Effect of ankle joint position on triceps surae contractile properties and motor unit discharge rates
Abstract
The length-tension relationship of the triceps surae (TS) can be altered by changing the knee joint position, ankle joint position or both. However, studies exploring the effect of muscle length on triceps surae (TS) neuromuscular properties have focused only on changes in knee joint position, affecting only 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, motor unit discharge rates (MUDRs) of the soleus, medial (MG) and lateral (LG) gastrocnemii were sampled during plantar flexion contractions at 25, 50, 75 and 100% MVC using indwelling tungsten electrodes. Peak twitch torque and MVC were lower by ~70% and 61%, respectively, whereas 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 at 100% MVC both muscles had 9% and 20% higher rates in the DF position, respectively.