Swing Velocity Profiles of Small Limbs Can Arise from Transient Passive Torques of the Antagonist Muscle Alone

In large limbs, changing motor neuron activity typically controls within-movement velocity. For example, sequential agonist-antagonist-agonist motor neuron firing typically underlies the slowing often present at the end of human reaches. In physiological movements of large limbs, antagonistic muscle passive torque is generally negligible. In small limbs, alternatively, passive torques can determine limb rest position, generate restoring movements to it, and decrease agonist-generated movement amplitude and velocity maxima. These observations suggest that in small limbs passive forces might also control velocity changes within movements. We investigated this issue in stick insect middle leg femur-tibia (FT) joint. During swing, the FT joint extensor muscle actively shortens and the flexor muscle passively lengthens. As in human reaching, after its initial acceleration, FT joint velocity continuously decreases. We measured flexor passive forces during imposed stretches spanning the ranges of FT joint angles, angular velocities, and movement amplitudes present in leg swings. The viscoelastic “transient” passive force that occurs during and soon after stretch depended on all three variables, and could be tens of times larger than the “steady-state” passive force commonly measured long after stretch end. We combined these data, the flexor and extensor moment arms, and an existing extensor model to simulate FT joint swing. To measure only passive (flexor) muscle-dependent effects, we used constant extensor activations in these simulations. In simulations using data from ten flexor muscles, flexor passive torque could always produce swings with, after swing initiation, continuously decreasing velocities. Antagonist muscle passive torques alone can thus control within-movement velocity.在大型肢体中，运动神经元活动的改变通常调控运动过程中的速度变化。例如，主动肌（agonist）-拮抗肌（antagonist）-主动肌依次放电的运动神经元活动模式，通常是人类伸手动作末期常见的速度减慢现象的成因。在大型肢体的生理性运动中，拮抗肌的被动扭矩通常可忽略不计。与之相反，在小型肢体中，被动扭矩可决定肢体的静息位置，产生回归该位置的复位运动，并降低主动肌所产生的运动幅度与最大速度。上述观察结果表明，在小型肢体中，被动力或许也能调控运动过程中的速度变化。我们以竹节虫中足的股胫（FT）关节为对象开展了相关研究。在摆动阶段，FT关节的伸肌（extensor）会主动缩短，而屈肌（flexor）则被动拉长。与人类伸手动作类似，FT关节的速度在初始加速后会持续下降。我们对腿部摆动过程中覆盖FT关节角度、角速度与运动幅度范围的被动拉伸过程中的屈肌被动力进行了测量。在拉伸过程中及拉伸结束后短期内出现的粘弹性“瞬态（transient）”被动力，取决于上述三个变量，其大小可比拉伸结束许久后通常测得的“稳态（steady-state）”被动力高出数十倍。我们将上述实验数据、屈肌与伸肌的肌力臂（moment arms），以及已有的伸肌模型相结合，对FT关节的摆动过程进行了模拟。为仅观测与被动（屈肌）相关的效应，我们在这些模拟中采用了恒定的伸肌激活水平。在使用10条屈肌的实验数据进行的模拟中，屈肌被动扭矩始终能够产生摆动起始后速度持续下降的摆动过程。由此可见，仅依靠拮抗肌的被动扭矩即可调控运动过程中的速度变化。