The effect of reinnervation on force production and power output in skeletal muscle

Failure to fully restore contractile function after denervation and reinner vation of skeletal muscle engenders significant disability in patients suff ering peripheral nerve injuries. This work tested the hypothesis that skele tal muscle denervation and reinnervation result in a deficit in normalized power (W/kg), which exceeds the deficit in specific force (N/cm(2)), and th at the mechanisms responsible for these deficits are independent. Adult Lew is rats underwent either transection and epineurial repair of the left pero neal nerve (denervation-reinnervation, n = 13) or SHAM: exposure of the per oneal nerve (SHAM, n = 13). After a 4-month recovery period, isometric forc e, peak power, and maximum sustained power output were measured in the left extensor digitorum longus (EDL) muscle from each animal. Isometric force m easurements revealed a specific force deficit of 14.3% in the reinnervated muscles. Power measurements during isovelocity shortening contractions demo nstrated a normalized peak power deficit of 25.8% in the reinnervated muscl es, which is accounted for by decreases in both optimal velocity (10.5%) an d average force curing shortening (13.7%). Maximum sustained power was simi lar in both groups. These data support our working hypothesis that both who le muscle force production and power output can be impaired in reinnervated muscle and that the relative deficits in power output exceed the deficits in force production. The mechanisms responsible for the deficits in force p roduction appear to be independent of those that result in changes in peak power output. The measurement of muscle power output may represent a clinic ally relevant variable for studies of the recovery of mechanical function a fter motor nerve injury and repair. (C) 1999 Academic Press.