Training and Exercise for Children and Teenagers - Part 2

Training and exercise for children and teenagers. This article is huge! So huge it has been split into a 4 part series. You can see the 4 parts below or click the continue link at the bottom of each part.

• Part 1: Introduction & body composition and development of tissue
• Part 2: Physiological responses and adaptation to exercise & training
• Part 3: Guidelines for exercise
• Part 4: The workouts

Physiological Responses to Acute Exercise

The function of almost all physiological systems improves until full maturity is reached or shortly before. After that physiological function plateaus for a period of time before starting to decline with the advancing age. Below are some of the changes in children and adolescents that accompany growth and development.

• Strength
• Cardiovascular and respiratory function
• Metabolic function, including aerobic capacity, running economy, and aerobic capacity

Strength.

Strength improves as muscle mass increases with age. Peak strength is usually attained by the age of 20 in women and between the ages of 20-30 in males. The hormonal changes that accompany puberty lead to marked improvements in strength in pubescent males because of the increased muscle mass. The extent of development and the performance capacity of muscle depend on the maturation of the nervous system. High levels of strength, power, and skill are impossible if the child has not reached neural maturity. Myelination of many motor nerves is incomplete until sexual maturity, so the neural control of muscle function is limited before that time.

Cardiovascular and Respiratory Function.

Cardiovascular function undergoes considerable change as children grow and age and there are changes during submaximal and maximal exercise.

Rest and Submaximal Exercise.

The blood pressure is lower in children than in adults at rest and during submaximal exercise, but increases progressively during the late teens. Blood pressure is directly related to body size and larger people generally have higher blood pressure. Also children have a greater blood flow to active muscle than adult due to there being less peripheral resistance. Cardiac output is a product of heart rate and stroke volume. A child’s smaller heart size and total blood volume result in a lower stroke volume both at rest and during exercise than in an adult. In an attempt to compensate for this, the child’s heart rate response to a given rate of submaximal training (such as cycling), where the absolute oxygen requirement is the same, is higher than in adults. As the child ages, heart size and blood volume increase along with body size, and consequently stroke volume also increases as the body size increases, for the same amount of training. However the child’s higher submaximal heart rate cannot completely compensate for the lower stroke volume. Because of this the child's cardiac output is also lower than the adults for the same rate of exercise or oxygen consumption.

Maximal Exercise.

The maximal heart rate is higher in children than in adults but decreases as children age. Children under the age of 10 yrs frequently have maximal heart rates exceeding 210 beats per minute, whereas the average 20 year old has a maximal heart rate of approx 195 beats per minute. As also seen with submaximal exercise the child's smaller heart and blood volume limit maximal stroke volume that he or she can achieve. Again, the elevated maximal heart rate cannot fully compensate for this, leaving the child with a lower maximal cardiac output than the adult. Lung function changes markedly with growth. All lung volumes increase until growth is complete. Peak flow rates follow the same pattern. The changes in these volumes and flow rates are matched by the changes in the highest ventilation that can be achieved during exhaustive exercise.

Aerobic Capacity.

The purpose of the basic cardiovascular and respiratory adaptations that occur in response to varying levels of exercise (rates of work) is to accommodate the muscles need for oxygen. Thus increases in cardiovascular and respiratory function that accompany growth suggested that aerobic capacity (VO2 max) similarly increases. Studies have shown that the VO2 max peaks between age the ages of 17-21 years and then decreases with age. Studies of girls have found the same trend, although in females the decrease begins at a much younger age, generally aged between 12-15 years which is thought to come about due to an assumption of a sedentary lifestyle. Aerobic capacity (VO2 max) when expressed in litres per minute is lower in children than in adults at similar levels of training. This is attributable primarily to a child's lower maximal cardiac capacity. When VO2 max values are expressed to reflect the difference in body size between children and adults there is no difference in aerobic capacity.

Anaerobic Capacity.

Children have a limited ability to perform anaerobic type activities. This is demonstrated in several ways, children cannot achieve adult concentrations of lactate in either muscle or blood for maximal or supramaximal rates of exercise, which indicates a lower glycolytic capacity (converting glucose into pyruvate). The lower lactate levels might reflect a lower concentration of phosphofructokinase (regulatory enzyme of glycolysis), the rate limiting enzyme of anaerobic glycolysis. Lactate dehydrogenase (An enzyme that catalyzes the conversion of lactate to pyruvate) activity also seems to be lower in children. However lactate threshold when expressed as a percentage of VO2 max, does not appear to be a limiting factor in children because children’s lactate thresholds are similar to, if not somewhat higher than those of similarly trained adults. Also children’s resting levels of adenosine tri-phosphate (ATP) and Phosphocreatine (PCr) are similar to those in children

Physiological Adaptations to Exercise Training

Training can improve the strength, aerobic capacity and anaerobic capacity of children but are physiologically different to adults. They adapt well to the training programs of adults but training routines for children should be designed around age groups.

Body Composition.

Adolescents and children respond to physical training similarly to adults with respect of changes in bodyweight and body composition. With both resistance and aerobic training, both boys and girls will decrease body weight and fat mass and increase fat free mass, although the increase in fat free mass 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. Studies suggest that the prepubertal years may be the most opportune time to increase the bone mass because of increase in bone density.

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 injuries may occur and so prematurely stopping the growth process. Scientists speculated that resistance training would have little or no effect on the prepubescent boys because of low levels of circulating androgens. Studies have concluded that resistance training offers protection against injury by strengthening the muscles that cross a joint, but a conservative approach is recommended in prescribing resistance exercise for children, particularly preadolescents. The mechanisms allowing strength changes in children are similar to those in adults, with one exception: Prepubescent strength gains are accomplished largely without any changes in muscle size. A comprehensive study of the mechanisms responsible for strength increase 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 very much the same way as adults, although any resistance training program must be carefully supervised by competent instructors who have been trained to work specifically with children. Resistance training should only be one part of a more comprehensive fitness program for the specific age group.

Basic Guidelines for Resistance Exercise Progression in Children:

If a child of any age begins a program with no previous experience, they should be started at the previous levels and move to the more advanced levels as exercise toleration, skill, and amount of training time, and understanding, permit.

Aerobic Capacity.

The question whether boys and girls benefit from aerobic training to improve their cardiorespiratory system has been a highly controversial area because several early studies indicated that training prepubescent children did not change their VO2 max values. Interestingly even without significant increases in VO2 max, the running performance of children studied did improve substantially. They could run a fixed distance faster following training. More recent studies have found 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 VO2 max 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 increase in aerobic capacity depends on heart growth.

Anaerobic Capacity.

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

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

When designing aerobic and anaerobic training programs for children and adolescents the standard training principles of adults can be applied, but it is prudent to be conservative to reduce the risk of injury and overtraining. This is also an appropriate time in time to focus on learning a variety of motor skills by exploring a number of activities and sports.

This article continues in part 3 >>