Biomechanics Related to Increased Softball Pitcher Shoulder Stress: Implications for Injury Prevention

Background: Softball pitchers exhibit high throwing shoulder distraction force, which is a theorized mechanism of throwing shoulder overuse injury. Windmill pitching involves a variety of highly individualistic pitching styles, and certain kinematics preceding ball release likely influence the amount of shoulder stress that a pitcher accrues. Purpose: To examine the association of trunk and throwing arm kinematics, kinetics, and timing variables with peak throwing shoulder distraction force in high school softball pitchers. Study Design: Descriptive laboratory study. Methods: A total of 37 high school pitchers (mean ± SD; height, 1.71 ± 0.06 m; weight, 75.53 ± 16.12 kg; age, 16 ± 2 years) threw 10 fastball pitches at regulation distance (43 ft [13.1 m]) and with maximum effort. Kinematic data were collected at 240 Hz using an electromagnetic motion capture system synced with motion analysis software. The 3 fastest pitches were averaged and analyzed. Kinematic, kinetic, and timing variables were entered into a stepwise linear regression analysis. Results: Four variables were included in the significant model ( F4,39 = 147.51; P < .001) and explained peak shoulder distraction force ( R2 = 0.944; adjusted R2 = 0.938; SE = 0.036): increased peak elbow distraction force ( t = 19.90; P < .001) and extension moment ( t = 3.63; P = .001), as well as decreased elbow flexion velocity ( t = −2.37; P = .023) and trunk flexion at foot contact of the pitch ( t = −3.00; P = .005). Conclusion: Elbow kinetics and angular velocity, as well as trunk positioning at foot contact, are associated with peak throwing shoulder distraction force in high school softball pitchers. Clinical Relevance: Variables associated with peak throwing shoulder distraction force provide information regarding injury mechanism and coachable cues that might reduce shoulder injury risk among pitchers.

Machine Learning and Statistical Prediction of Pitching Arm Kinetics

Background: Over the past decade, research has attempted to elucidate the cause of throwing-related injuries in the baseball athlete. However, when considering the entire kinetic chain, full body mechanics, and pitching cycle sequencing, there are hundreds of variables that could influence throwing arm health, and there is a lack of quality investigations evaluating the relationship and influence of multiple variables on arm stress. Purpose: To identify which variables have the most influence on elbow valgus torque and shoulder distraction force using a statistical model and a machine learning approach. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A retrospective review was performed on baseball pitchers who underwent biomechanical evaluation at the university biomechanics laboratory. Regression models and 4 machine learning models were created for both elbow valgus torque and shoulder distraction force. All models utilized the same predictor variables, which included pitch velocity and 17 pitching mechanics. Results: The analysis included a total of 168 high school and collegiate pitchers with a mean age of 16.7 years (SD, 3.2 years) and BMI of 24.4 (SD, 1.2). For both elbow valgus torque and shoulder distraction force, the gradient boosting machine models demonstrated the smallest root mean square errors and the most precise calibrations compared with all other models. The gradient boosting model for elbow valgus torque reported the highest influence for pitch velocity (relative influence, 28.4), with 5 mechanical variables also having significant influence. The gradient boosting model for shoulder distraction force reported the highest influence for pitch velocity (relative influence, 20.4), with 6 mechanical variables also having significant influence. Conclusion: The gradient boosting machine learning model demonstrated the best overall predictive performance for both elbow valgus torque and shoulder distraction force. Pitch velocity was the most influential variable in both models. However, both models also revealed that pitching mechanics, including maximum humeral rotation velocity, shoulder abduction at foot strike, and maximum shoulder external rotation, significantly influenced both elbow and shoulder stress. Clinical Relevance: The results of this study can be used to inform players, coaches, and clinicians on specific mechanical variables that may be optimized to mitigate elbow or shoulder stress that could lead to throwing-related injury.

Increased Shoulder Distraction Force and Shoulder Horizontal Abduction in Professional Baseball Pitchers With Discordant Torso Rotation Order

Background: Inefficient energy transfer from the pelvis and trunk has been shown to increase compensation at the level of the shoulder. Kinetic chain sequencing of the core segments is underexamined in professional baseball pitchers, especially as it relates to changes in upper extremity kinetics. Purpose: To evaluate elbow and shoulder kinetics in a cohort of professional pitchers differentiated by instances of discordant pelvic to upper torso sequencing during the pitch. Study Design: Descriptive laboratory study. Methods: 285 professional baseball pitchers were evaluated using 3D motion capture (480 Hz). Pitchers were divided into “chronological” and “discordant” groups based on whether maximum pelvic rotation velocity occurred before (chronological) or after (discordant) maximum upper torso rotation velocity during the pitch motion. Pelvic, upper torso, and shoulder kinematic parameters, shoulder distraction force, shoulder internal rotation torque, and pitch efficiency (PE) were compared between groups. Results: Pitchers with discordant torso sequencing (n = 30; 110 pitches) had greater shoulder horizontal adduction at maximum external rotation (mean difference, 3.6°; 95% CI, −5.2° to −2.0°; t = −4.5; P < .001) and greater maximum shoulder external rotation (mean difference, 3.7°; 95% CI, 5.7° to 1.5°; t = −3.5; P < .001) than chronological pitchers (n = 255; 2974 pitches). PE did not differ between groups ( P = .856), whereas ball velocity was significantly faster in the discordant group (mean difference, 0.6 m/s; 95% CI, −1.1 to −0.3 m/s; t = −3.3; P = .0012). Chronological pitchers had significantly reduced shoulder distraction force (mean difference, −4.7% body weight (BW); 95% CI, −7.9% to −1.5% BW; t = −2.9; P = .004) with no difference in shoulder internal rotation torque ( P = .160). These kinematic and kinetic differences were not observed when accounting for interpitcher variability. Conclusion: Between pitchers, those who had a discordant pelvic to upper torso sequence experienced significantly greater shoulder distraction forces, potentially compensating by increasing maximum shoulder external rotation and horizontal abduction. Achieving maximal pelvic rotation velocity before maximal rotation velocity may be advantageous in preventing compensation at the upper extremity and excessive throwing arm loading. Clinical Relevance: Identifying risk factors for increased upper extremity forces has potential implications in injury prevention. Specifically, mitigating shoulder distraction forces may be beneficial in reducing risk of injury.

Comparative Study of Biomechanical Properties between Needled Suture and Needleless Suture in Multi-Strand Tendon Graft Model

Objective: To compare the biomechanical properties of the Chinese finger (CF) suture, a needleless suture technique, with the baseball stitch (BS) suture, a needled suture technique, in a multi-strand model by using a 4-strand tendon model. Additionally, the BS was compared with the serial rolling hitch (RH), a locking needleless suture technique. Materials and Methods: 4-strand grafts, made from two 20-cm fresh porcine toe extensors, were used in all three groups. After the grafts were sutured, pretension was applied with a load of 100-N distraction force for five minutes. After the tendon elongation was measured before and after the pretension, the distraction force was continued until the constructed graft failed. Stress-strain relationship graphs were recorded by universal testing machine (UTM), distributing to the calculation of percentage on tendon elongation, stiffness, and load-to-failure. Results: The BS had significantly higher load of failure than the CF (p=0.001) but no significant difference when compared with the RH. Comparing between BS, CF, and RH, there were no significant difference in stiffness and percentage of tendon elongation. In modes of failure, there was evidence of knot slipping in CF in six of six cases and graft strangulation in RH in four of six cases. Conclusion: Multi-strand model BS, a needled suture, had a higher load to failure than CF, a needleless suture. Moreover, needleless sutures had serious modes of failure, which were knot slipping and strangulation of graft by the suture material. Therefore, needleless suture technique for multi-strand tendon graft preparation was not recommended. Keywords: Tendon preparation; Multi-Strand; Needled suture; Needleless suture; Chinese finger; Baseball stitch; Rolling Hitch; Biomechanical study; Graft elongation; Load to failure

Is there enough evidence to support hip capsular reconstruction? A systematic review of biomechanical studies

ABSTRACT The aim of this study was to review and summarize the available biomechanical data on hip capsular reconstruction to guide clinical decision-making. A literature search was completed in December 2020 using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines to identify biomechanical cadaver studies on hip capsular reconstruction, hip capsulectomy or hip capsular defect. The investigated parameters included maximum distraction force, capsular state affecting range of motion (ROM), rotation and translation. Four studies met al. the inclusion–exclusion criteria. The median effective force for resisting maximum distraction for the reconstruction state, capsular defect state and the intact state was 171, 111 and 206 N, respectively. The defect capsule force was significantly lower (P = 0.00438) than the intact capsule force. The reconstruction state had a higher distraction force than that of the capsular defect, but due to heterogeneity, the overall effect size was not statistically significant. The capsular reconstruction state reduced excess motion and the degree of instability compared to the capsular defect state but restored the hip close to its native capsular state in the cadaveric model. When compared to capsulectomy/defect state, hip capsular reconstruction significantly improved the rotational stability and effective force at maximum distraction and minimized translation. However, no conclusions can be made regarding the most effective protocol due to the high heterogeneity between the four studies. Further biomechanical studies are needed to test various types of grafts under the same protocol.

Association of Pitch Timing and Throwing Arm Kinetics in High School and Professional Pitchers

Background: Understanding the relationship between the temporal phases of the baseball pitch and subsequent joint loading may improve our understanding of optimal pitching mechanics and contribute to injury prevention in baseball pitchers. Purpose: To investigate the temporal phases of the pitching motion and their associations with ball velocity and throwing arm kinetics in high school (HS) and professional (PRO) baseball pitchers. Study Design: Descriptive laboratory study. Methods: PRO (n = 317) and HS (n = 54) baseball pitchers were evaluated throwing 8 to 12 fastball pitches using 3-dimensional motion capture (480 Hz). Four distinct phases of the pitching motion were evaluated based on timing of angular velocities: (1) Foot-Pelvis, (2) Pelvis-Torso, (3) Torso-Elbow, and (4) Elbow-Ball. Peak elbow varus torque, shoulder internal rotation torque, and shoulder distraction force were also calculated and compared between playing levels using 2-sample t tests. Linear mixed-effect models with compound symmetry covariance structures were used to correlate pitch velocity and throwing arm kinetics with the distinct temporal phases of the pitching motion. Results: PRO pitchers had greater weight and height, and faster ball velocities than HS pitchers ( P < .001). There was no difference in total pitch time between groups ( P = .670). PRO pitchers spent less time in the Foot-Pelvis ( P = .010) and more time in the Pelvis-Torso ( P < .001) phase comparatively. Shorter time spent in the earlier phases of the pitching motion was significantly associated with greater ball velocity for both PRO and HS pitchers (Foot-Pelvis: B = −6.4 and B = −11.06, respectively; Pelvis-Torso: B = −6.4 and B = −11.4, respectively), while also associated with increased shoulder proximal force (Pelvis-Torso: B = −76.4 and B = −77.5, respectively). Decreased time in the Elbow-Ball phase correlated with greater shoulder proximal force for both cohorts (B = −1150 and B = −645, respectively) with no significant correlation found for ball velocity. Conclusion: Significant differences in temporal phases exist between PRO and HS pitchers. For all pitchers, increased time spent in the final phase of the pitching motion has the potential to decrease shoulder distraction force with no significant loss in ball velocity. Clinical Relevance: Identifying risk factors for increased shoulder and elbow kinetics, acting as a surrogate for loading at the respective joints, has potential implications in injury prevention.

The influence of shoulder abduction and external rotation on throwing arm kinetics in professional baseball pitchers

Background The relationships between shoulder abduction and external rotation with peak kinetic values at the shoulder and elbow in professional baseball pitchers are not well established. Methods Professional pitchers ( n = 322) threw 8–12 fastballs under 3D motion analysis (480 Hz). Pitchers were stratified into quartiles by shoulder abduction and external rotation at distinct timepoints. Regression analyses were performed to quantify associations between shoulder position and kinetics. Results Shoulder abduction remained relatively consistent throughout the pitch (foot contact–ball release: 85.5 ± 11.1–90.7 ± 8.4°); shoulder external rotation increased dramatically (foot contact–ball release: 30.8 ± 24.6–165.2 ± 9.7°). For every 10° increase in maximum shoulder rotation, shoulder superior force increased by 2.3% body weight ( p < 0.01), shoulder distraction force increased by 5.9% body weight ( p < 0.01), and ball velocity increased by 0.60 m/s ( p < 0.01). Shoulder abduction was significantly associated with shoulder superior force at all timepoints but not with ball velocity ( p > 0.05). For every 10° increase in shoulder abduction at ball release, shoulder superior force increased by 3.7% body weight ( p < 0.01) and shoulder distraction force increased by 11.7% body weight ( p < 0.01). Conclusion Increased shoulder abduction at ball release and increased maximum shoulder external rotation were associated with greater superior and distraction forces in the shoulder. Pitchers can consider decreasing shoulder abduction at later stages of the pitch to around 80° in order to minimize shoulder superior force, with no impact on ball velocity.

Relationship Between Clinical Scapular Assessment and Scapula Resting Position, Shoulder Strength, and Baseball Pitching Kinematics and Kinetics

Background: Scapular assessment is important in examining overhead athletes, but there is inconsistency in scapular clinical assessment and its relation to pathology. Purpose: To determine the relationship between clinical scapular assessment and biomechanical scapula resting position, shoulder strength, and pitching shoulder kinematics and kinetics. Study Design: Descriptive laboratory study. Methods: Two clinicians performed scapular assessments and graded the scapula as presence or absence of scapular dyskinesis. Shoulder external rotation (ER) and internal rotation (IR) strength were collected. The 3-dimensional biomechanics of the scapula resting position (upward/downward rotation, IR/ER, and anterior/posterior tilt) were assessed while participants stood at rest, and pitching kinematics (maximum shoulder ER, shoulder abduction, shoulder horizontal abduction, shoulder rotation velocity) and kinetics (maximum shoulder distraction force) were assessed when participants pitched off the portable pitching mound that was engineered to meet major league specifications. Results: A total of 33 high school baseball pitchers (age, 16.3 ± 1.2 years; height, 184.0 ± 6.9 cm; weight, 76.8 ± 20.8 kg; hand dominance: left, 9 [27%]; right, 24 [73%]; pitch velocity, 34.7 ± 2.3 m/s) participated in this study. Of them, 15 participants had scapular dyskinesis, and 18 had normal scapulothoracic rhythm. No differences were observed for upward/downward rotation or anterior/posterior tilt, shoulder ER, shoulder abduction, or shoulder distraction force, based on the presence of scapular dyskinesis. Pitchers with scapular dyskinesis demonstrated significantly greater scapular resting IR position (effect size [ES], 0.80; 95% CI, 0.06 to 1.54; P = .020), greater nondominant shoulder ER to IR strength ratio (ES, 0.49; 95% CI, –0.02 to 1.00; P = .018), and decreased shoulder rotation velocity (ES, 14.66; 95% CI: 12.06 to 17.25; P = .016). Pitchers with greater anterior tilt demonstrated greater shoulder rotation velocity ( r = –0.48; P = .006). Conclusion: Pitchers with scapular dyskinesis had greater scapular IR, greater nondominant shoulder ER to IR strength ratio, and reduced shoulder rotation velocity. Clinical Relevance: Scapular assessment may be more influenced by differential IR than upward rotation or anterior tilt. Scapular dyskinesis has no competitive performance advantage among amateur athletes. Greater understanding is needed to decipher the critical threshold between beneficial and maladaptive scapular movement patterns.