Biomechanics of hang-time in volleyball spike jumps.
OBJECTIVE
This study examined whether "hang", an extended period of greatly reduced or zero vertical velocity of the head and trunk created by inter-segmental interactions, would be seen during skilled volleyball player spike jumps.


METHOD
Fifteen skilled volleyball hitters (eight men and seven women, age 23.26 ± 3.22 years, height 1.86 ± 0.08 m, mass 77.53 ± 10.45 kg) performed spike jumps in two hitting conditions, flexing their knees during flight as much as possible and not flexing their knees during flight. We analyzed the effect of knee flexion on the vertical and temporal components of the trajectories of the head, trunk, legs and wrist of the hitting arm to study the existence of "hang" and its underlying mechanisms.


RESULTS
With knee flexion, unlike no knee flexion, the head and trunk (HT) demonstrated "hang", characterized by a longer time of near-zero vertical velocity of the head and trunk near mid-flight (p < 0.001). Analysis of the influence of the timing and extent of knee flexion on the HT center of mass trajectory revealed significant effects (p < 0.001).Women demonstrated longer "hang" during flight than men. Athletes in this study hit the ball later in flight in the "hang" condition (p < 0.001).


CONCLUSION
An extended period of reduced vertical velocity of the head and trunk near mid-flight resulted from knee flexion and then extension. This additional time at the peak of the jump could be useful to adjust to ball trajectory and to decide where, when and how to hit the ball.