THE HYBRID MASS-SPRING PENDULUM MODEL OF HUMAN LEG SWINGING - STIFFNESS IN THE CONTROL OF CYCLE PERIOD

Human leg swinging is modeled as the harmonic motion of a hybrid mass- spring pendulum. The cycle period is determined by a gravitational com ponent and an elastic component, which is provided by the attachment o f a soft-tissue/muscular spring of variable stiffness. To confirm that the stiffness of the spring changes with alterations in the inertial properties of the oscillator and that stiffness is relevant for the co ntrol of cycle period, we conducted this study in which the simple pen dulum equivalent length was experimentally manipulated by adding mass to the ankle of a comfortably swinging leg. Twenty-four young, healthy adults were videotaped as they swung their right leg under four condi tions: no added mass and with masses of 2.27, 4.55, and 6.82kg added t o the ankle. Strong, linear relationships between the acceleration and displacement of the swinging leg within subjects and conditions were found, confirming the motion's harmonic nature. Cycle period significa ntly increased with the added mass. However, the observed increases we re not as large as would be predicted by the induced changes in the gr avitational component alone. These differences were interpreted as bei ng due to increases in the active muscular stiffness. Significant line ar increases in the elastic component (and hence stiffness) were demon strated with increases in the simple pendulum equivalent length in 20 of the individual subjects, with r(2) values ranging between 0.89 and 0.99. Significant linear relationships were also demonstrated between the elastic and gravitational components in 22 subjects, with individu al r(2) values between 0.90 and 0.99. This finding suggests stiffness is varied concomitantly with alterations in the inertial properties of the leg pendulum in a simplified mechanism of control.