Sway-dependent changes in standing ankle stiffness caused by muscle thixotropy.
Sway-dependent changes in standing ankle stiffness caused by muscle thixotropy.
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DOI：
10.1113/jp271137
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发表时间：
2016-02-01
期刊：
影响因子：
--
通讯作者：
Reynolds RF
中科院分区：
文献类型：
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作者：
Sakanaka TE;Lakie M;Reynolds RF
小腿肌肉的被动僵硬有助于站立平衡，尽管肌肉组织的特性是高度不稳定的。我们调查了摆动历史对踝关节固有刚度的影响，并证明在基线摆动增加的情况下，刚度降低高达43%。当使用低幅度刚度测量扰动时，这种摇摆依赖性最为明显，而在高摇摆期间，短距离刚度分量较小。这些特征与引起观察到的踝关节僵硬变化的小腿肌肉触变特性一致。增加的摇摆期损害了站立的被动稳定，需要更主动的神经平衡控制。安静站立是通过主动和被动机制的结合来实现的，分别由神经控制和踝关节固有的机械刚度组成。机械刚度部分是由小腿肌肉决定的。然而，肌肉的粘弹性是高度不稳定的，表现出对运动历史的强烈依赖。通过测量摇摆历史对踝关节刚度的影响，本研究确定这种不稳定性是否对人类站立的被动稳定有影响。10名受试者安静地站在旋转平台上，旋转平台的轴线与踝关节共线。通过以随机方式缓慢倾斜这个平台来增加脚踝的摆动，或者通过将身体固定在一块板上来减少脚踝的摆动。踝关节刚度通过使用相同的平台测量，同时施加小而短暂的扰动（<0.6度；140毫秒），同时记录产生的扭矩响应。结果表明，与身体固定的情况相比，增加摆动可以减少多达43%的踝关节僵硬。正常的安静姿态与中间值相关。当使用较小的扰动幅度来测量刚度（0.1对0.6度）时，效果最为明显。此外，扭矩响应呈现双相模式，包括最初的急剧上升，然后是较浅的增加。这种转变在踝关节摆动水平增加时发生得更早。这些结果与由小腿肌肉触变特性引起的被动踝关节僵硬的运动依赖性变化一致。结果是增加依赖于主动神经控制的时候，增加的摆动使踝关节僵硬低。小腿肌肉的被动僵硬有助于站立平衡，尽管肌肉组织的特性是高度不稳定的。我们调查了摆动历史对踝关节固有刚度的影响，并证明在基线摆动增加的情况下，刚度降低高达43%。当使用低幅度刚度测量扰动时，这种摇摆依赖性最为明显，而在高摇摆期间，短距离刚度分量较小。这些特征与引起观察到的踝关节僵硬变化的小腿肌肉触变特性一致。增加的摇摆期损害了站立的被动稳定，需要更主动的神经平衡控制。
The passive stiffness of the calf muscles contributes to standing balance, although the properties of muscle tissue are highly labile. We investigated the effect of sway history upon intrinsic ankle stiffness and demonstrated reductions in stiffness of up to 43% during conditions of increased baseline sway. This sway dependence was most apparent when using low amplitude stiffness‐measuring perturbations, and the short‐range stiffness component was smaller during periods of high sway. These characteristics are consistent with the thixotropic properties of the calf muscles causing the observed changes in ankle stiffness. Periods of increased sway impair the passive stabilization of standing, demanding more active neural control of balance. Quiet standing is achieved through a combination of active and passive mechanisms, consisting of neural control and intrinsic mechanical stiffness of the ankle joint, respectively. The mechanical stiffness is partly determined by the calf muscles. However, the viscoelastic properties of muscle are highly labile, exhibiting a strong dependence on movement history. By measuring the effect of sway history upon ankle stiffness, the present study determines whether this lability has consequences for the passive stabilization of human standing. Ten subjects stood quietly on a rotating platform whose axis was collinear with the ankle joint. Ankle sway was increased by slowly tilting this platform in a random fashion, or decreased by fixing the body to a board. Ankle stiffness was measured by using the same platform to simultaneously apply small, brief perturbations (<0.6 deg; 140 ms) at the same time as the resulting torque response was recorded. The results show that increasing sway reduces ankle stiffness by up to 43% compared to the body‐fixed condition. Normal quiet stance was associated with intermediate values. The effect was most apparent when using smaller perturbation amplitudes to measure stiffness (0.1 vs. 0.6 deg). Furthermore, torque responses exhibited a biphasic pattern, consisting of an initial steep rise followed by a shallower increase. This transition occurred earlier during increased levels of ankle sway. These results are consistent with a movement‐dependent change in passive ankle stiffness caused by thixotropic properties of the calf muscle. The consequence is to place increased reliance upon active neural control during times when increased sway renders ankle stiffness low. The passive stiffness of the calf muscles contributes to standing balance, although the properties of muscle tissue are highly labile. We investigated the effect of sway history upon intrinsic ankle stiffness and demonstrated reductions in stiffness of up to 43% during conditions of increased baseline sway. This sway dependence was most apparent when using low amplitude stiffness‐measuring perturbations, and the short‐range stiffness component was smaller during periods of high sway. These characteristics are consistent with the thixotropic properties of the calf muscles causing the observed changes in ankle stiffness. Periods of increased sway impair the passive stabilization of standing, demanding more active neural control of balance.