Passive responses in mouse hind leg locomotion

Mice are of a size at which passive joint and muscle forces should be important in leg movements. To investigate this issue we measured, in anesthetized mice, hind leg passive movements in response to changes in animal orientation relative to gravity and to manual deflections of the leg. Changing gravity orientation did not rotate leg joints to their physiological extremes, indicating that passive responses limit joint rotation range. The manual leg deflections were sufficient to achieve joint angles overlapping those present in published descriptions of mouse locomotion. Upon release from these deflections, the legs returned to intermediate postures. These results show that passive responses are 1) present at locomotory joint angles and 2) sufficiently large, at these angles, to move the leg. Return amplitude depended linearly on deflection amplitude. The slope of this dependence was the same across leg joints, suggesting it is evolutionarily or developmentally selected for. Combining the extremes of our passive response data and published descriptions of joint angles during mouse locomotion (e.g., most flexed passive response mouse with most extended published locomotion pattern) allowed determining when in a locomotory cycle passive responses could be definitely extending or flexing. In three of these four combinations, only extending passive responses could be definitely present in the locomotory patterns. In the fourth, alternatively, both extending and flexing passive responses could be definitely present. Passive responses thus likely act during mouse hind leg locomotion, but their amplitude and even sign may vary across individual mice.