® Hitting 2.0 unit positioned 4.3 meters from home plate. Peak values for the SBJ, TBJ, single leg LRJ, and exit velocity were used for analysis. Pearson-product moment correlations were used to determine bivariate associations between jump distances (cm) and exit velocity (mph). A forward multiple linear regression, including height (cm) and jump distances, was performed to determine the best predictor of exit velocity. Height was entered initially to estimate the proportion of variance accounted for by the anthropometric measure. The additional predictive value of each jump distance, above and beyond the predictive effects of height were also estimated (ΔR2). Statistical significance was set a priori to p \u3c .05. RESULTS: The mean peak exit velocity was 56.1 ± 8.03 mph. Bivariate correlations determined SBJ, TBJ, and single leg LRJ distances were all significantly and positively related to exit velocity (all p-values \u3c .001). The regression analysis indicated that body height accounted for 65.9% of the variance in exit velocity alone. The predictive model was improved by adding peak SBJ [ΔR2 = .090; R2 = 0.749, F(2, 48) = 71.7, p \u3c .001]. On average, the model predicted that a 1.1 mph increase in exit velocity for every 10 cm increase in peak SBJ (β = .376, p = \u3c .001). CONCLUSIONS: Lower body power is positively related to hitting performance in youth baseball hitters. Specifically, the SBJ had the highest correlation to exit velocity with height held constant

INCIDENCE OF PAIN IN YOUTH SOFTBALL PITCHERS AND POSITIONAL PLAYERS

Nicole M. Bordelon1, Peyton Gober1, Jessica Talmage2, Kyle Wasserberger1, Anthony Fava1, Kate Everhart1, Jeff Dugas3, Gretchen Oliver, FACSM1. 1Auburn University, Auburn, AL. 2Northern State University, Aberdeen, SD. 3Andrews Sports Medicine Institute, Birmingham, AL. BACKGROUND: The rise in fastpitch softball participation across the United States is associated with increased injury rates. Pitchers are 2.6 times more likely to sustain an injury than positional players; therefore, the purpose of this study was to compare the incidence of pain between youth softball pitchers and positional players. METHODS: 33 youth (\u3c18yrs) fastpitch softball pitchers (age: 13.7 ± 2.0yrs, height: 163.8 ± 8.2cm, weight: 63.7 ± 16.0kg) and 56 positional players (age: 14.9 ± 2.5yrs, height: 159.8 ± 17.6cm, weight: 63.1 ± 14.8kg) completed an online Qualtrics® survey during the 2021 season. The presence of pain was determined by answering “yes” to the question, “Do you currently experience any pain/discomfort?”. Those who answered “no” were considered “pain free”. A Chi-Square Test of Independence was used to analyze the association between position (pitcher or positional) and incidence of pain. Statistical significance was set a priori to p\u3c0.05. Descriptive analyses were performed to examine pain location, intensity [0(least)-10(most pain) scale], and the frequency of those who continue to play with the onset of pain. RESULTS: 30% (n = 10) and 38% (n = 21) of pitchers and positional players reported experiencing pain, respectively. However, the Chi-Square Test of Independence revealed no statistically significant association between position and pain incidence (χ (1) = .474, p=.491). The most frequently reported pain locations were the shoulder (40%) and elbow (30%) for pitchers and the shoulder (62%) and knee (33%) for positional players. The average pain intensity was 5.2 and 5.0 for pitchers and positional players, respectively. The percentage of pitchers and positional players who played through their experienced pain during practice or competition was 70% and 82%, respectively. CONCLUSIONS: Pitchers did not exhibit a higher incidence of pain than positional players; however, it is important to note the high incidence of pain in both pitchers (30%) and positional players (38%). Future research should identify upper extremity pain and injury risk factors in pitchers and positional players since the shoulder and elbow were two of the most frequently reported pain locations. Athlete monitoring and injury prevention strategies should be enhanced to reduce the rates of athletes playing with pain during practice and competition

SHOULDER KINETICS AND INFRASPINATUS EMG DURING DECELERATION PHASE IN YOUTH BASEBALL PITCHERS

Chris Novellino1, Anthony Fava1, Adam Nebel1, Christopher O\u27Grady2, Jack Iandoli3, Hillary Plummer4, Thomas van Hogerwou1, Gretchen Oliver, FACSM1. 1Auburn University, Auburn, AL. 2Florida Bone and Joint Specialists, Gulf Breeze, FL. 3OhioHealth Department of Orthopedic Surgery, Columbus, OH. 4San Diego Padres, San Diego, CA. BACKGROUND: Forces on the shoulder after ball release (BR) in baseball pitching are postulated to increase risk of injury to the rotator cuff due to translation of the humeral head. PURPOSE: To examine the relationship of peak shoulder forces on infraspinatus muscle activity during the deceleration phase of the pitch. METHODS: Thirty-three youth baseball pitchers [11.6±1.5y, 156.8±13.1cm, 49.4±12.0kg] active on a team roster and free from injury for the past six months participated. Electromyographic (EMG) data of the infraspinatus were collected and processed using a root-mean-square calculation with a period of 100ms and a sampling interval of 1/1500 seconds. Two, five second maximal voluntary isometric contractions (MVIC) of the infraspinatus were performed. Afterwards, pitchers threw five maximal effort four-seam fastballs to a catcher. Kinematic data were collected using an electromagnetic tracking system. The mean of the two MVIC peaks was used to normalize EMG data. Mean shoulder peak forces (distraction, anterior, and superior) as a percentage of body weight (BW) were analyzed during the deceleration phase (BR to maximum shoulder internal rotation). Linear regression was performed to predict infraspinatus activity from each kinetic variable at the shoulder. RESULTS: Shoulder superior force was -51.8±22.9%BW, distraction was 76.2±22.0%BW, and anterior was -51.4± 20.1%BW, while infraspinatus activity was 32.7±15.1%MVIC. There was a moderate correlation between superior force and infraspinatus activity (r=-.31 p=.08). Weak correlations were found between distraction force (r=.18 p=.32), and anterior force (r=.17 p=.35) on infraspinatus EMG activity. Superior force did not predict infraspinatus activity (F(1, 31)=3.23, p=.08, R2=.09, R2­adj=.07). Neither distraction force (F(1, 31)=1.02 p=.32) nor anterior force (F=.88 p=.35) predicted infraspinatus activity. CONCLUSIONS: Increased superior force was correlated with higher maximum infraspinatus EMG activity, yet the individual forces did not predict maximal muscle activation. The infraspinatus undergoes eccentric loading to decrease the rate of glenohumeral internal rotation and may play a minor role in mitigating superior translation of the humeral head. Examining shoulder forces and muscle activity in additional muscles can help explain mechanisms of injury