Kinematic Analysis of the Landing Phase onto an Elevated Surface in Box Drop Plyometric Push Ups

Abstract

Previous research concerning loading and kinematics of box drop plyometric push ups (DPU) has not assessed the vertical ground reaction forces (vGRF) and kinematics of the landing phase onto an elevated surface during DPU. Landing phase is defined as the time from contact with the boxes to propulsion off the boxes. PURPOSE: To compare the flight height, peak vGRF, and elbow flexion during the landing phase of DPU from 3.8cm (1.5 in), 7.6cm (3 in), and 10.2cm (4.5 in) heights. METHODS: Ten active male subjects (86.0±11.6 kg, 1.82±.07 m, 25±3 yrs) performed four repetitions of each DPU variation in a counterbalanced order. DPU were completed with each upper extremity placed over separate force plates. Boxes were placed on the individual force plates just outside of the subjects self-selected hand separation for the push ups. An electromagnetic tracking system (Motion Monitor, IST, Inc) collected kinematics of the trunk and dominant elbow. From the push up position, peak flight (maximal vertical trunk position during flight), peak vGRF, elbow flexion at ground contact (GC), and elbow displacement (difference between elbow position at GC and peak flexion) were calculated. RESULTS: Significant differences in peak flight (P<.001) and elbow flexion at GC (P<.001) were revealed between DPU variations. Post hoc analysis of peak flight demonstrated peak flight during 3.8cm DPU to be significantly less than 7.6cm DPU (P=.002, d=.58) and 10.2 DPU (P<.001, d=1.17), and 7.6 DPU to be significantly less than 10.2 DPU (P<.001, d=.66). Post hoc analysis of elbow flexion at GC revealed subjects had significantly greater elbow extension during the 3.8cm DPU compared to 10.2cm DPU (P<.001, d=.67). No significant differences were revealed for peak vGRF (P=.253) or elbow displacement (P=.074) between the DPU variations. CONCLUSION: Based on peak flight, as box height increases, the higher the subjects must propel their trunk to land on the boxes. In addition, as the box height increased the subjects landed with more elbow flexion. This may indicate that the difficulty of landing on the higher boxes prompted subjects to compensate with more elbow flexion but not enough to affect peak flight. Future research needs to examine DPU from greater heights to determine if kinematics and forces change with an increase in the height of the boxes in DPU.