Most of the available data on the aging of muscle function are cross-sectional in type. Static and dynamic muscle strength seem well preserved to about 45 years of age, but performance deteriorates by about 5% per decade thereafter. There is a parallel loss of lean tissue. Because muscle biopsy specimens are not always representative of an entire muscle, it is unclear whether there is a general hypotrophy or atrophy of the muscle fibres or a selective hypoplasia and degeneration of type II fibres associated with a loss of terminal sprouting. Other influences may include a deterioration of end-plate structures with impaired excitation-contraction coupling and impaired fibre recruitment. Both contraction time and half-relaxation time are prolonged with aging, and there is a decrease of maximal contraction velocity, more marked in the legs than in the arms. On the other hand, endurance at a fixed fraction of maximal force is increased. Potential factors leading to the enhanced endurance include a poorer maximal effort and an increased proportion of type I fibres; however, there is little evidence that muscle capillarity is altered. The loss of function is generally less in the arms than in the legs, but it remains unclear whether this is an inherent biological difference, or merely a reflection of differential changes in activity patterns between the upper and the lower limbs. The male/female strength ratio does not seem to change with age, but substantial slowing or reversal of the aging process is possible through appropriate activity programmes. The preservation of muscle function has important implications for the quality of life in the frail elderly, counteracting dyspnoea, stabilising joints and extending the period of independent living by up to 20 years.