Children's V ˙ O 2 max trainability deficit: A quantitative analysis and a qualitative hypothesis.

Background: The response to training of children's maximal aerobic power ( $\dot{\text{V}}$ O₂max/peak) has long been claimed to be significantly smaller than in adults and to be augmented by high-intensity training. Although several hypotheses have been put forth to explain children's lesser response, none has addressed both aspects, partly because a sufficiently large database for quantifying the phenomena and formulating a plausible hypothesis has not been established.

Purpose: To compile and analyze an unprecedentedly large literature database and use its findings to inform the formulation of a hypothesis for children's trainability deficit and its underlying causes.

Methods: Cohort size, age, pre-training $\dot{\text{V}}$ O₂max/peak, training intensity and duration, testing modality, and the training response were extracted from 650 training studies comprising 1046 experimental cohorts (children = 222, adults = 824), which were then compared between children and adults.

Results: Children's mean overall $\dot{\text{V}}$ O₂max training response was only 58% that of adults (p < 0.000001). High-intensity training was 25% more effective than low-moderate intensity in children (p = 0.041), but not in adults (-3.8%; NS). Nevertheless, children's high-intensity training was still only 67% as effective as in adults (p < 0.00001).

Interpretation and hypothesis: While some of the factors proposed to-date may account, in part, for children's trainability deficit, none has explained why high-intensity training is advantageous in children but not in adults. Presently, only child-adult differences in neuromuscular activation and muscle functional composition can, on their own or in conjunction with other factors, comprehensively account for all observations. An extensive body of evidence is presented in support of the hypothesis and data interpretation.