Mechanical load affects growth and maturation of skeletal muscle grafts

The purpose of our study was to determine whether the early patterns of growth and maturation of regenerating soleus muscle grafts are sensitive to alterations in mechanical load. We hypothesized that decreased and increased mechanical loading of grafts would reduce and accelerate, respectively, the rate and magnitude of growth and impair and enhance, respectively, the pattern of maturation. On day 0, soleus muscles were grafted and rats were assigned to one of three groups: cage sedentary (normal load), hindlimb suspension (decreased load), or ablation of synergist muscle (increased load). From days 7 to 35, graft mass in cage-sedentary rats increased at a rate of 1.85 mg mass/day. Rates were less for grafts of suspended rats and greater in grafts of ablated rats (-1.06 and 3.89 mg mass/day, respectively; P < 0.01). Neonatal myosin heavy chain (MHC) in grafts reached 10 +/- 1.6% of total MHC at day 7 for cage-sedentary rats, whereas in the suspended animals it reached 11 +/- 2.4% of total MHC at day 14. At days 21 and 35, grafts from the suspended animals had a lower proportion of slow MHC (45 +/- 2.4%) than did grafts from the control and ablated groups (95 +/- 1.5%; P < 0.05). Decreased mechanical load impaired the rate and degree of growth and maturation during regeneration, whereas increased mechanical load enhanced growth characteristics but not maturation.