Summary Aging is characterized by a progressive loss of muscle mass and impaired contractility (e.g., decline in force, velocity, and power). Although the slowing of contraction speed in aging muscle is well described, the underlying molecular mechanisms responsible for the decrement in speed are unknown. Myosin heavy chain (MHC) isoforms are the primary molecules determining contractile velocity; however, the contraction speed of single fibers within a given MHC isoform type is variable. Recent evidence proposes that the decline in shortening velocity (Vo) with aging is associated with a decrease in the relative content of essential myosin light chain 3f (MLC3f) isoform. In the current study, we first evaluated the relative content of MLC3f isoform and Vo in adult and old rats. We then used recombinant adenovirus (rAd) gene transfer technology to increase MLC3f protein content in the MHC type II semimembranosus muscle (SM). We hypothesized that (i) aging would decrease the relative MLC3f content and Vo in type II fibers, and (ii) increasing the MLC3f content would restore the age-induced decline in Vo. We found that there was an age-related decrement in relative MLC3f content and Vo in MHC type II fibers. Increasing MLC3f content, as indicated by greater % MLC3f and MLC3f/MLC2f ratio, provided significant protection against age-induced decline in Vo without influencing fiber diameter, force generation, MHC isoform distribution, or causing cellular damage. To the best of our knowledge, these are the first data to demonstrate positive effects of MLC3f against slowing of contractile function in aged skeletal muscle.

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The purpose of this study was to examine how non‐weightbearing (NWB) alters power generation of myosin heavy chain (MHC) IIB, IIXB, and IIX fibers from the plantaris (PL), tibialis anterior (TA), extensor digitorum longus (EDL) muscles and MHCI fibers from the soleus (SOL) muscle. NWB was achieved in rats by the hindlimb suspension technique (1 to 3 weeks). Single fiber power, force, and velocity were determined from force‐velocity curves and MHC isoforms were identified by SDS‐PAGE (total 387 fibers). Peak power (PP), peak force (Po) and velocity (Vmax) of MHCIIB and IIXB fibers from the TA and EDL did not change with NWB. In contrast, PP and Po of MHCII fibers from the PL were significantly reduced by 25–60% with NWB. Because NWB‐induced atrophy (3–23%) was present in the MHCII fibers from the PL, normalized power (NP) and specific force (SF) did not change with NWB. The contractile parameters (PP, NP, Po, SF) of MHCI fibers from the SOL were significantly reduced with NWB by 30–69%. The results indicate that the contractile properties of MHCII fibers from the TA and EDL, both ankle dorsiflexor muscles, are maintained during NWB. However, the contractile properties of MHCII fibers from the PL, an ankle plantarflexor, show NWB‐induced reductions. Collectively, MHC type II fibers adapt to NWB; however, the extent of adaptation is muscle‐specific. These findings are important when designing therapeutic exercise interventions.