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EFFECTS OF LEAD LIMB AND JUMP HEIGHT ON VERTICAL GROUND REACTION FORCES DURING DROP JUMPS

Abstract

BACKGROUND: The drop jump (DJ) is a commonly used exercise for training lower body propulsion and power. Previous literature has shown that drop influences vertical ground reaction force (vGRF) and temporal characteristics. Whether the lead limb, dominant (DOM) verses nondominant (NDOM) - effects vGRF and temporal characteristics remains unknown. The purpose of this study is to evaluate the effect of lead limb and drop heights on average vGRF, impulse, initial ground contact time, and ground off time. METHODS: 24 physically active males (24.2±2.1 yrs, 1.78±.06 m, 79.4±9.4 kg) with six or more months resistance training experience completed eight DJs from four drop heights (30, 45, 60, 75cm). Participants alternated between using DOM and NDOM as the lead limb (4 trials each) for the drop phase. Upon landing with each foot on adjacent force plates, participants immediately performed a maximal effort vertical jump. The recorded vGRF, normalized to body weight (BW), under the DOM and NDOM feet were used to compute vGRF average (AV), impulse (IMP), initial ground contact time (IGCT), and ground-off time (GOT) DOM-NDOM differences. Lead limb and height orders were randomly assigned for each participant. Height by lead limb repeated measures analysis of variance were conducted for each measure, followed by post hoc trend analyses (ordinal height) and pairwise comparisons (nominal limb). RESULTS: IGCT demonstrated a significant height by lead limb interaction (P<.001). Post hoc comparison of significant linear height trends within each lead limb demonstrated that as DJ height increased, the DOM lead limb (P<.001, d=.90) deviation from symmetry was significantly different (P<.001, d=.91) than the NDOM lead limb (P=.001, d=-.76) trend towards more symmetry. Despite the height and lead limb influence on IGC, only significant limb effects were revealed for vGRF AV (D:.04±.10BW, ND:-.09±.09BW, P<.001, d=1.3) and IMP (D:.03±.04BWs,ND:-.02±.04BWs, P<.001, d=1.3). There were no statistically significant GOT findings (P>.120). CONCLUSIONS: Asymmetry in the vGRF AV and IMP favored the lead limb and was unaffected by DJ height. Future research should consider a more comprehensive analysis of vGRF characteristics, such as loading rate and peak vGRF, as well as joint kinematic and kinetic patterns.

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