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Motor Unit Properties of Nerve-Intact Extensor Digitorum Longus Muscle Grafts In Young and Old Rats

^a Institute of Gerontology and Departments of, University of Michigan Health Systems, Ann Arbor
^b Surgery, University of Michigan Health Systems, Ann Arbor
^c Cell and Developmental Biology, University of Michigan Health Systems, Ann Arbor
^d Biomedical Engineering, University of Michigan Health Systems, Ann Arbor
^e Physiology, University of Michigan Health Systems, Ann Arbor

Decision Editor: Edward Masoro, PhD

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.