Emulating constant acceleration locomotion mechanics on a treadmill

Locomotion on an accelerating treadmill belt is not dynamically similar to overground acceleration. The purpose of this study was to test if providing an external force to compensate for inertial forces during locomotion on an accelerating treadmill belt could induce locomotor dynamics similar to real accelerations. Nine males (mean±sd age=26±4 years, mass=81±9 kg, height=1.8±0.05 m) began walking and transitioned to running on an accelerating instrumented treadmill belt at three accelerations (0.27 m s−2, 0.42 m s−2, 0.76 m s−2). Half the trials were typical treadmill locomotion (TT) and half were emulated acceleration (EA), where elastic tubing harnessed to the participant provided a horizontal force equal to mass multiplied by acceleration. Net mechanical work (WCOM) and ground reaction force impulses (IGRF) were calculated for individual steps and a linear regression was performed with these experimental measures as independent variables and theoretically derived values of work and impulse as predictor variables. For EA, linear fits were significant for WCOM (y=1.19x+10.5, P<0.001, R2=0.41) and IGRF (y=0.95x+8.1, P<0.001, R2=0.3). For TT, linear fits were not significant and explained virtually no variance for WCOM (y=0.06x+1.6, P=0.29, R2<0.01) and IGRF (y=0.10x+0.4, P=0.06, R2=0.01). This suggested that the EA condition was a better representation of real acceleration dynamics than TT. Running steps from EA where work and impulse closely matched theoretical values showed similar adaptations to increasing acceleration as have been previously observed overground (forward reorientation of GRF vector without an increase in magnitude or change in spatio-temporal metrics).