Effect Of Changes In Ball Weight On Shoulder Rotations During Upper Body Plyometrics

Abstract

PURPOSE To determine differences in shoulder kinematics with 3 different weighted balls during an upper body plyometric exercise. METHODS This study was conducted on 5 males (age 32±7 yrs) using standard 2, 6, & 12lb medicine balls. Subjects performed a plyometric chest pass with both arms against a rebounder for 10 consecutive trials. Each subject was instrumented with 23 retroreflective markers modified from Aguinaldo (2004). 3D motion data was collected with 8 cameras (Motion Analysis Corp.) at 240Hz. Joint centers were calculated with virtual markers within EvaRT (Motion Analysis Corp.). Marker trajectories were then exported to Matlab to calculate shoulder joint angles using the Joint Coordinate System method. ROM for each trial was calculated from the max. & min. angles from each shoulder rotation. A repeated measures ANOVA was utilized to determine the effect of ball weight on each dependent variable. RESULTS Shoulder flex/ext, abd/add & int/ext ROM were all significantly increased when the ball weight was increased (main effect of ball p<0.001, p<0.01, and p<0.01). There were NS differences in side-to-side measures from each shoulder joint rotation. Post-hoc analysis revealed significant change in shoulder flex ROM from each increase in weight (p<0.05). However, abd/add ROM was significantly increased only from the 2 to 6lb weight while int/ext rotation was significantly increased only from the 6 to 12lb weight. The reliability, intraclass correlation coefficient (ICC[3,1]), of the measurements was high (average over 3 weights: shoulder flex r=0.789, abd r=0.884 & int/ext r=0.912).TableCONCLUSIONS Increasing ball weight affected shoulder abd/add and int/ext rotation differently. Changes in abd/add occurred at low weight, while changes in int/ext rotation required higher loads. Therefore, when developing an upper extremity plyometric program, the patient's individual injury and resultant “at-risk” movements must be considered prior to deciding on a progression of weight.