Compound group I excitatory input is differentially distributed to human soleus motoneurons

ObjectiveWe studied whether the distribution of synaptic input from compound group I afferents onto the various-sized motoneurons in the human soleus muscle supports the size principle.MethodsThe subject lay prone on a physiotherapy table and electrical stimuli were delivered to the tibial nerve. The recordings were taken with surface electromyography (SEMG) and single motor unit (SMU) potentials. The relative sizes of SMUs were estimated using four different methods. After identifying the relative size of each SMU of the pair, normalised size of the H-reflex was determined using the extra spike per trigger (ESPT) method.ResultsIn total 33 SMU pairs were studied to compare results obtained in each pair. It was found that, although the stimulus intensity was identical for each pair, the ESPT values were statistically larger in the bigger SMUs compared with the relatively smaller SMUs (p < 0.05).ConclusionsWe conclude that, within the limits of this study, compound group I excitatory input to soleus motoneurons in human subjects does not support the size principle which governs the recruitment order of motoneurons in the reduced animal preparations.SignificanceThis study illustrates the importance of performing human experiments to confirm or reject principles obtained using reduced animal preparations.

► We studied whether the distribution of group I excitatory input to human soleus motoneurons supports the size principle of motor unit recruitment. ► Size of the H-reflex response was compared in simultaneously recorded motor units. ► We found that the size of the H-reflex is bigger in larger cells, which does not support the development of orderly recruitment of motor units in human soleus muscle.