Effect of Shoulder Compliance on Peak High Bar Forces During the Giant Swing

Abstract

When a female gymnast model does not include shoulder compliance, a simulated optimal performance of a giant swing on the uneven parallel bars is limited by the maximum force that can be exerted on the hands without slipping from the bar (Sheets and Hubbard 2005). To determine the effect of shoulder compliance on hand force, two four-segment gymnast models including an arm, a torso/head and two leg segments are compared: one in which the shoulder is a rigid pin joint, and one which includes shoulder compliance. Experimental values of shoulder stiffness and damping are determined to best describe 1-D vertical damped oscillations of the gymnast/bar system. Optimum shoulder and hip motions during the swing are calculated to complete the most dismount flight revolutions prior to the mass center (CM) passing a specified landing height. Optimization constraints include maximum bar/hand force, physiologic joint limitations, low bar avoidance, and minimum landing distance from the bar. Shoulder and hip motions result from time varying joint torques that are limited by joint angle, angular velocity, isometric strength, and activation factors. Bar release time and joint torque activations at ten nodes equally spaced throughout the swing are optimized using the downhill simplex method. Joint torque activations at all other times are approximated by cubic splines fit to the ten nodes. Performance limitations due to the slipping constraint are not reduced in the compliant model, even though it is active for a shorter period. The compliant shoulder model also produced fewer flight revolutions than the rigid one, 1.417 vs. 1.478, because the shoulder dissipated energy during the swing and stored it at bar release.