Relationships between hand skill and the excitability of motoneurons innervating the postural soleus muscle in human subjects.

The relation of hand skill to the excitability of motoneurons innervating the postural soleus muscle was studied in normal human subjects. The motoneuronal excitability was tested by the recovery curve of the Hoffmann reflex. The hand skill was assessed by the peg moving test. Females have been found to be better than males in hand skill. It was established that the H response recovery curve from the right leg was significantly lower than that from the left leg in right handed subjects. The opposite was found in left handers. In ambidexters, there was no significant difference between the heights of the left and right recovery curves. The laterality quotients of the H response recovery curves were normally distributed in the total sample with a mean significantly less than zero, indicating a left shift bais in the motoneuronal excitability. The possible factors contributing to the inverse relationship between hand skill and the excitability of motoneurons innervating the postural soleus muscle were discussed in light of genetic determinants of brainedness and the corticospinal inputs to postural motoneurons.