[Neuromuscular system and aging: involutions and implications]

In aged human, the number of muscle fibers and motor units decreases. The remaining motor units lose their functionality (decrease of the discharge frequency, greater fluctuation of the discharge) particularly those which contain type II fibers. The renewal of intracellular proteins declines which creates a negative balance between the daily protein losses and the capacities to renew them. The activity of the protein kinase (Akt) that stimulates the synthesis of regulation proteins (mTOR, p70S6, IGFBP-5) declines whereas the factors of degradation of proteins (NF-kappa B) are activated. Besides, the process of activation and proliferation of satellite cells is affected and the production of anabolic hormones and local factors is decreased. After a strength training program, muscle hypertrophy is linked to the protein synthesis at the level of myosin heavy chain (MHC) isoforms in older subjects. However, the transcription of the genes that code the MHC-I (slow form) increases and the transcription of the genes that code the MHC-II (fast form) decreases. Thus, the transition of the phenotype towards a slower form cannot be inverted by strength training during the advanced in age. Moreover, strength training enables to decrease the proportion of fibers containing MHC of hybrid form in the process of evolution. Hence, strength training can engender a stabilization of the muscular phenotype i.e. different isoforms of MHC. In addition, strength training counteracts the noxious effects mentioned above by generating muscular hypertrophy thanks to a reactive increase in the production of anabolic hormones. A program of aerobic training can induce an increase in the synthesis of ARN messengers coding isoforms related to the oxidative metabolism (MHC-I and to a lesser extent MHC-IIa) while the transcribed for the type MHC-IIx decrease.