Aging of the human neuromuscular system

Loss of cells from the motor system occurs during the normal aging process, leading to reduction in the complement of motor neurons and muscle fibers. The latter age-related decrease in muscle mass has been termed "sarcopenia" and is often combined with the detrimental effects of a sedentary lifestyle in older adults, leading to a significant reduction in reserve capacity of the neuromuscular system, which is the primary subject of this review. Clear evidence of this aging effect is seen when voluntary or stimulated muscle strength is compared across the adult lifespan, with a steady decline of approximately 1-2% per year occurring after the sixth decade. Interestingly, when compared with isometric contractions, the effect of aging is more pronounced for concentric movements and less for eccentric movements (i.e., muscle shortening versus lengthening). This phenomenon appears to be linked to the stiffer muscle structures and prolonged myosin crossbridge cycles of aged muscles. It is encouraging that the capability of physiological adaptations in the motor pathways remains into very old age--when an appropriate exercise stimulus is given--and long-term prevention strategies are advocated to avoid excessive physical impairments and activity restrictions in this age group.