Motor unit properties of nerve-intact extensor digitorum longus muscle grafts in young and old rats.

Impaired reinnervation has been implicated as the cause of the threefold disparity in the recovery of maximum force (P0) of standard muscle grafts in old compared with young rats. The specific, null hypothesis of this study is that compared with age-matched control extensor digitorum longus (EDL) muscles, nerve-intact EDL muscle grafts in young and old rats show no evidence of an age-related impairment in reinnervation. Nerve-intact grafts were performed in 3-month-old and 23-month-old rats and were evaluated 60 days postoperatively. Compared with age-matched control EDL muscles, nerve-intact grafts in young and old rats showed no difference in muscle mass or motor unit numbers. The mean motor unit P0 for nerve-intact graft muscles in both age groups was significantly lower than that of age-matched control muscles. These data support our hypothesis that if axons are allowed to regenerate in an endoneurial environment, there is no evidence of an age-related impairment in muscle reinnervation.