Skeletal muscles have substantial inertia that cause inertial forces working around joints. These inertial forces are not typically considered in musculoskeletal models used for sport biomechanics research, which can lead to considerable errors in estimated joint kinetics. How large these errors are in common sports movements is yet unclear. We therefore examined the role of shank muscle inertia on ankle joint moments during the swing phase of running at different speeds. Ankle moments were considerably affected when muscles were modelled as separate masses, with a general shift towards reduced dorsiflexion and higher plantarflexion moments. These results show that ignoring inertial muscle forces in musculoskeletal simulations can lead to under- or overestimations of structure-specific loads and possibly erroneous conclusions. We therefore encourage sport biomechanics researchers to consider the impact of muscle inertia on inverse dynamics calculations.
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