The ability to generate muscle power during daily living activities is reduced with advancing age. To date, determinants of maximal instantaneous muscle power during vertical jumping (optimal force and velocity of movement) have only been reported for groups of either young or older individuals, men or women, with no data in middle-aged subjects. The present study aimed at investigating age-related differences in the determinants of muscle power between young (Y), middle-aged (M) and older (O) individuals of both genders living in a city district. Seventy two volunteers were assigned to three gender-matched age groups: 24 Y aged 28±2 years, 24 M aged 48±2 years and 24 O aged 70±3 years. Maximal peak power, optimal force and velocity were estimated from counter-movement jumping on a force platform. Maximal peak power, optimal force and velocity in O (24.8±5.9 W kg^-1, 15.5±2.4 N kg^-1, 1.6±0.2 m s^-1) were significantly lower than in Y (52.0±14.7 W kg^-1, 24.8±16.4 N kg^-1, 2.6±1.3 m s^-1; P<0.01), while there were no differences in maximal power between M and Y; however, M showed significantly higher optimal force and lower optimal velocity (32.1±13.2 N kg^-1, 1.5±0.3 m s^-1) than Y (P<0.05). The present results suggest that decrease of optimal velocity represents a critical determinant of the age-related decline in maximal instantaneous muscle power. Middle-aged individuals would increase optimal force to overcome such a decline and maintain adequate levels of muscle power.