Neuromuscular adaptability of male and female rats to muscle unloading

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Muscle unloading has been shown to elicit a number of negative neuromuscular adaptations, including diminished muscular strength and alterations in power and fatigability (Bamman et al., 1998; Berg, Dudley, Hather, & Tesch, 1993; Berg, Larsson, & Tesch, 1997; Dudley et al., 1992; Hackney, Cook, & Ploutz‐Snyder, 2011). These functional modifications are associated with muscular atrophy, both at the whole muscle and cellular levels (Adams, Hather, & Dudley, 1994; Deschenes, McCoy, & Mangis, 2012; Widrick et. al., 2002), as well as depressed electromyographic (EMG) activity of maximally contracting muscles, suggesting reductions of neural drive during maximal efforts (Deschenes, McCoy, Holdren, & Eason, 2009; Deschenes et. al., 2012). Clearly, both morphological and functional remodeling occurs in response to muscle unloading whether that condition is presented as bed rest, crutch‐assisted ambulation, limb immobilization, or the microgravity of spaceflight (Adams, 2002; Berg, Dudley, Haggmark, Ohlsen, & Tesch, 1991; Berg et. al., 1993; Deschenes et. al., 2009; Deschenes et. al., 2012; Hortobagyi et. al., 2000: Rozier, Elder, & Brown, 1979).
Recently, it has been reported that when humans are subjected to muscle unloading, females experience significantly greater decrements in neuromuscular function (i.e., strength), than their male counterparts even when the same model of muscle unloading is imparted, and for the same period of time (Deschenes et. al., 2012; Yasuda, Glover, Phillips, Isfort, & Tarnopolsky, 2005). This sex‐specific response to muscle unloading has stimulated additional interest in identifying the mechanism(s) that may account for the more severe reductions in neuromuscular function evident in females compared to males as a result of muscle unloading. Accordingly, the objective of the present investigation was to assess the morphological adaptations of male and female neuromuscular systems consequent to the same, relatively brief 2‐week period of muscle unloading imparted by the same model (hind limb suspension). Such sex‐specific structural remodeling of the neuromuscular system upon exposure to muscle unloading would provide important insight into the previously observed sex‐related disparities in neuromuscular function brought about by unloading.
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