Aerobic and Anaerobic Development
Raphael Brandon explores the benefits of aerobic and anaerobic training for young athletes.
Cardio-respiratory function develops throughout childhood. Lung volume and peak flow rates steadily increase until full growth. For example, maximum ventilation increases from 40 L/min at five years to more than 110 L/min as an adult (Wilmore & Costill, 1994). This means that children have higher respiratory rates than adults, 60 breaths/min compared to 40 breaths/min for the equivalent level of exercise (Sharp, 1995). The ventilatory equivalent for oxygen is also higher in children, VE/V02=40 for an eight-year-old compared to 28 for an 18-year-old. This means that children have inferior pulmonary functions to adults.
Cardiovascular function is also different for children. They have a smaller heart chamber and lower volume than adults. This results in a lower stroke volume than adults, both at rest and during exercise. Chamber size and blood volume gradually increase to adult values with growth. Children compensate for the smaller stroke volume by having higher maximal heart rates than adults have. For a mid-teenager, max heart rate could be more than 215 beats/min compared to a 20-year-old whose max heart rate will be around 195-200 bpm (Sharp, 1995). However, the higher heart rates cannot fully compensate for the lower stroke volume and so children's cardiac output, measured in L/min, is lower than adults (Wilmore & Costill 1994). Children can compensate a little again, as their arterial-venous oxygen difference is greater. This suggests that a greater percentage of the cardiac output goes to the working muscles than in adults (Wilmore & Costill, 1994).
Because of the fact that lung and heart capacity increase with age, one would expect aerobic capacity to increase accordingly. This is true in absolute terms. V02max, measured in L/min, increases from 6 to 18 years for boys and from 6 to 14 for girls. However, when V02max is normalised by body weight, little change is observed with age in boys, and in girls, there is a slight decline after puberty. Therefore, relative to body weight, children have a Cardiorespiratory system for effective aerobic exercise. This is demonstrated by the fact that children can run quite well compared to adults. Indeed 10-year olds have completed marathons in very respectable times.
For the young athlete, an inferior V02max, expressed in L/kg/min does not limit running endurance performance. In fact, young pre-pubescent girls have an advantage before their relative body fat increases. Instead, endurance performance is limited by a poor running economy. This means that for a given pace a child requires higher oxygen consumption than an adult. Children have shorter limbs and a smaller muscle mass, resulting in a lower mechanical power. They have disproportionately long legs, meaning that they are biomechanically out of balance and potentially less coordinated. In addition, they have a greater surface area to mass ratio. All these factors reduce biomechanical efficiency. Physiologically, children have inferior cooling mechanisms, due to low blood volume and high skin temperature. They also expend more energy per kilogram of body weight. Children have a higher VE/V02 ratio due to their inferior lung function and they rely more on fat metabolism because of a lack of muscle glycogen and glycolytic enzymes.
All these factors reduce physiological efficiency. Combined, these biomechanical and physiological limitations lead to a reduced running economy, though this seems to improve with age from 8 to 20 years (Wilmore & Costill, 1994). Although they are biomechanically and physiologically inefficient, children rely heavily on aerobic metabolism for exercise. Sharp (1995) describes them as aerobic animals. The anaerobic capacity for both boys and girls increases with age but is not fully developed until around 20 years. The main reason for this is probably the lack of muscle mass. However, children also have less glycogen stored per gram of muscle along with less phosphofructokinase (PFK), an important glycolytic enzyme. They also have lower creatine phosphate stores per gram of muscle (Sharp, 1995). Children are thus unable to generate low blood pH or high blood lactate values that are associated with anaerobic work (Malina, 1991). This means that the natural fatigue mechanisms from intense work that adults possess do not exist with children. This, along with the fact that they tend to overheat more than adults, are the major risk factors that coaches need to be aware of when training young athletes at high intensities. For instance, on sprint interval training, while they may appear to be able to keep going in that they have not developed high acidosis, their muscles will still be fatigued, and they may be hot if it is warm weather or indoors.
As children are naturally more aerobic, it would be useful to know if aerobic capacity is trainable in them. Unfortunately, few studies have shown that aerobic capacity in children improves with aerobic training. However, Rowland (1992) argued that no study has been done that included all the following criteria: at least 12 weeks training, three times a week training, heart rate at least 160 bpm for at least 20 minutes, and using a large group plus matched controls. This would be the equivalent of an adult aerobic training programme in a well-controlled study. Rowland found in his study of children that, when adult-type training in terms of intensity was performed, V02max improved between 7 and 26%. This suggests that children can improve their aerobic fitness from a training programme of adult-like intensity.
The argument for doing this is probably valid. Sharp (1995) shows that, because of lower lactate production, the anaerobic threshold for children is normally at pulse rates around 165 to 170 bpm, similar to that of trained endurance adults. With sedentary adults, the anaerobic threshold will vary from 120 to 150 bpm. Thus, the optimal heart-rate-training stimulus may be relatively higher for sedentary children than for sedentary adults. Other evidence supporting the high-intensity stimulus theory is the fact that activity levels in children are not related to V02max (Rowland, 1992). While children may not be as active now as they were in the past, they are still as aerobically fit (Armstrong & Welsman, 1994). This shows that general activity does not provide a training stimulus and suggests that children have a natural fitness. Thus, to improve their natural fitness, a reasonably tough training programme is required.
It's useful for coaches to know that aerobic capacity is probably trainable in children with a sufficient training stimulus. This makes aerobic training worthwhile since it will improve their performance. However, the training effect will not be as great as is possible with adults because the lower stroke volume in children prior to full growth will limit the potential cardiac output increases with training. In addition, until after puberty, a poor running economy limits running endurance.
Thus, as before, it is probably best to wait until the young athlete reaches adolescence before starting tough aerobic training, as this is the age when the athlete will truly benefit. Tough anaerobic training is of even more limited use for children since they possess little anaerobic capacity. In my opinion, the most important areas of training for children are strength, speed, coordination, sport-specific skills, and agility. These are areas where improvements can be made through enhanced neuromuscular recruitment, laying down the skills for adulthood. As the nervous system develops, it seems that the potential for improvement in skills is the greatest. Training for aerobic and anaerobic endurance can be improved from adolescence when the body has reached its natural capacity and responses from this kind of metabolic training are greatest.
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