The influence of age-associated motor unit loss on contractile strength was investigated in a representative sample of healthy, active young and older men and women. In 24 younger subjects (22–38 yr) and 20 older subjects (60–81 yr) spike-triggered averaging was employed to extract a sample of surface-recorded single motor unit action potentials (S-MUAPs) from the biceps brachii and brachialis muscles. The amplitude of the maximum compound muscle action potential of the biceps brachii and brachialis muscles was divided by the mean S-MUAP amplitude to estimate the numbers of motor units present. The maximum isometric twitch contraction (MTC) and maximum voluntary contraction (MVC) of the elbow flexors were also recorded in 18 of the younger subjects and in all older subjects. The estimated numbers of motor units were significantly reduced (47%, P < 0.001) in older subjects with a mean value of 189 +/- 77 compared with a mean of 357 +/- 97 in younger subjects. The sizes of the S-MUAPs, however, were significantly larger in older subjects (23%, P < 0.01). Significant but less marked age-associated reductions in the MTC (33%, P < 0.05) and MVC (33%, P < 0.001) were also found and were similar for both men and women. These results suggest that motor unit losses, even in healthy active individuals, are a primary factor in the age-associated reductions in contractile strength.