Physiological adaptations to exercise training for children

Training can improve strength, aerobic capacity, and anaerobic capacity for the children. Generally, youngsters adapt well to the same type of training routine used by adults. But training programs for children and adolescents should be designed specifically for each age group, keeping in mind the developmental factors associated with that age. In this section, we look at traininig-induced changes in each of the following:

• Body composition
• Strength
• Aerobic capacity
• Anaerobic capacity.

Then, where appropriate, we discuss proper training procedures to optimize performance gains and reduce the risk of injury.

Body composition

The child and adolescent respond to physical training similarly to adults with respect to changes in body weight and composition. With both resistance and aerobic training, both boys and girls will decrease body weight and fat mass increase fat-free mass, although the increase in fat-free mass is attenuated in the child compared with the adolescent and adult. There is also evidence of significant bone growth as a result of exercise training, above that seen with normal growth. In fact, Bass suggested that the prepubertal years may be the most opportune time to increase bone mass because of increases in bone density and periosteal expansion of cortical bone.

There is presently an epidemic of obesity in the United States, Canada, much of Europe, and other westernized countries. This is true not only in adults but in children and adolescents as well. Physical training and an active lifestyle are critical throughout the growing years to maintain a healthy body composition and establish a lifelong habit of exercise and activity.

Strength

For many years, the use of resistance training to increase muscular strength and endurance in prepubescent and adolescent boys and girls was highly controversial. Boys and girls were discouraged from using free weights for fear that they might injure themselves and prematurely stop the growth process. Furthermore, many scientists speculated that resistance training would have little or no effect on the muscles of prepubescent boys because their levels of circulating androgens were still low. Is resistance training in children and adolescents dangerous or risky? Even if it is safe, are there any benefits?

Studies on animals suggest that heavy resistance exercise can lead to stronger, broader, and more compact bones. But these studies have not contributed much to our understanding of the benefits or risks associated with this form of activity for humans because it is nearly impossible to load these animals to the same extent as youngsters can be loaded. Fortunately, several studies have been conducted in which both prepubescent and adolescent children have participated in resistance training. From these studies, Kraemer and Fleck concluded that the risk of injury is very low. In fact, resistance training might offer some protection against injury, for example by strengthening the muscles that cross a joint. Still, a conservative approach is recommended in prescribing resistance exercise for children, particularly preadolescents.

Now that we have established that resistance training is relatively safe, does it increase strength? A number of studies have now been conducted on both children and adolescents and have clearly demonstrated that resistance training is very effective in increasing strength. The increase is largely dependent on the volume and intensity of training. Further, the percentage increases for the children and adolescents are similar to those for young adults.

How are these increases in strength accomplished? The mechanism allowing strength changes in children are similar to those for adults, with one minor exception: prepubescent strength gains are accomplished largely without any changes in muscle size. A comprehensive study of the mechanisms responsible for strength increases in prepubescent boys concluded that the likely determinants of the strength gains achieved are improved motor skill coordination, increased motor unit activation, and other undetermined neurological adaptations. Strength gains in the adolescent result primarily from neural adaptations and increases in both muscle size and specific tension.

For actual training programs, resistance training for children should be prescribed in much the same way as for adults. Specific guidelines have been established by a number of professional organizations, including the American Academy of Pediatrics, American Otrhopaedic Society for Sports Medicine, the American College of Sports Medicine, the National Athletic Trainers’ Association, the National Athletic Trainers’ Association, the President’s Council on Physical Fitness and Sports, the U.S. Olympic Committee, and the Society of Pediatric Orthopaedics. Basic guidelines have been established for the progression of resistance exercise in children, which are presented in table below. Further information on resistance training program designs for children is available.

Basic guidelines for resistance exercise progression in children
Age	Considerations
7 years or younger	Introduce child to basic exercises using little or no weight; develop the concept of a training session; teach exercise technique; progress from body weight calisthenics, partner exercises, and lightly resisted exercises; keep volume low.
8-10 years	Gradually increase the number of exercises; practice exercises technique in all lifts; start gradual progressive loading of exercises; keep exercises simple; gradually increase training volume; carefully monitor tolerance of the exercise stress.
11-13 years	Teach all basic exercise techniques; continue progressive loading of each exercise; emphasize exercise techniques; introduce more advanced exercises with little or no resistance. Progress to more advanced youth programs in resistance exercise; add sport-specific components; emphasize exercise techniques; increase volume
14-15 years	Progress to more advanced youth programs in resistance exercise; add sport-specific components; emphasize exercise techniques; increase volume.
16 years or older	Move child to entry-level adult programs after all background knowledge has been mastered and a basic level of training experience has been gained.

Any youth resistance training program must be carefully supervised by competent instructors who have been trained specifically to work with children. Furthermore, resistance training should be only one part of a more comprehensive fitness program for this age-group.

Aerobic capacity

Do prepubescent boys and girls benefit from aerobic training to improve their cardiorespiratory systems? This also has been a highly controversial area because several early studies indicated that training prepubescent children did not change their VO₂max values. Interestingly, even without significant increases in VO₂max, the running performance of the children studied did improve substantially. They could run a fixed distance faster following the training program. More recent studies have shown small increases in aerobic capacity with training in prepubescent children, but these increases are less than would be expected for adolescents or adults – about 5% to 15% in children compared with about 15% to 25% in adolescents and adults.

More substantial changes in VO₂max appear to occur once children have reached puberty, although the reason for this is unknown. Because stroke volume appears to be the major limitation to aerobic performance in this age-group, it is quite possible that further increases in aerobic capacity depend on heart growth. Also, scaling of these variables is an issue.

Anaerobic capacity

Anaerobic training appears to improve children’s anaerobic capacity. Following training, children have:

• Increased resting levels of PCr, ATP, and glycogen;
• Increased phosphofructokinase activity;
• Increased maximal blood lactate levels.

Ventilatory threshold, a noninvasive marker of lactate threshold, also has been reported to increase with endurance training in 10- to 14-year-old boys.

When one is designing aerobic and anaerobic training programs for children and adolescents, it appears that standard training principles for adults can be applied. Children and adolescents have not been well studied, but what we do know suggests that they can be trained in a manner similar to that for adults. Again, because children and adolescents are not adults, it is prudent to be conservative to reduce the risk of injury, overtraining, and loss of interest in sport. The approach outlined earlier for resistance training is a good model to use for aerobic and anaerobic training. This is also an appropriate time in life to focus on learning a variety of motor skills by having children explore a number of activities in sports.