Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada.
KITE, Toronto Rehabilitation Institute-University Health Network, Toronto, Ontario, Canada.
J Neurophysiol. 2020 Jun 1;123(6):2343-2354. doi: 10.1152/jn.00123.2020. Epub 2020 May 13.
Plantarflexors such as the soleus (SOL) and medial gastrocnemius (MG) play key roles in controlling bipedal stance; however, how the central nervous system controls the activation levels of these plantarflexors is not well understood. Here we investigated how the central nervous system controls the plantarflexors' activation level during quiet standing in a cosine tuning manner where the maximal activation is achieved in a preferred direction (PD). Furthermore, we investigated how spinal cord injury affects these plantarflexors' activations. Thirteen healthy adults (AB) and thirteen individuals with chronic, incomplete spinal cord injury (iSCI) performed quiet standing trials. Their body kinematics and kinetics as well as electromyography signals from the MG and SOL were recorded. In the AB group, we found that the plantarflexors followed the cosine tuning manner during quiet standing. That is, MG was most active when the ratio of plantarflexion torque to knee extension torque was ~2:-3, whereas SOL was most active when the ratio was ~2:1. This suggests that the SOL muscle, despite being a monoarticular muscle, is sensitive to both ankle plantarflexion and knee extension during quiet standing. The difference in the PDs accounts for the phasic activity of MG and for the tonic activity of SOL. Unlike the AB group, the MG's activity was similar to the SOL's activity in the iSCI group, and the SOL PDs were similar to those in the AB group. This result suggests that chronic iSCI affects the control strategy, i.e., cosine tuning, for MG, which may affect standing balance in individuals with iSCI. Soleus muscle shows a tonic activity whereas medial gastrocnemius muscle shows a phasic activity during quiet standing. Cosine tuning and their preferred direction account for the different muscle activation patterns between these two muscles. In individuals with chronic incomplete spinal cord injury, the preferred direction of gastrocnemius medial head is affected, which may result in their deteriorated standing balance.
跖屈肌（如比目鱼肌和腓肠肌内侧头）在控制双足站立中起着关键作用；然而，中枢神经系统如何控制这些跖屈肌的激活水平还不是很清楚。在这里，我们研究了中枢神经系统如何以余弦调谐方式控制跖屈肌在安静站立时的激活水平，在这种调谐方式中，最大激活发生在首选方向（PD）。此外，我们还研究了脊髓损伤如何影响这些跖屈肌的激活。13 名健康成年人（AB）和 13 名患有慢性不完全性脊髓损伤的个体（iSCI）进行了安静站立试验。记录了他们的身体运动学和动力学以及腓肠肌和比目鱼肌的肌电图信号。在 AB 组中，我们发现跖屈肌在安静站立时遵循余弦调谐方式。即，当跖屈力矩与膝伸力矩的比值约为 2:-3 时，腓肠肌最活跃，而当比值约为 2:1 时，比目鱼肌最活跃。这表明，尽管比目鱼肌是单关节肌，但在安静站立时，它对踝关节跖屈和膝关节伸展都很敏感。PD 的差异解释了腓肠肌的相性活动和比目鱼肌的紧张性活动。与 AB 组不同，iSCI 组中腓肠肌的活动与比目鱼肌的活动相似，比目鱼肌的 PD 与 AB 组相似。这一结果表明，慢性 iSCI 影响了 MG 的控制策略，即余弦调谐，这可能会影响 iSCI 患者的站立平衡。在安静站立时，比目鱼肌呈现紧张性活动，而腓肠肌内侧头呈现相性活动。余弦调谐及其首选方向解释了这两块肌肉之间不同的肌肉激活模式。在患有慢性不完全性脊髓损伤的个体中，腓肠肌内侧头的首选方向受到影响，这可能导致他们站立平衡恶化。
