Michelle Sabick

Biomechanics of the Shoulder in Youth Baseball Pitchers Implications for the Development of Proximal Humeral Epiphysiolysis and Humeral Retrotorsion

Background The effects of repetitive throwing on the shoulders of developing athletes are not well understood because of the paucity of data describing the biomechanics of youth pitchers and the plasticity of the developing skeleton. Hypothesis The direction and magnitude of the stresses that exist at the proximal humeral physis during the fastball pitching motion are consistent with the development of proximal humeral epiphysiolysis (Little League shoulder) and/or humeral retrotorsion. Study Design Descriptive laboratory study. Methods A total of 14 elite youth baseball pitchers (mean age, 12.1 ± 0.4 years) were filmed from the front and dominant side while throwing fastballs in a simulated game. The net force and torque acting on the humerus throughout the throwing motion were calculated using standard biomechanical techniques. Results The external rotation torque about the long axis of the humerus reached a peak value of 17.7 ± 3.5 N.m (2.7% ± 0.3% body weight × height) just before maximum shoulder external rotation. A shoulder distraction force of 214.7 ± 47.2 N (49.8% ± 8.3% body weight) occurred at, or just after, ball release. Conclusion Shear stress arising from the high torque late in the arm-cocking phase is large enough to lead to deformation of the weak proximal humeral epiphyseal cartilage, causing either humeral retrotorsion or proximal humeral epiphysiolysis over time. The stresses generated by the external rotation torque are much greater than those caused by distraction forces generated during the pitching motion of youth baseball pitchers. Clinical Relevance The motion of throwing fastballs by youth baseball pitchers results in force components consistent with proposed mechanisms for 2 clinical entities.

Humeral Torque in Professional Baseball Pitchers

Background Spontaneous fracture of the humeral shaft in throwers is a rare but well-known phenomenon. Although it has been hypothesized that the biomechanics of the throw cause such fractures, it is not clear how or when the fractures occur in the pitching motion. Methods The torque acting about the long axis of the humerus was calculated in 25 professional baseball pitchers throwing in game situations. Results Peak humeral axial torque reached a mean value of 92 ± 16 Nm near the time of maximum shoulder external rotation at the end of the cocking phase. This torque tended to externally rotate the distal end of the humerus relative to its proximal end. The direction of the torque was consistent with the external rotation spiral fractures of the humerus noted to occur in throwers. The magnitude of the peak humeral torque averaged 48% of the theoretical torsional strength of the humerus, suggesting that repetitive stress plays a role in humeral shaft fractures. Conclusions Fractures are most likely to occur near the time of maximum shoulder external rotation when humeral torque peaks. Pitchers whose elbows were more extended at stride foot contact tended to have lower peak humeral torques.

The Effects of a Commercially Available Warm-Up Program on Landing Mechanics in Female Youth Soccer Players

The purpose of this study was to examine lower extremity kinematics following implementation of the Sportsmetrics Warm-Up for Injury Prevention and Performance (WIPP) training program. The hypothesis was that there would be no difference in landing mechanics between 2 groups of female youth soccer players (9–11 years of age), with 1 group (Treatment) completing the 8-week–duration (2 days per week) WIPP program and the other serving as a Control group. We recruited 21 female youth soccer players. Treatment (n = 12) and Control (n = 9) groups were established. Using the Sportsmetrics Software for Analysis of Jumping Mechanics, we analyzed lower extremity movement during landing after subjects jumped off a 30.5-cm box and immediately went into a vertical jump. No significant changes in knee separation values were observed in the Treatment group after 8 weeks of WIPP training. The results indicate that 8 weeks of WIPP training did not significantly alter landing strategies.

A Biomechanical Analysis of Youth Pitching Mechanics

Background: It is estimated that nearly 6% of youth baseball participants seek medical attention for injuries sustained during play. Most injuries are overuse injuries, and 26% are to the shoulder or upper arm. By quantifying youth pitching biomechanics, knowledge can be gained concerning the manner in which these injuries are sustained during play. Methods: Sixteen healthy right hand-dominant baseball pitchers participated in this study. After digitization of 21 bony landmarks, kinematic calculations were conducted using the 3-dimensional coordinates from each video frame. Data were time normalized, forcing major temporal components of the movement to occur at specific intervals. Segment-based reference frames were established, and resultant joint kinetics were projected onto each reference frame. Kinetic data were normalized and calculated along or about the anterior/posterior, medial/lateral, and proximal/distal axes. Results: Maximum trunk rotation and external shoulder rotation were observed during arm cocking. Each of the remaining kinematic parameters peaked after ball release. All maximum values for joint kinetics were measured during arm cocking with the exception of compressive forces experienced at the shoulder and elbow, which peaked after the instant of ball release. Conclusions: Data produced in this study indicate that youth pitchers initiate trunk rotation early in the movement, which can lead to shoulder hyperangulation. Opposing torques at each end of the humerus also produce a large net torque about the longitudinal axis of the humerus during late arm cocking and may increase humeral retrotorsion in youth pitchers. Underdeveloped musculature in the rotator cuff may lead to difficulty controlling throwing-arm deceleration, causing an increase in horizontal adduction across the torso. Clinical Relevance: An improved understanding of youth pitching mechanics is gained from the data collected, analyzed, and discussed in this study. Through increases in the knowledge pertaining specifically to the mechanics of youth pitchers, the opportunity to develop pitching mechanics specifically designed for preventing injuries in little league pitchers arises. Level of Evidence: This study is a Level 4 study describing youth pitching biomechanics and how they relate to possible injuries.

Lower extremity biomechanics during weightlifting exercise vary across joint and load.

Kipp, K, Harris, C, and Sabick, MB. Lower extremity biomechanics during weightlifting exercise vary across joint and load. J Strength Cond Res 25(5): 1229-1234, 2011-The purpose of this study was to determine the effect of load on lower extremity biomechanics during the pull phase of the clean. Kinematic and kinetic data of the 3 joints of the lower extremity were collected while participants performed multiple sets of cleans at 3 percentages: 65, 75, and 85% of 1 repetition maximum (1RM). General linear models with repeated measures were used to assess the influence of load on angular velocities, net torques, powers, and rates of torque development at the ankle, knee, and hip joint. The results suggest that the biomechanical demands required from the lower extremities change with the lifted load and to an extent depend on the respective joint. Most notably, the hip and knee extended significantly faster than the ankle independent of load, whereas the hip and ankle generally produced significantly higher torques than the knee did. Torque, rate of torque development (RTD), and power were maximimal at 85% of 1RM for the ankle joint and at 75% of 1RM for the knee joint. Torque and RTD at the hip were maximal at loads >75% of 1RM. This study provides important novel information about the mechanical demands of a weightlifting exercise and should be heeded in the design of resistance training programs.

Effect of Loading Condition on Traction Coefficient between Shoes and Artificial Turf Surfaces

ASTM F2333 is a test method for quantifying traction characteristics between an athletic shoe and a sports surface. This standard calls for normal loads of 500—3000N to be applied between a footform and a playing surface. To assess the effect of varying the normal load on the traction coefficients between cleated athletic shoes and artificial turf surfaces, a new testing device was developed and used to collect traction data. Four different models of cleated athletic shoes were tested on FieldTurf™ at normal loads ranging from 222N to 1776N. Static, dynamic, and peak traction coefficient values were calculated for each condition. There was a significant difference in the slope of the load versus traction coefficient curve for loads below and above 888N for all three variables measured. No significant differences in traction characteristics were found between shoes for loads below 888N. Significant differences between the shoes were seen with loads above 888N. However, buckling and potential permanent damage to the turf surface were seen at loads of 1776N. The results suggest that traction data obtained on FieldTurf at loads below one body weight are not sensitive to different shoe designs. Therefore, the measurement of traction between cleated shoes and FieldTurf should be conducted at a load of at least 888N, which is, in part, consistent with the default normal load of 1000N, in ASTM F2333. However, a normal force of 3000N defined in the standard for studying stopping may not be feasible without permanently damaging the turf surface.

Anterior shoulder forces in professional and Little League pitchers.

Background The developing musculoskeletal system of a youth pitcher is substantially different from that of the adult professional pitcher, predisposing the younger players to a different set of injuries. Methods High-speed videography of 39 professional and 13 youth pitchers were obtained. High-speed motion analysis was performed to calculate average anterior forces and arm positions at maximal force generation. Results Professional players generated an average of 33.8±14.4 N/kg maximal anterior force, corresponding to 151.9±17.0 degrees of external rotation. Youth pitchers generated 16.2±3.8 N/kg of anterior forces, corresponding to 118.0±23.4 degrees of external rotation. The degree of coronal abduction and horizontal abduction between the 2 groups were not significantly different—92.4±9.0 degrees in professionals versus 91.7±7.9 degrees in the youth and 11.1±11.1 degrees of horizontal abduction in professionals versus 7.8±14.1 degrees in the younger throwers. Professional pitchers exerted higher internal rotation torque at 19.4±4.1 Nm/kg versus 5.6±1.0 Nm/kg in youth, and compressive forces were found to be 121.7±21.7 N/kg in professional pitchers compared with 47.5±7.6 N/kg in the youth pitchers. Conclusions Youth pitchers experience significant anterior shoulder forces and internal rotation torques, although these are lower than professional pitchers. Clinical Relevance Overhead throwing as a youth can lead to shoulder injuries and can predispose the shoulder to more significant injuries as an adult pitcher.

Influence of towing force magnitude on the kinematics of supramaximal sprinting.

Clark, DA, Sabick, MB, Pfeiffer, RP, Kuhlman, SM, Knigge, NA, and Shea, KG. Influence of towing force magnitude on the kinematics of supramaximal sprinting. J Strength Cond Res 23(4): 1162-1168, 2009-The purpose of this study was to determine the influence of towing force magnitude on the kinematics of supramaximal sprinting. Ten high school and college-age track and field athletes (6 men, 4 women) ran 60-m maximal sprints under 5 different conditions: Nontowed, Tow A (2.0% body weight [BW]), Tow B (2.8% BW), Tow C (3.8% BW), and Tow D (4.7% BW). Three-dimensional kinematics of a 4-segment model of the right side of the body were collected starting at the 35-m point of the trial using high-speed (250 Hz) optical cameras. Significant differences (p < 0.05) were observed in stride length and horizontal velocity of the center of mass during Tow C and Tow D. For Tow D, a significant increase (p = 0.046) in the distance from the center of mass to the foot at touchdown was also observed. Contact time decreased significantly in all towing conditions (p < 0.01), whereas stride rate increased only slightly (<2.0%) under towed conditions. There were no significant changes in joint or segment angles at touchdown, with the exception of a significant decrease (p = 0.044) in the flexion/extension angle at the hip during the Tow D condition. We conclude that towing force magnitude does influence the kinematics of supramaximal running and that potentially negative training effects may arise from towing individuals with a force in excess of 3.8% BW. Therefore, we suggest that coaches and practitioners adjust towing force magnitude for each individual and avoid using towing forces in excess of 3.8% of the athletes BW.

Weightlifting Performance is Related to Kinematic and Kinetic Patterns of the Hip and Knee Joints

Abstract Kipp, K, Redden, J, Sabick, MB, and Harris, C. Weightlifting performance is related to kinematic and kinetic patterns of the hip and knee joints. J Strength Cond Res 26(7): 1838–1844, 2012—The purpose of this study was to investigate the correlations between biomechanical outcome measures and weightlifting performance. Joint kinematics and kinetics of the hip, knee, and ankle were calculated while 10 subjects performed a clean at 85% of 1 repetition maximum (1RM). Kinematic and kinetic time-series patterns were extracted with principal components analysis. Discrete scores for each time-series pattern were calculated and used to determine how each pattern was related to body mass–normalized 1RM. Two hip kinematic and 2 knee kinetic patterns were significantly correlated with relative 1RM. The kinematic patterns captured hip and trunk motions during the first pull and hip joint motion during the movement transition between the first and second pulls. The first kinetic pattern captured a peak in the knee extension moment during the second pull. The second kinetic pattern captured a spatiotemporal shift in the timing and amplitude of the peak knee extension moment. The kinematic results suggest that greater lift mass was associated with steady trunk position during the first pull and less hip extension motion during the second-knee bend transition. Further, the kinetic results suggest that greater lift mass was associated with a smaller knee extensor moments during the first pull, but greater knee extension moments during the second pull, and an earlier temporal transition between knee flexion-extension moments at the beginning of the second pull. Collectively, these results highlight the importance of controlled trunk and hip motions during the first pull and rapid employment of the knee extensor muscles during the second pull in relation to weightlifting performance.

Muscle Synergies During a Single-Leg Drop-Landing in Boys and Girls

The purpose of this study was to investigate muscle activation patterns during a landing task in boys and girls through the use of muscle synergies. Electromyographical data from six lower extremity muscles were collected from 11 boys and 16 girls while they performed single-leg drop-landings. Electromyographical data from six leg muscles were rectified, smoothed, and normalized to maximum dynamic muscle activity during landing. Data from 100 ms before to 100 ms after touchdown were submitted to factor analyses to extract muscle synergies along with the associated activation and weighing coefficients. Boys and girls both used three muscle synergies. The activation coefficients of these synergies captured muscle activity during the prelanding, touchdown, and postlanding phases of the single-leg drop-landing. Analysis of the weighing coefficients indicated that within the extracted muscle synergies the girls emphasized activation of the medial hamstring muscle during the prelanding and touchdown synergy whereas boys emphasized activation of the vastus medialis during the postlanding synergy. Although boys and girls use similar muscle synergies during single-leg drop-landings, they differed in which muscles were emphasized within these synergies. The observed differences in aspects related to the muscle synergies during landing may have implications with respect to knee injury risk.