Joint kinetics of accelerated running overground vs. running on an accelerating motorized treadmill belt

Literature shows that running on an accelerated motorized treadmill is mechanically different from accelerated running overground. Overground, the subject has to increase the net fore-aft force impulse proportional to acceleration in order to overcome linear whole body inertia, whereas on treadmill this force impulse remains zero regardless of belt acceleration. Therefore, it can be expected that changes in kinesiology of the human body (joint kinetics and kinematics) associated with acceleration also are proportional to acceleration overground, whereas no changes according to belt acceleration are expected on treadmill. This study documents the kinesiology of accelerated running overground and running on an accelerated motorized treadmill belt for ten young healthy subjects. When accelerating overground, ground reaction forces accommodate less braking and more propulsion, generating a more forward oriented ground reaction force vector and a more forwardly inclined body. This change in body orientation as such is partly responsible for the changed force direction. Besides this, more pronounced hip and knee flexion at initial contact, higher hip extension velocity (and power generation), smaller knee flexion velocity (and power absorption) and smaller initial plantarflexion velocity are associated with less braking. Higher knee extension and plantarflexion velocity result in larger propulsion. Altogether, during stance, joint moments are not significantly influenced by acceleration overground. Therefore, we propose that accelerated running overground might be the neuromusculoskeletal system doing essentially the same thing, but in a different orientation. However, it can be speculated that the effort on the muscular level might be different as the acceleration-dependent joint kinematics indicate that muscles operate at different conditions in their force-length-velocity relationship according to acceleration. On a treadmill, ground reaction forces are not influenced by acceleration and, compared to overground, only smaller kinesiological adaptations to an accelerating belt are observed. Consequently, adaptations to acceleration during running differ from treadmill to overground and should therefore be studied in the condition of interest