Purpose: We determined the metabolic and biomechanical effects of adding mass to the running-specific prosthesis (RSP) and biological foot of individuals with a unilateral transtibial amputation (TTA) during running.Methods: 10 individuals (8 males, 2 females) with a TTA ran on a force-measuring treadmill at 2.5 m/s with 100 g and 300 g added to their RSP alone or to their RSP and biological foot while we measured their metabolic rates and calculated peak vGRF, stance-average vGRF, and step

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... symmetry indices.Results: For every 100 g added to the RSP alone, metabolic power increased by 0.86% (p = 0.007) and for every 100 g added to the RSP and biological foot, metabolic power increased by 1.74% (p < 0.001) during running. Adding mass had no effect on peak vGRF (p = 0.102), stance-average vGRF (p = 0.675), or step time (p = 0.413) symmetry indices. We also found that the swing time of the affected leg was significantly shorter than the unaffected leg across conditions (p < 0.007).Conclusions: Adding mass to the lower limbs of runners with a TTA increased metabolic power by more than ~1%, which has been reported for those without an amputation. We found no effect of added mass on biomechanical asymmetry, but the affected leg had consistently shorter swing times than the unaffected leg. This suggests that individuals with a TTA maintain biomechanical asymmetries despite changes in RSP mass and that lightweight prostheses could improve performance by minimizing metabolic power without affecting biomechanical asymmetry.