Relationship between ankle joint and ground contact time in athlete drop jump performance

The purpose of this investigation was to observe the influence of ankle joint angle and ground contact time (GCT) on jump height performance (JH). Eighteen male junior college football players performed depth jumps in four distinct jump conditions: dorsiflexed-fast (DFF), dorsiflexed-slow (DFS), plantarflexed-fast (PFF) and plantar-flexed-slow (PFS). The four jump conditions were evaluated against the dependent variables GCT, JH, reactive strength index (RSI), ankle angle at contact (AAC), peak ankle flexion angle (PAFA), ankle range (AR), knee angle at contact (KAC), peak knee flexion angle (PKFA), knee range (KR), hip angle at contact (HAC), peak hip flexion angle (PHFA), hip range (HR), initial GRF and peak GRF. The fast conditions showed significant main effects for decreased GCT, decreased joint range for the knee and hip, decreased peak flexion at all 3 joints, as well as increased initial and peak GRF (all p .05). Contrary to our hypotheses, the foot condition was not related to a significant main effect for GCT or JH. However, the foot condition did result in a significant main effect for ankle angle at contact, ankle range, initial GRF and peak GRF. Neither the foot nor the speed conditions displayed a significant main effect for jump height. The results indicate that depth jump GCT can be reduced without a significant corresponding decrease in jump height. A ground contact time of 0.3 s may be the minimal GCT possible to elicit maximum jump height. However, depth jumps over 45 cm may elicit eccentric loads that are too great for subjects to achieve fast stretch-shorten cycles, less than 250 ms. Increased dorsiflexion of the ankle at ground contact does not significantly affect ground contact time or jump height, thus, it is not recommended at this time. Future research should revisit these four jump conditions at depth jump heights above and below 45 cm. Additionally, the longitudinal effects of depth jump training should be examined.