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.

小鼠体型适宜，其腿部运动中关节与肌肉的被动作用力理应占据重要地位。为探究该问题，本研究对麻醉状态下的小鼠开展测量，记录其后腿在两种条件下的被动运动：一是躯体相对于重力方向的姿态变化，二是对腿部施加手动偏转。改变重力朝向并未使腿部关节旋转至生理极限，表明被动响应会限制关节的旋转活动范围。手动腿部偏转所能达到的关节角度，与已发表的小鼠运动相关文献中记载的关节角度存在重叠。解除手动偏转后，后腿会回归至中间姿态。本研究结果证实，被动响应具备两个特征：其一，在运动相关的关节角度下存在被动响应；其二，在此类角度下，被动响应的强度足以带动腿部运动。腿部回归的幅度与偏转幅度呈线性相关。该线性关系的斜率在各腿部关节间保持一致，提示这一特性可能是进化或发育选择的结果。将本研究被动响应数据的极值与已发表的小鼠运动关节角度描述相结合（例如，将被动响应时最屈曲的小鼠姿态与已发表运动模式中最伸展的姿态进行配对），即可确定在运动周期的哪些阶段，被动响应必然表现为伸肌或屈肌活动。在上述四种配对组合中，有三种组合可明确判定运动模式中仅存在伸肌型被动响应；剩余一种组合则可明确判定运动模式中同时存在伸肌型与屈肌型被动响应。由此可见，被动响应极有可能参与小鼠后腿的运动过程，但其幅度甚至响应方向（伸/屈）可能因个体小鼠而异。