Changes of activity patterns in slow and fast muscles during postnatal development

Abstract The motor activity patterns of the slow soleus and the fast extensor digitorum longus (EDL) muscles of the rat were studied by electromyography (EMG) in the awake unrestrained animal during postnatal development. In the adult soleus, tonic, low frequency activity of motor units was recorded at rest and during movement. In animals less than 12 days old the muscle was activated phasically, the motor unit firing frequency was slightly lower than in the adult, and tonic discharges were absent at rest. After this time, tonic postural activity gradually developed and resembled the adult pattern by 3 weeks of age. The adult EDL muscle was silent at rest and was activated phasically at high frequencies during movement. In animals of less than 12 days of age the muscle was also activated phasically but at lower frequencies. The firing frequency increased to adult values during the second and third week of postnatal development. The discharge properties of motor units from soleus and EDL were compared in rats of different ages. There was a greater variability of interspike intervals during firing in motor units from very young animals than in adults. Whereas in adult animals it was possible to discern a graded recruitment of motor units, animals less than 10 days old showed no regular recruitment order and some motor units appeared to be activated synchronously.