Exercise Physiology in Children

Aerobic vs. Anaerobic Exercise

Exercise can be classified into two forms (i.e., anaerobic and aerobic) based on the dominant metabolic energy sources used during the activity. Anaerobic activities are characterized by higher intensities of muscular contraction. Contractions are sustained by the phosphagen and anaerobic glycolytic systems to produce lactic acid and energy in the form of adenosine triphosphate (i.e., ATP). Anaerobic activities include sprinting, power lifting, hockey, and some motions during basketball and racquet sports.

Anaerobic fitness refers to the ability to work at a very high level during these activities for relatively short periods (5-30 s).

Aerobic activities are characterized by lower rates of muscular contraction. These contractions are usually more prolonged in duration and use carbohydrates, fats, and some protein for oxidation by mitochondria within the muscle. Aerobic metabolism is the primary method of energy production during endurance activities such as running, cycling, rowing, swimming, soccer, and ultra-endurance events. Aerobic fitness (VO2max) indicates the endurance capacity of the individual’s heart, lungs, and muscles that allows him/her the ability to offset fatigue over the course of an activity. It is crucial to note that these and similar activities often include short bursts of anaerobic metabolism. The distinction between the two types of exercise is important because of their different effects on blood glucose concentration. For example, many individuals find that aerobic-type exercise causes blood glucose to decrease both during and post activity. On the contrary, anaerobic activities, which may only last for seconds, tend to cause dramatic increases in blood glucose levels.

Fuel metabolism and mechanisms of glucose regulation during exercise

To understand the possible metabolic responses to exercise in children with diabetes, it is useful to first briefly describe the mechanisms of glucose regulation in non-diabetic youth.Non-diabetic children and adolescents

In healthy children, precise autonomic and endocrine regulation allows blood glucose levels to remain relatively stable, except for a transient decrease in blood glucose at the start of exercise. At rest, the body uses primarily free fatty acids (FFAs) as fuel which are delivered from adipose tissue. During the transition to exercise, muscles draw upon a complex mixture of circulating FFAs, muscle triglycerides, muscle glycogen, and blood glucose derived from liver glycogen. Although there are no studies specifically conducted in children, under most circumstances, protein oxidation represents <5% of the overall energy utilization and thus has a negligible effect on performance. Fuel metabolism during exercise is under complex neuroendocrine control and includes the hormones insulin, glucagon, catecholamines, growth hormone, and cortisol. The proportions of substrate depend on the intensity and duration of the activity. In general, at lowto-moderate intensities, plasma-derived FFAs predominate, while both plasma glucose and muscle glycogen make up the majority of fuel as the exercise intensifies. During heavy exercise, total carbohydrate utilization may be as great as 1.0-1.5 g/kg body mass per hour in healthy adolescents and in adolescents with diabetes. As the exercise duration increases, there is a greater reliance on fuels from outside of the muscle, including plasma FFAs and blood glucose.  This greater dependence on fuels from outside the muscle, as the duration of exercise increases, can have dramatic effects on blood glucose levels, particularly for the child with T1DM. Compared with adults, children and adolescents utilize less carbohydrate and more fat during exercise performed at the same relative intensity, possibly because they have less endogenous carbohydrate stores. Hypo- and hyperglycemia are rare in healthy children who do not have diabetes because insulin secretion is lowered and counterregulatory hormones are elevated, thereby causing glucose production by the liver to match utilization by the working muscles (Fig. 1A).