Two Motor Neuron Synergies, Invariant Across Ankle Joint Angles, Activate the Triceps Surae During Plantarflexion

Recent studies have suggested that the nervous system generates movements by controlling groups of motor neurons (synergies) that do not always align with muscle anatomy. In this study, we determined whether these synergies are robust across tasks with different mechanical constraints. We identified motor neuron synergies using principal component analysis (PCA) and cross-correlations between smoothed discharge rates of motor neurons. In part 1, we used simulations to validate these methods. The results suggested that PCA can accurately identify the number of common inputs and their distribution across active motor neurons. Moreover, the results confirmed that cross-correlation can separate pairs of motor neurons that receive common inputs from those that do not receive common inputs. In part 2, 16 individuals performed plantarflexion at three ankle angles while we recorded EMG signals from the gastrocnemius lateralis (GL) and medialis (GM) and the soleus (SOL) with grids of surface electrodes. The PCA revealed two motor neuron synergies. These motor neuron synergies were relatively stable, with no significant differences in the distribution of motor neuron weights across ankle angles (P = 0.62). When the cross-correlation was calculated for pairs of motor units tracked across ankle angles, we observed that only 13.0% of pairs of motor units from GL and GM exhibited significant correlations of their smoothed discharge rates across angles, confirming the low level of common inputs between these muscles. Overall, these results highlight the modularity of movement control at the motor neuron level, suggesting a sensible reduction of computational resources for movement control.imageKey pointsThe CNS might generate movements by activating groups of motor neurons (synergies) with common inputs.We show here that two main sources of common inputs drive the motor neurons innervating the triceps surae muscles during isometric ankle plantarflexions.We report that the distribution of these common inputs is globally invariant despite changing the mechanical constraints of the tasks, i.e. the ankle angle.These results suggest the functional relevance of the modular organization of the CNS to control movements. Abstract figure legend Here, we aimed to test the robustness of motor neuron synergies across isometric plantarflexion tasks performed at different ankle angles. We found with simulations that our methods, i.e. principal component analysis and correlation of smoothed discharge rates, can accurately identify the number of common inputs and whether pairs of motor neurons receive or do not receive a significant proportion of common inputs. Thus, we identified two main common inputs driving motor neurons innervating the gastrocnemius lateralis (GL), gastrocnemius medialis (GM) and soleus (SOL), with no significant systematic changes in the distribution of weights across ankle angles. We also found that a low percentage of pairs of motor units from GL and GM received a significant proportion of common inputs. Key points The CNS might generate movements by activating groups of motor neurons (synergies) with common inputs. We show here that two main sources of common inputs drive the motor neurons innervating the triceps surae muscles during isometric ankle plantarflexions. We report that the distribution of these common inputs is globally invariant despite changing the mechanical constraints of the tasks, i.e. the ankle angle. These results suggest the functional relevance of the modular organization of the CNS to control movements.