Lower extremity stiffness during hopping in different frequencies

Abstract

The purpose of the study was to determine leg stiffness and low extremity joint stiffness over different frequency ranges during hopping in place. Eighteen healthy male subjects participated in this study. Single-leg hopping was performed in three different frequencies (2.0 Hz, 2.5 Hz, and the frequency preferred by each subject). Kinematic data were obtained using the 3-D motion capture system (VICON Motion Systems Ltd, UK) synchronized with two AMTI force plates (AMTI, USA). EMG activities were recorded from six leg muscles using DELSYS(Trigno Wireless System, USA). The subjects were asked to hop twelve times. The maximum GRF was found in the middle of the stance phase during the maximum leg compression. Leg stiffness was determined as the ratio of the peak ground reaction force(GRF) and the center of mass(COM) displacement to the maximum leg compression, and the joint stiffness was calculated as the ratio of the joint moment to the joint angular displacement. During landing phase, both ankle joint angular displacement and peak ankle joint moment were relatively larger than those of the knee joint in three frequencies. Ankle joint power was larger than that of knee joint in three frequencies. During landing phase, mean EMG activities except for BF were decreased with an increase hopping frequencies. Knee joint stiffness was larger than that of ankle in three hopping frequencies. But knee joint stiffness was no significant difference with an increase hopping frequencies. Ankle joint stiffness and leg stiffness increased with an increase of hopping frequency. Leg stiffness had an approximately linear relationship with ankle joint stiffness.