Moving against inertia, friction, gravity: different modalities, same consequences on animals’ propulsive force capacities

The ability to generate propulsive force at different velocities is essential for animal locomotion but has often been depreciated. This study explored animals' locomotion under varying mechanical constraints by addressing whether force capacities measured during accelerations on level ground are representative of propulsion capacities exerted during steady velocity uphill running or running against a resistance. We hypothesised that locomotion against resistances induced by inertia, friction or gravity would lead to similar propulsive force capacities, step length, and step frequency. Nineteen human-participants performed 3 accelerated, 6 resisted, and 10 uphill sprints while their instantaneous velocity, step length, and step frequency were measured. The propulsive force capacities decreased linearly with velocity. This individual relationship was preserved among the disparate mechanical constraints, 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) where 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-animals 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 capacities and allometric scaling. Using humans as an experimental paradigm, we provided a framework for interpreting how environmental stressors affect movement strategies in many terrestrial species. In sports science, this study opens practical implications for the design of training and research protocols.