A specific force deficit exists in skeletal muscle after partial denervation

Skeletal muscle demonstrates a specific force deficit after repair of injured peripheral nerves, microneurovascular muscle transfer, and normal aging. Because atrophy cannot account for deficits in specific force, other, unknown, mechanisms are responsible for the resulting muscle contractile dysfunction under these circumstances. We tested the hypothesis that a subpopulation of denervated fibers is partially or completely responsible for the specific force deficit after partial denervation of the rat extensor digitorum longus muscle (EDL). Adult Fisher rats underwent either sham exposure or partial transection of 80% of the cross-sectional area of the left deep peroneal nerve. After a 2-week recovery period, maximum isometric force (F(0)) was measured in situ and maximum specific force (sF(0)) was calculated for EDL from both control (n = 8) and partial denervation (n = 7) groups. Innervated fiber cross-sectional area (CSA(inn)) was measured directly from whole EDL cross sections after immunohistochemical labeling for neural cell adhesion molecule (NCAM), a marker of muscle fiber denervation. A corrected specific force value (sF(0-inn)) was calculated by normalizing F(0) to CSA(inn). Partial skeletal muscle denervation resulted in significant reductions in muscle mass, F(0), and sF(0). The percentage of muscle fibers expressing NCAM in the extrajunctional sarcolemma increased from 1.0 +/- 0.8% in control to 49 +/- 15% in partially denervated EDL muscles. A 62.7% deficit in EDL specific force was observed after partial denervation. Denervated muscle fibers accounted for 59.3% of this deficit, but sF(0-inn) still differed significantly between control and partially denervated muscles, with a 25.5% difference between groups. In partially denervated muscles, the specific force deficit is partially but not fully explained by a subpopulation of noncontractile, denervated fibers.