MECHANICAL OUTPUT FROM INDIVIDUAL MUSCLES DURING EXPLOSIVE LEG EXTENSIONS - THE ROLE OF BIARTICULAR MUSCLES

The main result of this study is that biarticular leg muscles contribu te significantly to the work done at joints, due to transfer of power during explosive leg extensions. In particular, a net power transfer w as shown from hip to knee joint during jumping and sprinting. Seven el ite athletes performed explosive one legged jump and sprint push-offs. Kinematics, ground reaction forces and electromyography (EMG) of leg muscles were recorded. The mechanical output of six individual muscle groups was estimated by using Hill-based muscle models. The EMG and ki nematics served as input to these models. For jumping as well as for s printing, the model estimated similar results for the relative work co ntribution done about a joint due to transfer of power by the biarticu lar muscles. Rectus femoris showed a power transfer from hip to knee j oint, while in contrast hamstrings showed a power transfer from knee t o hip joint. Regardless of these opposite directions of power transfer , a net transfer occurred from the hip to the knee joint. The relative work contribution of hamstrings done in hip extension was 7% in jumpi ng and 11% in sprinting. For rectus femoris, the relative work contrib ution done in knee extension was 21% in jumping and 31% in sprinting. Power transferring actions by gastrocnemius from knee to ankle contrib uted 25% in jumping and 28% in sprinting to the work done in plantar f lexion. These results support the hypothesis that the action of biarti cular muscles contributes to a net transfer of power from proximal to distal joints during explosive leg extensions. This action of the biar ticular muscles causes an efficient conversion of body segment rotatio ns into the desired translation of the body centre of gravity.