Raphael Brandon explores the benefits of endurance training for young athletes.
The science of developmental physiology can supply answers to certain important questions regarding the training of children. One such question is: should children perform adult-type endurance training in reduced quantities, or should they be performing a different type of training that is tailored to their physiology? Science suggests the latter is true and that the type and intensity of training that is most effective for developing endurance in the young will be different from that used by adults. In this article I want to discuss some of the evidence that points to this. The average adult model for endurance training involves an intensity of 75% of max heart rate maintained for 20 to 30 minutes. If this is performed 3 to 5 times a week, then the average adult can expect a 25% improvement in VO2 max. Both an increase in stroke volume and an improvement in O2 respiration and metabolism in the working muscles due to increased capillaries, mitochondria and enzyme activity cause this improvement in fitness.
Lower in children
Several training studies have been carried out on children to find out what effect a cardiovascular (CV) training programme will have on fitness levels. In general, the research shows that if children follow a 3 to 5 times a week routine of at least 20 minutes continuous activity for 12 weeks, then improvements in VO2 max of 7 to 26% is possible. On average, though, and the results of some of the better-controlled experiments support this, a child can expect a 10% improvement in VO2 max after following an 'adult-like' CV training programme. The consensus from the research is that children can improve their aerobic fitness but not to the same degree as adults, when following a similar training programme.
Why is this so?
Some scientists have hypothesised that the reason for this diminished training effect in children is that a 'hormonal trigger' exists which limits CV trainability until puberty.
It seems reasonable that until growth hormone levels, such as testosterone, rise, then increasing the size of the heart through endurance training may be limited, just as increasing the size of the muscles through strength training is limited until post-puberty. A child's heart is smaller than an adult's and does not achieve its natural full size until full height is reached.
Thus, stroke volume, which is the amount of blood the heart can pump with one beat, is lower in children and it may be that this limits any further improvements in VO2 max.
Evidence to support the theory that immaturity limits trainability can be obtained from observations of elite endurance-trained children. It is rare that an elite child athlete has a VO2 max greater than 65 ml/kg/min. compared to elite adults who can achieve VO2 max scores above 80 ml/kg/min. This suggests that even with well-trained individuals there is a ceiling on possible improvements. In addition, studies analysing VO2 max development in young endurance-trained athletes have shown that they benefit from a jump in VO2 max levels around puberty of sometimes as much as 10 points. This observed hike in fitness levels supports the idea that puberty is a crucial time for the trainability of VO2 max. Another factor that could explain the diminished training effect in children is that the pre-training status of the average child is higher than the pre-training status of the average adult. Children have VO2 max scores of around 40 to 50 ml/kg/min whereas the untrained adult scores in the 35 to 40 ml/kg/min range. Children are naturally fit and will remain fit independent of their activity levels until 14 years in girls and 18 years in boys. Thereafter, CV training is required to maintain fitness. Thus, it seems logical that if children have higher fitness levels than adults to start with, they will gain fewer benefits when following the average 'adult' CV training plan.
Train at a higher level?
Research has proved that for adults who have been training consistently for a long period and whose fitness levels are already high, the basic level of endurance training (3 x week, 20 minutes, HR 75% max) will not bring about any further improvements. This is why elite endurance athletes build up to training 10 to 14 times a week and use high-intensity interval training at maximal heart rates alongside the moderate-intensity continuous training. By extension, the same may be true for children and that to significantly improve their already good 'natural' fitness they may need to train to a higher level than the average adult model prescribes. Related to this idea that children may need to train quite hard to improve their VO2 max is a third factor which may explain the reduced trainability of children observed in the research. This is the fact that children have higher Anaerobic Thresholds (AT) than adults do and therefore may require higher intensities of cardiovascular training for optimum benefits. It is accepted that training at one's AT when performing continuous training is potentially the best intensity for fitness benefits because it is the maximum intensity that one can maintain before lactate starts to accumulate. The average adult will have an AT around 75% of max heart rate, but research has shown that children's AT is around 85% of max heart rate, suggesting that higher intensity training will be more appropriate for children. If we assume that a child's maximum heart rate is 205 bpm, then the optimum training heart rate for continuous CV training will be 174 bpm (205 x 0.85) which is considerably higher than the rate normally recommended for the average adult.
Children burn more fat
One of the major physiological differences between adults and children is that between aerobic and anaerobic metabolism. Children have limited anaerobic glycolysis capabilities until post-puberty because they have much lower glycolytic enzyme activity. For example, Eriksson et al. (1973) in their famous study showed that 11 to 13 year-old boys have at least half the PFK enzyme activity of an adult. This means that children cannot produce as much energy through anaerobic glycolysis and rely more on aerobic metabolism. To aid this, children have greater aerobic enzyme activity than adults and burn a greater proportion of fats during aerobic exercise. Because children are naturally aerobic and are good fat-burners, it thus makes sense that higher intensity training which taxes the glycolytic system, rather than the fatty acid system, would be more useful, since this is the physiological area in which children are limited.
Eriksson et al. (1973) showed that high-intensity endurance training can significantly increase the PFK enzyme activity and the peak lactate response to exercise in children, which suggests that the anaerobic glycolysis function can improve with training. Arguably, improvements in aerobic capacity rely on the development of anaerobic metabolism, since anaerobic glycolysis is the starting point for aerobic glycolysis. Glycogen is first broken down into pyruvate via anaerobic glycolysis and then, with sufficient oxygen present, the pyruvate enters the Kreb's cycle to be burned in the mitochondria. In this way, anaerobic and aerobic metabolisms are inextricably linked and the aerobic metabolism of glycogen, which is the most efficient and important fuel for endurance performance, cannot improve until anaerobic glycolysis develops. To support this argument, research shows that in pre-pubertal children anaerobic power, measured on the Wingate test, and aerobic power, measured with a VO2 max test, is highly correlated. This suggests that, at a young age, the two systems are related and possibly dependent on each other.
In simple terms, all this physiological discussion relates to the fact that the most effective endurance training for children will involve high heart rates achieving at least AT. The goal should be to challenge the glycogen-burning capabilities and recruit the Type IIa fibres in the child. It may also be possible that, pre-puberty, one could improve endurance performance by using only one short burst of anaerobic-type training, although it is impossible to say whether this type of training alone would be sufficient. The condensed version is this: on average, pre-puberty children can improve their VO2 max but not as much as adults can, after following a normal CV training plan (3 to 5 x week, 75% max HR, 20 minutes). After puberty it seems that greater improvements in VO2 max are possible and this may be related to sexual maturation or simply the child's limited cardiac output. It is also possible that children benefit less from adult-type CV training because they have higher initial fitness. Children, pre-puberty, seem to be naturally quite fit, and so CV training at this stage is not necessarily a priority. During and after puberty, when the benefits from training are greater, may be the most appropriate time to start serious endurance training.
One could argue that anaerobic short-burst interval training may be more beneficial for pre-pubertal children, as they can benefit greatly from this type of training via improvements in anaerobic glycolysis, which is limited at a young age. The most effective kind of endurance training for children will be high-intensity continuous or interval training, where heart rates reach AT and above. It is likely that children have an AT around 85% of max HR and, for elite young endurance athletes, it may be higher still.
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