Factors to shorten the contact time in rebound drop jump : With special reference to work done by the lower limb joints and anticipation of the landing

Abstract

Rebound drop jump index [RDJ_&ltindex&gt=(1/8・g・RDJt_a^2)/RDJt_c] was developed to evaluate the ability to perform the ballistic stretch. shortening cycle (SSC) movement. The RDJ_&ltindex&gt consists of ability to jump higher (RDJt_a) and that to shorten the contact time (RDJt_c) in rebound drop jump (RDJ), a typical SSC movement. The former is affected by leg strength and counter movement jump ability but the factors affecting the latter case have not yet been well established. This study examined the factors to shorten the contact time with special reference to two important views, i.e. work done by the lower limb joints and anticipation of the landing. 1. Relationships between work done by the lower limb joints and RDJ_&ltindex&gt, RDJt_c, and RDJt_a in RDJ from height of 0.3m were examined in ten college male athletes. There was a significant correlation between the ratio of negative work at the ankle to total work done by the lower limb joints and RDJ_&ltindex&gt (r=0.726, p&lt0.05), and RDJt_c (r=-0.823, p&lt0.01) but not RDJt_a (r=0.226,ns). Furthermore, there was no significant correlation between the ratio of negative work at the ankle and maximum plantar flexion strength (r=-0.329,ns). These results suggested that the rate of energy absorption at the ankle joint in former contact phase was one important factor to shorten the contact time in RDJ but not affected by plantar flexion strength. 2. RDJ_&ltindex&gt, RDJt_c and RDJt_a in two RDJs with or without visual information to inhibit temporal and spatial anticipation of landing were compared in six college male athletes. As compared without and with visual information, RDJt_c was longer, RDJt_a was shorter and RDJ_&ltindex&gt was lower, significantly. These changes were greater in subjects showing the higher RDJ_&ltindex&gt than those showing the lower RDJ_&ltindex&gt. Furthermore, changes of RDJ_&ltindex&gt, RDJt_c and RDJt_a in series of nine RDJs without visual information at thirty seconds of rest intervals were compared between subject A showing high RDJ_&ltindex&gt and subject B showing low RDJ_&ltindex&gt. RDJt_c decreased and RDJt_a increased slightly, and RDJ_&ltindex&gt increased by repeated trials even without visual information in subject A but not in subject B. These results suggested that temporal and spatial anticipation of the landing were another important factors to shorten the contact time in RDJ. These finding seemed to be beneficial for establishing strength and power training methods for jumper and ballgame players who are required ballistic stretch-shortening cycle movement.