A treadwheel mounted on a torque motor was used to study changes in step coordination as a function of the magnitude and direction of external forces resisting and assisting leg retraction. The apparent contradiction in the literature concerning the shift in the posterior extreme point is shown to depend upon the magnitude of the load. External forces resisting leg retraction generally shift step end-points rostrally and those assisting leg retraction shift them caudally, shifting leg movement so that the active muscles can provide greater force. However, this primary trend reverses for external forces equivalent to or less than the weight of the body. Together, these results indicate that the decision to end stance does not simply depend upon the leg's loading but rather on its efficiency in contributing propulsion. As expected, loads resisting leg retraction decrease retraction speed and increase step period. They also decrease protraction speed, increase swing duration and shift phase relationships of adjacent ipsilateral legs away from alternation towards a metachronal pattern. Moderate to large assisting loads evoke similar changes: they increase swing duration less but shift the phase of middle and rear legs more than the corresponding resisting loads. Thus, by itself, a reduction in the force required during retraction does not transform the gait from metachronal to tripod. The mean phase of contralateral leg pairs is unaffected by load: these legs continue to alternate.

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