Velocity of the body center of mass during walking on split-belt treadmill

Walking on split-belt treadmills (each belt rotating at a different velocity) has inspired a growing number of researchers to study gait adaptation and rehabilitation. An overlooked peculiarity of this artificial form of gait is that the mean velocity adopted by the participant, considered as a whole system represented by the body Center of Mass (CoM), can be different from the mean velocity of the two belts. Twelve healthy adults (21-34 years) were requested to walk for 15 minutes on a treadmill with belts rotating at 0.4 and 1.2 m s-1, respectively (mean 0.8 m s-1). Each belt was supported by four 3D force sensors. For each participant, six strides were analyzed during the 1st and the 15th minute of the trial. The mean CoM velocity was computed as the sum of the velocities of each belt weighted by the percentage of time during which the resulting forces, underlying the accelerations of the CoM, originated from each belt. Across early and late observations, the median CoM velocities were 0.72 m s-1 and 0.67 m s-1, respectively (p<0.05). Therefore, the real velocity of the CoM and its time course should be individually assessed when studying walking on split-belt treadmills.