Aim To analyse aerobic fitness and its relationship with sport participation, exercise training and habitual physical activity (HPA) during youth.

Methods Studies were located through computer searches of Medline, SPORT Discus and personal databases. Systematic reviews of time trends in aerobic fitness/performance, and exercise training and peak oxygen uptake (peak VO₂) are reported.

Results Peak VO₂ increases with age and maturation. Boys' peak VO₂ is higher than girls'. Despite data showing a decrease in performance test estimates of aerobic fitness there is no compelling evidence to suggest that young people have low levels of peak VO₂ or that it is declining over time. The primary time constant of the VO₂ kinetics response to moderate and heavy intensity exercise slows with age and the VO₂ kinetics response to heavy intensity exercise is faster in boys. There is a negative correlation between lactate threshold as a percentage of peak VO₂ and age but differences related to maturation or sex remain to be proven. Young athletes have higher peak VO₂, a faster primary time constant and accumulate less blood lactate at the same relative exercise intensity than their untrained peers. Young people can increase their peak VO₂ through exercise training but a meaningful relationship between aerobic fitness and HPA has not been demonstrated.

Conclusions During youth the responses of the components of aerobic fitness vary in relation to age, maturation and sex. Exercise training will enhance aerobic fitness but a relationship between young people's current HPA and aerobic fitness remains to be proven.