Several mechanisms contributing to the etiology of sarcopenia (age-related loss of muscle size) have been postulated. One of these attributes the loss of muscle mass to a preceding age-related denervation of myofibers. The aim of this study was to determine if signs of denervation were apparent at the neuromuscular junction (NMJ) before fiber atrophy, or fiber type conversion could be documented, and to reveal if a muscle’s activity level impacts its sensitivity to age-related denervation. Plantaris and soleus muscles were obtained from young adult (10 mo) and early aged (21 mo) rats. Pre- and post-synaptic NMJ morphology was quantified with cytofluorescent staining of nerve terminal branches and endplate regions, respectively. Myofiber profiles (fiber size and fiber type composition) were assessed with histochemical procedures. Results show that in the lightly recruited plantaris, significant (P < 0.05) signs of denervation were noted in aged rats, while the same muscles displayed no change in myofiber profile. In the heavily recruited soleus, however, there was little evidence of denervation, and again no alterations in myofiber profile. These results indicate that age-related denervation occurs before myofiber atrophy, and that high amounts of neuromuscular activity may delay the onset of age-related denervation and sarcopenia.

Muscle unloading results in decreased strength. This is partly attributed to reduced neural activation. This study examined whether men and women experienced different adaptations to muscle unloading. Ten men (21.4 +/- 0.8 years; mean +/- SE) and ten women (20.9 +/- 0.2 years) were subjected to 1 week of muscle unloading. Muscle function was quantified during knee extensions before, and following unloading. Electromyography (EMG) was assessed during maximal isometric contractions before, and after unloading. Results showed greater (P < 0.05) unloading-induced decrement in muscle performance in women than men. The loss of peak torque was significantly correlated (r = 0.69) with the decline in EMG consequent to unloading, and women displayed significantly larger reductions in EMG. These findings indicate that: (1) women are more affected by muscle unloading than men, and (2) gender-related differences in strength declines are associated with similar disparities in the nervous system's capacity to maximally stimulate muscle.

During exercise of the same relative submaximal intensity, some physiological parameters responded likewise in young men and young women, whereas others did not. Among those variables that demonstrated significant gender-related differences, all but one (plasma lactate) were obscured within 5 min of postexercise recovery.