Direction Control in Standing Horizontal and Vertical Jumps

The purpose of this study was to perform a detailed kinematic, kinetic, and electromyographic comparison of maximal effort horizontal and vertical jumping. It was of particular interest to identify factors responsible for the control of jump direction. Eight male subjects performed maximal horizontal jumps (HJ) and vertical jumps (VJ) from a standing posture with a counter movement. Three-dimensional motion of the trunk, pelvis, and bilateral thigh, shank, and foot segments were recorded together with bilateral ground reaction forces and electromyographic (EMG) activity from seven right leg muscles. Relative to the VJ, the trunk is displaced further forward at the beginning of the HJ, through greater ankle joint dorsiflexion and knee extension. The activity of the biarticular rectus femoris and hamstrings were adapted to jump direction and helped to tune the hip and knee joint torques to the requirements of the task. The primary difference in joint torques between the two jumps was for the knee joint, with the extension moment reduced in the HJ, consistent with differences in activation levels of the biarticular rectus femoris and hamstrings. Activity of the mono-articular knee extensors was adapted to jump direction in terms of timing rather than peak amplitude. Overall results of this study suggest that jump direction is controlled by a combination of trunk orientation at the beginning of the push-off and the relative activation levels of the biarticular rectus femoris and hamstring muscles during the push-off.