Purpose: Much like running on a slope, running against/with a horizontal traction force which either hinders/aids the forward motion of the runner creates a shift in the positive and negative muscular work, which in turn modifies the bouncing mechanism of running. The purpose of the study is to (1) investigate the energy changes of the centre of mass and the storage/release of energy throughout the step during running associated with speed and increasing hindering and aiding traction forces; and (2) compare these changes to those observed when running on a slope.
Methods: Ground reaction forces were measured on eight subjects running on an instrumented treadmill against different traction forces at different speeds.
Results: As compared to unperturbed running, running against/with a traction force increases/decreases positive external work by ~ 20-70% and decreases/increases negative work by ~ 40-60%, depending on speed and traction force. The external power to maintain forward motion against a traction is contained by increasing the pushing time and step frequency. When running with an aiding force, the external power during the brake is limited by increasing braking time. Furthermore, the aerial time is increased to reduce the power required to reset the limbs each step.
Conclusion: Our results show that the bouncing mechanism of running against/with a hindering/aiding traction force is equivalent to that of running on a positive/negative slope.
Keywords: Body centre of mass; External work; Locomotion; Propulsive and braking force; Pulling force.