Hannah Smedley, Mackenzie Wall, Caroline Phelps, Josh Shoaf, John Fox. Methodist University, Fayetteville, NC.

PURPOSE: The purpose of this study was to compare the effect of increasing mass centrally or peripherally on bipedal hopping. METHODS: Ten females and five males were randomly assigned to either a trunk (TR) (N=5), thigh (TH) (N=5) or shank (SH) (N=5) group. Mass was increased by wrapping small bags of sand at half the length of the trunk, thigh or shank, depending on the assigned group. Each participant performed 2, 30 second trials of hopping with no increased mass, 10% of trunk, thigh or shank mass, and 20% of trunk, thigh or shank mass added. Participants hopped on two Bertec force plates. The amount of time in which force was greater than body weight was considered half the period of the hop. Therefore, hopping frequency was computed as the inverse of twice the half period of each hop. Center of mass (COM) displacement was estimated by double integrating force. Stiffness was computed at the ratio of peak force to COM displacement. Mean frequency and stiffness were computed for each trial in each condition. These data points were then used in a 3 (groups) x 3 (conditions) x 2 (trials) mixed factorial ANOVA to evaluate effects of added mass and mass location on hopping frequency and system stiffness. RESULTS: Mixed factorial ANOVA revealed a significant main effect of loading condition on frequency (F(2,24) = 5.124, p = 0.014) and a significant group by condition interaction (F(4,24) = 3.969, p = 0.013). Pairwise comparisons showed that frequency with 10% segment mass added was significantly greater than in the 20% mass added (p = 0.011). Additionally the shank group hopped with significantly higher frequency with 10% shank mass added compared to no mass added (p = 0.037). No main effects or interaction effects were found for stiffness. CONCLUSION: This study demonstrates that adding mass to the shank has an effect on hopping frequency, while adding mass to the thigh and trunk does not. Because the amount of mass added was a percentage of segment mass, differences found in the shank group were due to the least change in mass. These results have applications in rehab, return to sport and load distribution in training.

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