The fiber-type-specific effect of inactivity and intermittent weight- bearing on the gastrocnemius muscle of 30-month-old rats

Objective: To characterize single skeletal muscle fiber contractile properties from the gastrocnemius muscle that occur during inactivity and intermittent weight-bearing in 30-month-old animals. Design: Randomized control trial. Setting: A controlled laboratory environment. Subjects: Eighteen 30-month-old male Fisher 344 Brown Norway F1 Hybrid rats were randomly assigned to control (C), hindlimb unweighted (HU), and hindlimb unweighted with intermittent weight-bearing (HU-X) groups. Interventions: The HU and HU-X rats were suspended for 7 days. The HU-X animals were unsuspended for four 15-minute bouts of weight-bearing. Main Outcome Measures: Single skeletal muscle fiber contractile properties (diameter, peak active force [P₀], peak specific tension [P₀/CSA], and maximal shortening velocity [V₀] by fiber type) were determined from the deep portion of the lateral head of the gastrocnemius muscle (RG). Results: In comparison to C animals, the ratio of gastrocnemius weight to body weight decreased by 18% and 14% following HU and HU-X, respectively. Diameter and P₀ of type I fibers from the RG were reduced after HU. Attenuation of the decline in diameter and P₀ was observed in type I fibers from the RG with HU-X. P₀ was reduced in type IIa fibers and type I-IIa fibers with HU. Attenuation of the decline in P₀ by intermittent weight-bearing in type IIa fibers and type I-IIa fibers did not occur. Conclusions: Inactivity altered the contractile properties of single skeletal muscle fibers from the gastrocnemius muscle of 30-month-old animals. The inactivity-induced alterations were present in the three fiber types. Therapeutic intervention of weight-bearing attenuated the inactivity-induced changes in the type I fibers from the gastrocnemius, but not the other fiber types.