PERFORMANCE PHYSIOLOGY

adaptations of their athletes. It is this type of information that is needed to be shared between coaches, sports scientists and physiotherapists.

One future area of research is the conse- quences of prolonged exercise on the lower muscle glycogen stores of children and whether this leads to relatively earlier depletion than seen in adults during exer- cise (13). Linked to this potential research is the suggestion that children have a greater utilisation of fat stores during exer- cise (possibly as a consequence of lower glycogen stores) (14,15).

Anaerobic or sprint type training Physiological adaptations due to anaerobic or sprint type training are mainly confined to the skeletal muscle and nervous system and have not been studied extensively in children. This is mostly due to method- ological reasons because studies require invasive techniques such as a muscle biopsy. Although this technique has been used on children, firstly by Eriksson in the 1970s (16) and later by Fournier (17), Haralambie (18) and Glenmark (19), the results have been equivocal. There is some evidence that the enzyme profile begins to change during puberty but whether this is a more growth mediated or hormonal change, or both is still unclear. It has been shown that profound changes to testosterone levels in boys occurs during the pubertal growth spurt but other hor- mones such as growth hormone (GH), insulin-like growth factor 1 (IGF-1) cannot also be ignored, however relating the increase to improvements in performance is difficult to demonstrate.

Invasive methods have shown ATP and CP levels to be similar at rest compared to adults, however newer non-invasive methods using magnetic resonance spectroscopy (MRS) has shown some differences in children compared to adults during intensive exercise (unpublished data, CHERC). Although short duration sprint exercise (<10 seconds) appears to be easily accommodated by children, longer sprint exercise between 30 and 180 seconds appears to be limited compared to adults. Recent work from our laboratory has also attempted to link the fatigue of anaerobic metabolism and recovery from sprint exercise (20). Although it is assumed children require a shorter rest period than adults when participating in

10 TABLE 2. BASES RECOMMENDATIONS FOR RESISTANCE TRAINING (BASES 2004)

1. All young people should be encouraged to participate in safe and effective resistance exercise at least twice a week.

2. Resistance exercise should be part of a balanced exercise and physical education programme.

maximal sprint type exercise, the method- ologies designed to examine this question have not addressed the issue of unequal amounts of work done between children and adults.

Strength training Strength training has remained a contro- versial topic in relation to children and training, mainly due to concerns about the potential for injury. The initial consensus was that children were not able to adapt to the strength training stimuli. The design of experiments examining strength training in children has suffered similar methodologi- cal issues as outlined earlier. One of the key pieces of evidence that was used to suggest that children could not benefit from strength training was a lack of hyper- trophy. Although strength often improved, it was postulated that this was due to an improvement in the nervous system. More recent work in this area has concluded that relative strength (expressed as a percent- age change pre to post training values) is as trainable in adults and children. To date, the differences between adults and children in terms of the absolute strength gains have been hypothesised to be due to muscle mass differences.

More research work is required to differen- tiate the neurological and the hypertrophy effects due to training, as this appears to be a key difference between children and adult responses to training. It is not clear whether prepubertal children lack the abil- ity to sufficiently recruit all their muscles and therefore gain maximal activation from the training overload. Alternatively, neuro- logical developments might take place with age which are then superimposed onto the increases in muscle mass and hormonal influences to substantially increase strength and effect performance. The British Association of Sport and Exercise Sciences has recently published guidelines for resistance exercise in young people (21). BASES advocate that providing resis- tance exercise is developmentally appropri- ate, carefully supervised and taught posi- tively for both sport and health reasons, it can be beneficial to youngsters.

INJURIES As children and adolescents are participat- ing in intensive sports at younger and younger ages, there are concerns about the increased risk of injury. However, the pub- lished literature does not support such a proposition. Case study reports and anec- dotal information are common but a much more thorough approach is warranted to track these young athletes. Injury studies have tended to conclude that injury rates are similar in adults and younger groups (22) and similar between male and female young athletes (23) but surveillance injury methodologies do vary. However, it is acknowledged that the causes of the injuries between adults and children differ.

Due to the differences in the growing bone of young athletes and adult bone, epiphy- seal fractures or avulsion injuries are more common than ligamentous damage (24). Similarly, as the metaphysic of the long bone is more resilient and elastic than in adult bone, greenstick fractures are more common. Excessive repetitive sporting movements can lead to subchondral stress fractures, as even though the articular car- tilage of growing bones in children is of a greater depth than adults it can undergo remodelling. These fractures can regress to osteochondrosis dissecans in the articular cartilage of the elbow, knee and ankle if untreated. The other area of concern is the relatively weak tendon bone attachment which if over exerted can lead to apophysi- tis eg. Osgood-Schlatters or Severs disease. Factors which contribute to the incidence of apophyseal injuries, have been identi- fied; including weakened growth cartilages compared to the supporting muscle-tendon attachment, poor flexibility and increased tracking on the tendon insertion sites (25). The American College of Sports Medicine has suggested that many of the injuries diagnosed in adolescence are preventable and advocate pre-participation screening and regular visits to sports medicine personnel (26). However, before this strategy is embarked upon, sports scientists must make a more unified attempt to quantify and monitor the training volume at the time of rapid growth

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