Aging effects in skeletal muscle recovery after reinnervation

To investigate the influence of age on the process of muscle recovery after nerve repair, the nerves of the right extensor digitorum longus (EDL) and the right soleus muscles of 63 2-month-old and 61 15-month-old rats, respectively, were transsected and resutured. At four, eight, 16, and 24 weeks after nerve repair, functional recovery of the muscle was assessed by electromyographic (EMG) recordings and isometric muscle contraction. Muscle weight, morphological, and morphometric studies were also done. At four and eight weeks after nerve repair the younger age groups showed higher rates of recovery compared with the control side (left EDL and soleus) (recovery rate (%) = operated/control x 100) than the older age groups, and the recovery rates of soleus (slow twitch muscle) in both age groups were higher than EDL (fast twitch muscle). However, the differences between the two age groups decreased at 16 and 24 weeks after nerve repair in both muscles. We therefore conclude that earlier differences were the effects of nerve regeneration on the muscle between different ages and the reason for the reduced differences at later stages was that after reinnervation began, not only the nerve but also the muscle recovered successfully in older age groups, and slow motor units reinnervated faster than the fast units in both age groups. Our present study shows that the older age groups also have a good prognosis for recovery of muscle function after nerve repair.