Mechanisms Enabling Greater Vertical Displacement In Skipping Despite Lower Muscle Force Impulses Than In Running

Skipping is a common speed training exercise and a transitional rehabilitation activity between walking and running. Skipping has a greater metabolic cost than running, which is associated with greater vertical displacement of the body’s center of mass (COM). Muscle forces perform mechanical work to raise the COM, yet our recent work revealed lower muscle force impulses in skipping compared to running, particularly in the ankle plantarflexors. PURPOSE: To determine the mechanisms enabling greater vertical displacement in skipping despite lower muscle force impulses. METHODS: Running and skipping cycles were simulated in OpenSim for 5 adults (22.4 ± 2.2 y) using motion capture and ground reaction force (GRF) data collected at 2.5 m/s on an instrumented dual-belt treadmill. A segmental power analysis quantified muscle contributions to vertical mechanical power, which were integrated over the stance phase of running (Run) and the hop step of skipping (Skip). Paired t-tests compared vertical COM kinematics, vertical GRF impulses, and muscle contributions to vertical work between gaits. RESULTS: Skip had lower negative landing and greater positive take-off COM velocities than Run (Fig 1A, p < 0.05) and greater COM displacement during the stance and flight phases (Fig 1B, p < 0.05). GRF impulses were not different between gaits (Fig 1C). In Skip, the plantarflexors, quadriceps, and gluteal muscles generated greater positive vertical work while the hamstrings and tibialis anterior generated greater negative work than in Run (Fig 1D, p < 0.05). CONCLUSION: Due to lower negative vertical landing velocity in Skip, less work was required to redirect the COM, which began to move upward earlier in stance. Thus, the plantarflexors (despite producing lower force impulses), quadriceps and gluteal muscles generated greater net positive vertical work in Skip, which produced greater changes in vertical COM position during stance, take-off velocity, and COM position during flight.