KNEE STIFFNESS MEASUREMENT DURING HOPPING AT DIFFERENT FREQUENCIES

Abstract

1506 The purpose of this study was to develop and verify a theoretical model concerning the measurement of human knee stiffness during the hopping stance phase. The knee joint during the hopping stance phase is modeled as a rotational spring. This model predicted that there were three different stiffness values during the hopping stance phase. These stiffnesses were separated by two different equilibrium points. These equilibrium points were defined as the knee angles at maximum knee flexion and extension angular velocity during weight acceptance and propulsion phase respectively. The three stiffness values include two for the knee flexion phase (before and after the first equilibrium point) and one for the knee extension phase (before the second equilibrium point). Knee joint stiffness values were defined as the slopes of the knee joint angle squared vs. angular velocity squared graph of each phase. Kinematic data of hip, knee and ankle joint markers were captured with high-speed video cameras (120 Hz) while subjects hopped at three different frequencies: preferred, 120% and 80% of the preferred hopping frequency. Five consecutive hopping cycles were collected. Video images were digitized and 2D kinematics (knee joint angle and angular velocity) were calculated. The first (third) knee joint stiffness value decreased (increased) significantly with different hopping frequencies. No significant change was observed for the knee stiffness of the second phase. Compared to previous literature, this model provides more detailed information regarding knee joint function during the hopping stance phase.