Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers

We tested the hypothesis that denervated muscle fibers account for part of the specific force (sF(0)) deficit observed in muscles from old adult (OA) mammals, Whole muscle force (F-0) was quantified for extensor digitorum lon gus (EDL) muscles of OA and young adult (YA) rats. EDL muscle sF(0) was cal culated by dividing F-0 by either total muscle fiber cross-sectional area ( CSA) or by innervated fiber CSA. Innervated fiber CSA was estimated from ED L muscle cross sections labeled for neural cell adhesion molecules, whose p resence is a marker for muscle fiber denervation. EDL muscles from OA rats contained significantly more denervated fibers than muscles from YA rats (5 .6% vs 1.1% of total CSA), When compared with YA muscle, oil muscle demonst rated deficits of 34.1% for F-0, 28.3% for sF(0), and 24.9 % for sF(0) calc ulated by using innervated CSA as the denominator, Denervated muscle fibers accounted for 11.3% of the specific force difference between normal YA and OA skeletal muscle, Other mechanisms in addition to denervation account fo r the majority of the sF(0) deficit with aging.