Increase of push-off propulsion to compensate heel strike loss during step-to-step transition is limited at faster gait speeds

The role of push-off propulsion as a major source of energy generation to compensate for heel strike collisions has been emphasized analytically and experimentally. The predicted increase of push-off forces with gait speeds, however, may be physiologically limited by intrinsic muscle mechanics. In this study, we examined whether the contribution of push-off propulsion to step-to-step transition is restricted with gait speed. Seven healthy young subjects participated in over the ground walking experiments. Ground reaction forces (GRFs) were measured at four different gait speeds ranging from self-selected to the maximum gait speed. The results showed that the vertical push-off force showed saturation, whereas the horizontal push-off forces proportionally increased with speed. The configuration of the trailing leg showed greater forward leaning during faster gait trials, which increases the horizontal push-off force without a significant increase in the total push-off forces. In contrast, both vertical and horizontal heel strike forces increased with speed. The lack of energy supplied by push-off to counter the heel strike collision loss during the double support phase was followed by an increase in the GRF work during the single support phase. These results imply that the intrinsic muscle mechanics limit the compensatory increase in the push-off force against the heel strike collision loss at faster gait speeds, which then leads to changes in the leg configuration to guarantee horizontal propulsion so as to maintain a steady forward gait speed.