Faculty of Health Sciences, School of Kinesiology, The University of Western Ontario, London, ON, Canada.
Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, ON, Canada.
Physiol Rep. 2021 Jan;8(24):e14680. doi: 10.14814/phy2.14680.
The triceps surae (TS) length-tension relationship can be altered by changing the knee joint position, ankle joint position or both. However, studies exploring the effect of muscle length on neuromuscular properties have focused only on knee joint position changes affecting two of the three muscle components of the TS. Thus, the purpose of this study is to compare the neuromuscular properties of the three TS muscles during plantar flexion contractions at two ankle joint positions, 20° dorsiflexed (DF) and 20° plantar flexed (PF). Maximal isometric voluntary strength (MVC), voluntary activation, and evoked contractile properties of the ankle plantar flexors were compared between both ankle joint positions. Additionally, soleus, medial (MG), and lateral (LG) gastrocnemii motor unit discharge rates (MUDRs) were sampled during plantar flexion contractions at 25%, 50%, 75%, and 100% MVC using indwelling tungsten electrodes. MVC and peak twitch torque were lower by 61% and 70%, respectively, whereas the maximal rate of torque relaxation was 39% faster in the PF compared with the DF position. Voluntary activation (95%) was unaffected by changes in ankle joint position. LG MUDRs showed no differences between ankle joint positions, regardless of contraction intensity. Submaximal MG and soleus MUDRs showed no differences between the two ankle joint positions, however both muscles had 9% and 20% higher MUDRs in the DF position, respectively. These results provide further evidence for the differential activation among the three components of the TS with the greatest increases in soleus MUDRs compared with the gastrocnemii when the muscles are lengthened.
比目鱼肌（TS）的长度-张力关系可以通过改变膝关节位置、踝关节位置或两者同时改变来改变。然而，研究肌肉长度对神经肌肉特性影响的研究仅集中在改变膝关节位置对 TS 的三个肌肉成分中的两个的影响上。因此，本研究的目的是比较在两种踝关节位置（20°背屈（DF）和 20°跖屈（PF））下进行跖屈收缩时 TS 的三个肌肉的神经肌肉特性。在两种踝关节位置下，比较了最大等长自愿收缩力（MVC）、自愿激活和踝跖屈肌的诱发收缩特性。此外，在 25%、50%、75%和 100% MVC 下，使用植入钨电极在跖屈收缩期间采样比目鱼肌、内侧（MG）和外侧（LG）腓肠肌运动单位放电率（MUDR）。与 DF 位置相比，PF 位置的 MVC 和峰值单次收缩扭矩分别降低了约 61%和 70%，而最大扭矩松弛率快了 39%。自愿激活（~95%）不受踝关节位置变化的影响。LG MUDR 在踝关节位置之间没有差异，无论收缩强度如何。亚最大 MG 和比目鱼肌 MUDR 在两种踝关节位置之间没有差异，但在 DF 位置，这两块肌肉的 MUDR 分别增加了 9%和 20%。这些结果进一步证明了 TS 的三个成分之间的差异激活，当肌肉拉长时，比目鱼肌的 MUDR 增加最大，比腓肠肌增加 9%和 20%。
