MECHANISM OF LEG STIFFNESS ADJUSTMENT FOR HOPPING ON SURFACES OF DIFFERENT STIFFNESSES

When humans hop in place or run forward, leg stiffness is increased to offset reductions in surface stiffness, allowing the global kinematic s and mechanics to remain the same on all surfaces. The purpose of the present study was to determine the mechanism for adjusting leg stiffn ess. Seven subjects hopped in place on surfaces of different stiffness es (23-35,000 kN/m) while force platform, kinematic, and electromyogra phic data were collected. Leg stiffness approximately doubled between the most stiff surface and the least stiff surface. Over the same rang e of surfaces, ankle torsional stiffness increased 1.75-fold, and the knee became more extended at the time of touchdown (2.81 vs. 2.65 rad) . We used a computer simulation to examine the sensitivity of leg stif fness to the observed changes in ankle stiffness and touchdown knee an gle. Our model consisted of four segments (foot, shank, thigh, head-ar ms-trunk) interconnected by three torsional springs (ankle, knee, hip) . In the model, an increase in ankle stiffness 1.75-fold caused leg st iffness to increase 1.7-fold. A change in touchdown knee angle as obse rved in the subjects caused leg stiffness to increase 1.3-fold. Thus b oth joint stiffness and limb geometry adjustments are important in adj usting leg stiffness to allow similar hopping on different surfaces.