Monitoring training is important in order to follow normal growth and maturation and to avoid possible overtraining in youth athletes. Overtraining is not a single condition but rather a continuum of related factors such as functional overreaching, non-functional overreaching, and overtraining syndrome. The monitoring process should follow a multilevel approach and include specific psychometric and hormonal markers in addition to performance assessment on a regular basis in growing athletes. In psychometric monitoring, stress and recovery should be measured simultaneously, and monitoring should not be restricted to the stress component alone. Endocrine mechanisms that participate in the response to high training stress are the growth hormone (GH)–insulin-like growth factor-1 (IGF-1), hypothalamic–pituitary–gonadal, and hypothalamic–pituitary–adrenal axes, and peripheral factors of energy homeostasis that also include specific adipose, muscle, and bone tissue markers. The function of the GH–IGF-1 axis is suppressed during high training stress demonstrating a catabolic hormonal environment, which is attributed to low energy availability. In female athletes, low energy availability is the major factor that alters reproductive hormone secretion, rather than high training stress or an increase in exercise energy expenditure. Circulating ghrelin and irisin may also be used as indicators of energy balance in female athletes after menarche, while increased cortisol level indicates possible overreaching/overtraining syndrome. In certain circumstances, leptin, ghrelin, and irisin concentrations can be used to characterise low energy availability and consequently high training stress in growing athletes.
