The upper marker of the heavy exercise intensity domain is the maximal lactate steady state (MLSS, the highest metabolic rate at which exercise can be sustained without an accumulation of blood lactate33) or, more often in young people, the critical power (CP, the highest metabolic rate at which V˙O2 can be stabilised below peak V˙O236 and 37). Exercise above MLSS or CP but below

peak V˙O2 is in the very heavy exercise domain and exercise above peak V˙O2 is in the severe exercise domain.38 With young participants it has been noted that small selleck compound breath-to-breath variations are inherent to children’s response profiles.39 This reduces the confidence with which pV˙O2 kinetic responses can be estimated and confidence intervals are likely to be beyond acceptable limits unless sufficient identical transitions are aligned and averaged to improve the signal to

noise ratio.40 Rigorously determined and interpreted data from young people are available in the moderate, heavy and very heavy intensity exercise domains.41, 42 and 43 The pV˙O2 response to a step transition has three phases. At the onset there is an immediate increase in cardiac output which occurs prior to the arrival at the lungs of venous blood from the exercising muscles. This cardiodynamic phase (phase I) which, in children, lasts ∼15 s is independent of V˙O2 at the muscle (mV˙O2) and reflects an increase in pulmonary SCR7 datasheet blood flow with exercise. Phase II, the primary component, is a rapid exponential increase in pV˙O2 that arises with hypoxic and hypercapnic blood from the exercising muscles arriving at the lungs. Phase II kinetics are described by the time constant (τ  ) which is the time taken over to achieve 63% of the change in pV˙O2. In phases I and II ATP re-synthesis cannot be fully supported by oxidative phosphorylation and the additional energy requirements of the exercise are met from body oxygen stores, PCr and glycolysis. During moderate intensity exercise with children pV˙O2 reaches a steady state (phase III) within about 2 min. In the heavy intensity exercise domain, the primary phase II oxygen cost is similar to that observed during moderate

intensity exercise but the overall oxygen cost of exercise increases over time as a slow component of pV˙O2 is superimposed upon the primary component and the achievement of a steady state might be delayed by ∼10–15 min. 44 In adults, at exercise intensities above the MLSS or CP the slow component of pV˙O2 rises rapidly over time and eventually reaches peak V˙O2 but this phenomenon has not been observed in children. 37 and 45 The mechanisms underlying the pV˙O2 slow component remain speculative but it has been established that ∼86% have been accounted for at the contracting muscles.46 During exercise above the TLAC the pV˙O2 slow component is associated with a progressive recruitment of additional type II muscle fibres with the low efficiency contributing to the increased oxygen cost of exercise.