Loram Ian D, Lakie Martin
Applied Physiology Research Group, School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK.
J Physiol. 2002 Dec 15;545(3):1041-53. doi: 10.1113/jphysiol.2002.025049.
During quiet standing the human "inverted pendulum" sways irregularly. In previous work where subjects balanced a real inverted pendulum, we investigated what contribution the intrinsic mechanical ankle stiffness makes to achieve stability. Using the results of a plausible model, we suggested that intrinsic ankle stiffness is inadequate for providing stability. Here, using a piezo-electric translator we applied small, unobtrusive mechanical perturbations to the foot while the subject was standing freely. These short duration perturbations had a similar size and velocity to movements which occur naturally during quiet standing, and they produced no evidence of any stretch reflex response in soleus, or gastrocnemius. Direct measurement confirms our earlier conclusion; intrinsic ankle stiffness is not quite sufficient to stabilise the body or pendulum. On average the directly determined intrinsic stiffness is 91 +/- 23 % (mean +/- S.D.) of that necessary to provide minimal stabilisation. The stiffness was substantially constant, increasing only slightly with ankle torque. This stiffness cannot be neurally regulated in quiet standing. Thus we attribute this stiffness to the foot, Achilles' tendon and aponeurosis rather than the activated calf muscle fibres. Our measurements suggest that the triceps surae muscles maintain balance via a spring-like element which is itself too compliant to guarantee stability. The implication is that the brain cannot set ankle stiffness and then ignore the control task because additional modulation of torque is required to maintain balance. We suggest that the triceps surae muscles maintain balance by predictively controlling the proximal offset of the spring-like element in a ballistic-like manner.
在安静站立时，人体“倒立摆”会不规则地摆动。在之前让受试者平衡一个真正的倒立摆的研究中，我们探究了内在机械性踝关节刚度对实现稳定性的贡献。利用一个合理模型的结果，我们认为内在踝关节刚度不足以提供稳定性。在此，我们使用一个压电转换器，在受试者自由站立时对其足部施加微小、不显眼的机械扰动。这些短时长的扰动在大小和速度上与安静站立时自然发生的动作相似，并且未产生比目鱼肌或腓肠肌有任何牵张反射反应的证据。直接测量证实了我们早期的结论；内在踝关节刚度不足以稳定身体或倒立摆。平均而言，直接测定的内在刚度为提供最小稳定性所需刚度的91±23%（平均值±标准差）。刚度基本恒定，仅随踝关节扭矩略有增加。在安静站立时，这种刚度无法通过神经调节。因此，我们将这种刚度归因于足部、跟腱和腱膜，而非被激活的小腿肌肉纤维。我们的测量表明，小腿三头肌通过一个本身过于柔顺而无法保证稳定性的弹簧状元件来维持平衡。这意味着大脑不能设定踝关节刚度然后忽略控制任务，因为需要额外调节扭矩来维持平衡。我们认为小腿三头肌通过以类似弹道的方式预测性地控制弹簧状元件的近端偏移来维持平衡。
