Changes in net joint torques during accomodation to change in surface compliance in a drop jumping task

This study investigated how subjects adjust the timing of joint torques during accomodation to an unfamiliar compliant surface in order to maximise height achieved in a drop jumping task (dropping from a height onto a surface and jumping for maximum height). Estimates of the energy absorbed and work done by the subject during the period of contact were obtained using a mathematical model of the compliant surface. Power output of joints and net joint torques were also calculated. In the case of all subjects there was a strong relationship (p < 0.001) between maximum energy absorbed and height achieved. Subjects also improved jump height by increasing positive work (p < 0.001). There was a decrease in energy absorbed at the hip, knee, and ankle joints following landing on the compliant surface. Improvement was related to changes in the timing and sequencing of net joint torques rather than to increases in peak torques. With practice there was an increase in net joint torque tending to extend the hip at the beginning of the period of contact and the timing of peak knee torques was advanced. As a result there was a change from the established hip-knee-ankle sequence of peak joint torques and power output to a knee-hip-ankle sequence. However, the hip-knee-ankle sequence of joint kinematics became more distinct.