The purpose of this study was to compare the VO₂ kinetic and mechanical power responses of boys and men to all out 90 s sprint cycle exercise. Eight boys (14.6 ± 0.3 y) and eight men (33.8 ± 6.5 y) volunteered to participate and completed a ramp test (to determine VO_2peak and ventilatory threshold, VT) and then on subsequent days, two 90 s all out cycle sprints on an isokinetic cycle ergometer. During each test, breath-by-breath pulmonary gas exchange and power output were measured. Parameters from the power output profiles were derived from the average response of the two tests including peak power (PP, highest power output in 1 s), end power (EP_60-90, power over the last 30 s), and mean power over the 90 s (MP₉₀). Independent pairwise and dependent t-tests were used to compare the data from tests between adults and boys subject groups. Significant differences between adults and boys were found for absolute PP (881.4 ± 60.7 vs 533.6 ± 50.7 W), EP_60-90 (288.6 ± 25.7 vs 134.3 ± 17.6 W) and MP₉₀ (434.5 ± 27.4 vs 238.4 ± 17.3 W, p =0.001) respectively. Relative to body mass significant differences between adults and boys were found for EP_60-90, MP₉₀ and total work (p < 0.002). The boys attained 90 s VO₂ values that were closer to VO_2peak than their adult counterparts (93.3 ± 2.6 vs 84.9 ± 2.3 %, p = 0.03). They also demonstrated faster VO₂ kinetics (10.8 ± 1.5 vs 17.6 ± 1.0 s, p < 0.01). In conclusion, during all out 90 s cycle sprinting boys were able to attain VO₂ values that were closer to VO_2peak and a faster time constant than adult men. These findings provide insight into the contribution and speed of response of the aerobic system during an ‘anaerobic’ test.

• The results of this study confirm the significant contributions of the aerobic energy systems during so called ‘anaerobic tests’.

• Boys were able to attain VO values from an all out 90 s sprint cycle that were closer to their aerobic VO peak test than adults. More detailed studies are required to investigate the limiting factors that prevent VO peak being reached in an all out sprint cycle.

• All out tests of a duration > 30 s and coupled with gas and power analyses offer paediatric physiologists considerable scope to examine the contributions of the anaerobic and aerobic energy systems until more ethically viable methods are found.