Abstract
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.
Highlights
The length-tension relationship indicates that the maximal voluntary isometric force produced by a muscle depends on the length of the muscle (Gordon et al 1966)
Despite no difference in voluntary activation between the DF and plantar flexed (PF) ankle joint positions (p = 0.33), plantar flexion maximal voluntary contractions (MVCs) torque was 61% lower in the PF compared with the DF position (p < 0.001)
Half relaxation times (HRT) were ~37% slower in the DF compared with the PF ankle joint positions (p < 0.001), whereas time-to-peak torque (TPT) was ~5% slower in the DF compared with the PF position (p = 0.16)
Summary
The length-tension relationship indicates that the maximal voluntary isometric force produced by a muscle depends on the length of the muscle (Gordon et al 1966). Submaximal MUDRs were higher when the muscle was shortened (Vander Linden 1991; Pasquet et al, 2005) whereas maximal MUDRs at maximal voluntary contraction forces showed no difference between muscle lengths (Bigland-Ritchie et al, 1992). In the hamstrings, both submaximal and maximal MUDRs were higher in the shortened compared with a lengthened position (Kirk & Rice, 2017).
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