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

The aim of the study was to investigate the coordination of the activity of the leg muscles during performance conditions simulating vertical jumping. Jumps were carried out in different body postures with no balance requirement. Five skilled male subjects performed maximum drop jumps with small (QDJ) and large stretching amplitude (DDJ) on a special sledge dynamometer. The inclination of the dynamometer could be adjusted to vary the load to between 40% and 110% of body weight. Increased load produced a higher negative angular velocity of the knee joint and higher maximum peak force (P < 0.001). The QDJ conditions caused significantly higher force results than the DDJ (p < 0.001). The main result of the present study was that the leg muscle activity coordination model was found to be consistent in all the different test conditions. The maximum peak activity of the agonist muscles was characterized in the model by a sequential proximodistal chain. The QDJ conditions potentiated muscle activity in the hip, knee and ankle joint during the eccentric work phase but decreased it after the transient point (p < 0.05). Especially in the eccentric phase, increased agonist activity in the hip and knee joint and, inversely, decreased activity in the ankle joint was linked to heavier load (p < 0.05). The present results suggest that the coordination program of the vertical jumping model had been so highly practiced that it had become an automatic reflex-like movement now named "skill-reflex". The programmed skill-reflex seemed to guid the execution of the jump irrespective of temporary voluntary orders from the central nervous system.