Purpose: The aim of this investigation was to assess alterations of pulmonary oxygen uptake (VO2) and muscle deoxygenation on-kinetics during heavy-intensity cycling in youth cyclists over a period of 15 months. Methods: Eleven cyclists (initial age, 14.3 [1.6] y; peak VO2, 62.2 [4.5] mL·min−1·kg−1) visited the laboratory twice on 3 occasions within 15 months. Participants performed an incremental ramp exercise test and a constant workrate test within the heavy-intensity domain during the first visit and second visit, respectively. Subsequently, parameter estimates of the VO2 and muscle deoxygenation on-kinetics were determined with mono-exponential models. Results: The phase II time constant decreased from occasion 1 (34 [4] s) to occasion 2 (30 [4] s, P = .005) and 3 (28 [4] s, P = .010). However, no significant alteration was observed between occasions 2 and 3 (P = .565). The VO2 slow component amplitude either expressed in absolute values (ie, L·min−1) or relative to end exercise (ie, %) showed no significant changes throughout the study (P = .972 and .996). Furthermore, the muscle deoxygenation on-kinetic mean response time showed no significant changes throughout the study (18 [8], 18 [3], and 16 [5] s for occasions 1, 2, and 3, respectively; P = .279). Conclusion: These results indicate proportional enhancements of local muscle oxygen distribution and utilization, which both contributed to the speeding of the VO2 on-kinetics herein.