Lower extremity control and dynamics during backward angular impulse generation in backward translating tasks.

Observation of complex whole-body movements suggests that the nervous system coordinates multiple operational subsystems using some type of hierarchical control. When comparing two backward translating tasks performed with and without backward angular impulse, we have learned that task-specific modifications in trunk-leg coordination contribute to the regulation of total-body center of mass (CoM) position relative to the reaction force (RF). In this study, we hypothesized that task-specific differences in trunk-leg coordination would affect the control of the lower extremity joints during the impulse-generation phase of the tasks. Eight highly skilled performers executed a series of backward translating jumps with and without backward rotation (back somersault and back timer, respectively). Sagittal plane kinematics, RFs and electromyograms of lower extremity muscles were acquired during the take-off phase of both tasks. Lower extremity joint kinetics was calculated using inverse dynamics. The results indicate that between-task differences in the relative angles between the lower extremity segments and the net joint forces/RF contributed to significant reductions in knee-extensor net joint moments and increases in hip-extensor net joint moments during the push interval of the back somersault as compared to the back timer. Between-task differences in backward trunk angular velocity also contributed to the re-distribution of work done by the lower extremity net joint moments. Between-task differences in lower extremity joint kinetics were associated with synergistic activation of the bi-articular muscles crossing the knee and hip. These results indicated that task-specific control of CoM relative to the RF in order to regulate the backward angular-impulse-involved modification in the control and dynamics of the knee and hip joints. These results indicate that between-task differences in the control objectives at the total-body level (position of CoM relative to the RF) alters the control and dynamics of the multi-joint lower extremity subsystem.