Child-Adult Differences in the Recovery From High-Intensity Exercise

Methodological Difficulties

Recovery Criteria

Intense exercise results in acute physiological responses of the metabolic, cardiovascular, respiratory, endocrine, immune, and neuromuscular systems. During the recovery from such exercise, numerous physiological processes take place, aimed at restoring homeostasis and functional capacity ( Table 2 ). These processes are not linear (e.g., see Ref. 3), and their kinetics are widely different (e.g., preexertion muscle glycogen stores take considerably longer to restore than ATP or plasma volume). The kinetics of these processes have mostly been studied in adults, although some comparative pediatric data do exist ( Table 2 ). As the recovery of performance capacity is clearly a function of physiological recovery, growth-or maturity-related differences in the rate of physiological processes could explain part of the faster performance recovery observed in children.^[7,11,14] However, the definition of recovery and its study are somewhat complicated by the fact that a complete return to homeostasis or to the physiological preexertion state is not necessary for a complete recovery of performance capacity. That is, residually elevated levels of factors such as heart rate (HR), core temperature, metabolic rate, or even [La] may not harm and often even benefit subsequent performance. This and the differential time axes of the various processes make it important to distinguish and clearly specify what kind of recovery is discussed, namely, physiological or that of the actual performance capacity.

Exercise Intensity

As pointed out earlier, maximal short-term performance is not comparable between children and adults. That is, although the exertion can be of maximal subjective intensity, the body mass-relative power output will still be considerably lower in children.^[7,8,11] The same will be true for any exertion at a given percentage of maximal power. Due to the considerable age-related size differences, the use of identical power outputs is not a methodological option. A realistic alternative is to normalize power output to total body, lean body, or muscle mass. However, in such a case, children's power output will constitute a considerably higher percentage of their maximal power capacity. Another possible approach is to use allometric scaling. However, because different body mass and height exponents have been suggested in the literature with no apparent consistency, this possibility remains ambivalent at the moment. Most studies have used maximal performance for comparison. The disadvantages of this approach are the significant differences in the deviation from homeostasis after that exertion and hence different states from which children and adults would have to recover. In addition, the prerequisite of maximal motivation is a crucial factor that cannot be a priori presumed identical in children and adults. This can normally be overcome by sufficient familiarization and habituation to the exercise tests and by proper encouragement and incentive to exercise at maximal effort.

Resting Values

Resting values of many physiological variables (e.g., HR and oxygen consumption, O₂) are higher in children compared with adults. Therefore, even if both groups attain similar values at maximal effort, the difference between resting and peak values will be smaller for the children. For example, if both groups reach a postexercise HR of 190 beats · min^-1, children's full recovery would be a return to 80—90 beats · min^-1, whereas the corresponding adult value could be some 15 beats · min^-1 lower than that.

"Peak" Values

Peak physiological values attained by children in response to maximal exercise are typically different from the corresponding adult values. Notable examples are peak [La] which has consistently been shown to be much lower in children^[3,7,11,14] or peak HR after maximal exhausting exertions (e.g.,^[3,7]). Again, this means that children may not start their recovery from a point identical or readily comparable with that of adults. This complicates any attempt of comparing recovery on common grounds.

A possible approach of managing the different resting and peak HR values is to define deviations from homeostasis as net change or as percentages of HR reserve (peak HR - resting HR), as has already been done in some studies.^[3,7]

Still, it is both practically and methodologically impossible to simultaneously control for all exercise and recovery variables, and it is normally necessary to equate groups by a single process or variable.

Exerc Sport Sci Rev. 2006;34(3):107-112. © 2006  American College of Sports Medicine

Cite this: Child-Adult Differences in the Recovery From High-Intensity Exercise - Medscape - Jul 01, 2006.