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A decreased speed of contraction in old age has been reported in various mammals including man. Mechanical properties of fast-and slowtwitch muscles and single motor units and the calcium transport activity and volume of the sarcoplasmic reticulum(SR) was measured in single fast-and slow-twitch muscle fibers in young and old rats in an attempt to clarify the mechanisms underlying this age-related impairment. The results from these experiments show that the reduced speed of isometric contraction is primarily related to changes in the contractile properties of both fast-and slow-twitch motor units. In fast-twitch motor units, the dominant underlying mechanisms for the the age-related decrease in speed of contraction is the decrease in volume and intrinsic function of the SR, ie a decreased specific activity of the Ca2+-pump. In slow-twitch fibres, SR volume and function is similar in young (3 months) and old (23-24 …