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

BACKGROUND: The purpose of this study was to evaluate the influence of added mass on human hopping. Given the effect of increased mass on a linear spring, it was hypothesized that system stiffness and center of mass (COM) displacement amplitude would increase and decrease, respectively. 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. GRF was integrated twice to obtain displacement of the center of mass. System stiffness was estimated by taking the ratio of force to peak displacement. A 1 (group) x 3 (conditions) x 2 (trials) factorial ANOVA was used to estimate the influence of loading condition on system stiffness and COM amplitude. RESULTS: There was a significant loading condition by trial interaction (F(2, 2560.40) = 20.8598, p < 0.001) on stiffness. Post hoc analysis showed stiffness decreased between the first and second trials in the 20% mass added condition (p = 0.0007). There was no effect of condition or trial on COM amplitude. CONCLUSIONS: This study provides evidence that adding enough mass to the trunk may lead to decreased stiffness 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 or sport rehabilitation.

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