Differential Modulation of Motor Unit Properties from the Separate Components of the Triceps Surae in Humans

The triceps surae is comprised of the soleus, and medial (MG) and lateral (LG) gastrocnemii. Modulation of triceps surae motor units (MUs) is context- and muscle-dependent, yet it is unknown how the disparate components of the triceps surae work together to achieve the common goal of high-intensity voluntary isometric plantar flexion torque gradation. Thus, the purpose was to assess the interrelationships between MU recruitment thresholds (MURTs) and MU discharge rates (MUDRs) among these three muscles during contractions from low to high intensities. We sampled 157 MU action potential trains from the MG (68), LG (38) and soleus (51) using fine-wire intramuscular electromyography (EMG) during voluntary ramp isometric contractions up to 100% maximal voluntary contraction (MVC). The soleus exhibited 41% and 54% lower MURTs compared to the MG (p < 0.0001) and LG (p < 0.0001), respectively, whereas MG MURTs were 22% lower than the LG (p < 0.0001). Initial MUDRs were 35% and 26% greater for the LG compared with the MG (p < 0.0001) and soleus (p < 0.0001), but no difference was detected between the MG and soleus (p = 0.28). Finally, initial MUDRs displayed a positive relationship with MURTs for each independent triceps surae component (p ≤ 0.002). The relative differences in MU properties of each muscle in this synergistic group illustrate that MU control strategies are likely optimized with respect to the relative contribution of each muscle to plantar flexion torque or functional roles.