Dynamics of force and muscle stimulation in human vertical jumping.

PURPOSE The purpose of this study was to gain insight into the importance of stimulation dynamics for force development in human vertical jumping. METHODS Maximum height squat jumps were performed by 21 male subjects. As a measure of signal dynamics, rise time (RT) was used, i.e., the time taken by the signal to increase from 10% to 90% of its peak value. RT were calculated for time histories of smoothed rectified electromyograms (SREMG) of seven lower extremity muscles, net moments about hip, knee, and ankle joints, and components of the ground reaction force vector. RESULTS Average RT values were 105-143 ms for SREMG signals, 90-112 ms for joint moments, and 120 ms for the vertical component of the ground reaction force (Fz). A coefficient of linear correlation of 0.88 was found between RT of SREMG of m. gluteus maximus (GLU) and RT of Fz. To explain this correlation, it was speculated that for an effective transfer from joint extensions to vertical motion of the center of mass (CM), the motion of CM needs a forward component during the push-off. Given the starting position, only the hip extensor muscles are able to generate such a forward acceleration of CM. To preserve the forward motion of CM, RT of knee and ankle joint moments need to be adjusted to RT of the hip joint moment. Thus, the greater RT of the hip joint moment and RT of GLU-SREMG, the greater RT of Fz. CONCLUSIONS Overall, it was concluded that the time it takes to develop muscle stimulation has a substantial effect on the dynamics of force development in vertical jumping, and that this effect should not be neglected in studies of the control of explosive movements.

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