Grafting autologous cultured satellite cells in irreversibly injured rat extensor digitorum longus EDL muscle leads to myofiber regeneration at the grafting site. In this study, we investigated whether cell grafts induced functional improvement and correlated mechanophysiological findings with histological observations. In cell grafted muscles, the number of myofibers did not differ significantly between 2 wk and 3 mo, whereas no regenerating myofibers were observed in ungrafted controls. During this period, the total number of myofibers in the cell grafted muscles represented 48.2-51.9% of that in normal muscles. The mean diameter of regenerated myofibers increased with time, reaching a maximum (32 microns) at the second mo and remained smaller than that of normal myofibers (47 microns). Muscle function was measured by mechanophysiological recordings of muscle response to supramaximal electrical stimulation of the nerve in situ. Cell grafted muscles exhibited a progressive improvement of all contractile parameters. After 3 mo, a 4-fold increase in absolute values of twitch and tetanic tension outputs was measured in cell grafted muscles when compared to ungrafted controls. However, these parameters remained much lower than in normal muscles (23.4% and 22.3% of control, respectively). This study showed that myogenic cell grafts replace degenerated myofibers and form functional myofibers. Functional improvement observed, between 2 wk and 3 mo after cell grafting, correlated with the development, differentiation, and maturation of the regenerated myofibers rather than with an increase in the number of regenerated myofibers.