We hypothesized that the maximum mechanical power outputs that can be maintained during all-out sprint cycling efforts lasting from a few seconds to several minutes can be accurately estimated from a single exponential time constant (k_cycle) and two measurements on individual cyclists: the peak 3 s power output (P_{mech max}) and the maximum mechanical power output that can be supported aerobically (P_aer). Tests were conducted on seven subjects, four males and three females, on a stationary cycle ergometer at a pedal frequency of 100 rpm. Peak mechanical power output (P_{mech max}) was the highest mean power output attained during a 3 s burst; the maximum power output supported aerobically (P_aer) was determined from rates of oxygen uptake measured during a progressive, discontinuous cycling test to failure. Individual power output-duration relationships were determined from 13 to 16 all-out constant load sprints lasting from 5 to 350 s. In accordance with the above hypothesis, the power outputs measured during all-out sprinting efforts were estimated to within an average of 34 W or 6.6 % from P_{mech max}, P_aer, and a single exponential constant (k_cycle = 0.026 s^-1) across a 6-fold range of power outputs and a 70-fold range of sprint trial durations (R² = 0.96 vs. identity, n = 105; range: 180 to 1136 W). Duration-dependent decrements in sprint cycling power outputs were two times greater than those previously identified for sprint running speed (k_run = 0.013 s^-1). When related to the respective times of pedal and ground force application rather than total sprint time, decrements in sprint cycling and running performance followed the same time course (k = 0.054 s^-1). We conclude that the duration-dependent decrements in performance are set by the fractional duration of the relevant muscular contractions.