Effects of Body Mass on Leg and Vertical Stiffness in Running Humans

Numerous cross-species comparisons have examined the scaling of gait parameters with respect to body mass (i.e., allometry), but few have done so within humans. This study examined how leg and vertical stiffness, force, displacement, and leg spring angle scaled in 64 healthy adults of varying body masses during slow and fast leg-length-adjusted running speeds. We calculated scaling patterns for stiffness and its components via kinematic and kinetic data using log-log regressions with 95% confidence/highest density intervals. To determine if the chosen statistical method influenced conclusions about scaling patterns, we compared regression results across three statistical methods, ordinary least squares (OLS) regression, linear mixed models (LMM), and Bayesian linear mixed models (BLMM). We also performed sex-specific analyses to determine if each sex revealed similar scaling patterns as the pooled sample. In the pooled sample, all variables scaled according to the isometric expectations, suggesting that different-sized humans move in a similar manner. Sex-specific analyses revealed similar patterns of isometry in all variables, except for vertical stiffness, which displayed slight negative allometry (i.e., lower than expected stiffness) in both sexes at the slow speed and negative allometry in females during fast running. Model choice did not significantly affect results, and scaling patterns were the same regardless of the statistical method employed.