Amanda Ransom, Blake Justice, Matthew Condo, Matthew Foreman, John Fox. Methodist University, Fayetteville, NC

BACKGROUND: The purpose of this study was to evaluate the influence of added mass on braking and propulsion time in human hopping. Given the effect of increased mass on a linear spring, it was hypothesized that braking and propulsion time would increase with increased mass. METHODS: A total of 14 volunteers between 18 and 40 years of age (mass = 77.87 kg, sd = 11.31 kg; height = 1.72m, sd = 0.07 m) performed two 20 second trials of bipedal hopping at preferred frequency under 3 loading conditions. Loading conditions consisted of no added mass, 10% body mass, and 20% body mass added. First, participants performed unweighted hopping. Conditions involving 10% and 20% added body mass were ordered randomly. Mass was added via a weighted vest on the trunk. Ground reaction force (GRF) was measured via two Bertec force plates. Given that equilibrium in hopping occurs when GRF is equal to bodyweight, contact time was considered to be the time in which GRF was greater than bodyweight. Therefore, braking time was the time from the first equilibrium point to peak force. Propulsion time was the time from peak force to the second equilibrium point. A 1 (group) x 3 (conditions) x 2 (trials) factorial ANOVA was used to estimate the influence of loading condition on braking and propulsion time. RESULTS: There was a significant main effect of trial on braking time (F(1, 12.90) = 5.667, p < 0.033). Participants decreased braking time in the second trial. There was a significant main effect for trial (F(1, 11.91) = 7.550, p < 0.018) and a significant loading condition by trial interaction (F(2, 1869.48) = 14..4137, p < 0.001) on propulsion time. Post hoc analysis found propulsion time decreased between the first and second trials. Specifically, propulsion time significantly decreased in the 20% mass added condition from the first to second trial (p < 0.001). CONCLUSIONS: This study provides evidence that adding enough mass to the trunk may lead to decreased braking and propulsion time over multiple hops and trials. Given the extra mass and number of hops this change in stiffness could be due to fatigue. In general, these results suggest that large amounts of mass must be added to the system to alter spring-mass parameters. These results may inform future clinical applications in obesity management.

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