Effect of postural and load variation on the coordination of the leg muscles in concentric jumping movement.

Coordination of the lower extremity muscles was studied under different load conditions (40-110% of the body weight). Skilled male subjects performed vertical jumps with maximum effort in four body postures (105-165 degree angle of the hip joint) without any balance requirements. The maximum peak force values closely followed the variation in the body weight (p < 0.001). Furthermore, a more erect body posture reduced the force level during the first half and increased it during the second half of the jump (p < 0.001) in consequence of the force-length relationship of the thigh muscles. The shift toward an erect body posture significantly increased the activity of the rectus femoris (p < 0.001) and slightly that of the vasti muscles (n.s.). On the other hand, the gastrocnemius medialis reacted clearly (p < 0.01) and the gluteus maximus and gastrocnemius lateralis muscle slightly to the variation in body weight. Hence, the concentric jump conditions divided the agonist muscles into two categories: the posture sensitive muscles guided by the rectus femoris and the load sensitive muscles guided by the gastrocnemius medialis. The central nervous system (CNS) seems basically to favour the muscle coordination of an erect body posture through the function of two-joint muscle extending the knee joint. In addition according to the requirements of the increasing load, the CNS postpones the timing of the concentration of the activity of the hip and ankle joint agonists. But in the vertical jump as a whole, the activity pattern of the leg muscles was not essentially influenced by the different posture or load conditions (n.s.).