Age-related alterations in muscular endurance

Aging often results in a decline of most physiological systems of the body. However, the maintenance of some appropriate level of neuromuscular function into old age is critical if we expect the elderly to be able to maintain normal daily activity and functional independence. It is well established that muscular strength declines in old age, but an equally important parameter of neuromuscular function muscular endurance, has received only minimal attention in the literature. Important information regarding age-associated changes in muscular endurance can be obtained from both animal and human research models, each having their own set of limitations and advantages. One problem in trying to interpret past research dealing in this area is the fact that muscular endurance can be expressed in a variety of ways and can be measured by a variety of techniques. It seems that conflicting reports arise from substantial differences in research design, statistical analyses or the ability to control extraneous influences such as physical activity levels and dietary intakes. This review will examine both human and animal literature in an attempt to elucidate methodological concerns in the assessment of muscular endurance and the controversial evidence regarding changes in muscle morphology and muscle metabolism that may be responsible for age-related changes in muscular endurance. Issues that address the possible selective loss of fibre type and the declines in both total fibre number as well as fibre size will be presented, since muscle quantity and quality have obvious links to endurance capacity. In conjunction with the alterations reported in skeletal muscle tissue, muscle blood flow and the relationship between fibre and capillary numbers and their possible influences on substrate availability will also be discussed in relation to muscle endurance capabilities. Finally, the adaptive ability of aged skeletal muscle to improve muscular endurance by different training regimens and through different physiological mechanisms will be investigated.