Moving against inertia, friction and gravity: different modalities, same consequences for animal propulsive force capacity.

The ability to generate propulsive force at different velocities is essential for animal locomotion but has often been overlooked. This study explored animal locomotion under varying mechanical constraints by addressing whether force capacity measured during acceleration on level ground is representative of propulsion capacity exerted during steady velocity uphill running or running against a resistance. We hypothesised that locomotion against resistance induced by inertia, friction or gravity would lead to similar propulsive force capacity, step length and step frequency. Nineteen human participants performed three accelerated, six resisted and ten uphill sprints while their instantaneous velocity, step length and step frequency were measured. The propulsive force capacity decreased linearly with velocity. This individual relationship was preserved among the disparate mechanical constraints, with humans just shifting along this curve. Trivial (-2.0±21.7%, P=0.43) and small differences (-6.1±21.5%, P=0.24), and positive correlation (P<0.001) were indeed found between force capacities at similar velocities among uphill/accelerated (r=0.94) and resisted/accelerated (r=0.91) conditions, respectively. Spatio-temporal variables did not differ between conditions (<2%). Conducting similar analysis in a 12-animal dataset from the literature revealed that different experimental modalities are associated with similar propulsive force-velocity relationships within the same species. Extending the analogy between accelerated, uphill and resisted running to the animal kingdom enabled comparisons between species based on propulsive force capacity and allometric scaling. Using humans as an experimental paradigm, we provide a framework for interpreting how environmental stressors affect movement strategy in many terrestrial species. In the field of sports science, this study has practical implications for the design of training and research protocols.