Myosin content of single muscle fibers following short-term disuse and active recovery in young and old healthy men

Short-term disuse and subsequent recovery affect whole muscle and single myofiber contractile function in
young and old. While the loss and recovery of singlemyofiber specific force (SF) following disuse and rehabilitation
has been shown to correlatewith alterations inmyosin concentrations in young, it is unknownwhether similar
relationships exist in old. Therefore, the purpose of the present study was to examine the effect of 14 days
lower limb disuse followed by 28 days of active recovery on single muscle fiber myosin content in old (68 yrs)
and young (24 yrs) recreationally physically active healthy men. Following disuse, myosin content decreased
(p b 0.05) in MHC 1 (young −28%, old −19%) and 2a fibers (young −23%, old −32%). In old, myosin content
decreased more (p b 0.05) in MHC 2a vs 1 fibers. Following recovery, myosin content increased (p b 0.05) and
returned to pre-disuse levels for both young and old in both fiber types, with MHC 2a fibers demonstrating an
overshooting in young (+31%, p b 0.05) but not old. Strong correlationswere observed between myosin content
and single fiber SF in both young and old, with greater slope steepness in MHC 2a vs 1 fibers indicating an enhanced
intrinsic contractile capacity of MHC 2a fibers. In conclusion, adaptive changes in myofiber myosin content
appear to occur rapidly following brief periods of disuse (2 wks) and after subsequent active recovery
(4 wks) in young and old, which contribute to alterations in contractile function at the single muscle fiber
level. Changes inmyosin content appear to occur independently of age, while influenced by fiber type (MHC isoform)
in young but not old.