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Effects of intensive training on growth and maturation are not established
Parents, coaches, sport administrators, healthcare professionals, and the broader public have been alarmed by reports that intensive physical training in female athletes, initiated at young ages, may delay subsequent growth and maturation, and perhaps even reduce final adult stature.
GROWTH, MATURATION, AND DEVELOPMENT
Whereas growth specifically refers to the increase in the size of the body as a whole, and of its parts, maturation refers to progress towards the biologically mature state. Maturation differs from growth in that, although various biological systems mature at different rates, all individuals reach the same end point, becoming fully mature. Maturation therefore has two components, timing and tempo. Development refers to the acquisition of behavioral competence and is culture specific. Growth, maturation, and development occur simultaneously and interact. Growth and maturation are characterised by individual variation and, although under genetic and neuroendocrine control, environmental factors, including sport, may also have an influence.1 Our understanding of the effect that sports training has on the growing child is limited because of the difficulty in distinguishing the independent effects of training from those of normal growth.2 Only when a child is repeatedly measured from childhood through to adolescence can independent effects be identified. To date, there are limited numbers of such longitudinal studies, and hence most of our knowledge has been gained from cross sectional studies. The cross sectional nature of such studies obviously has made inferences that training delays puberty or reduces adult stature problematic.
On average, young female athletes from most sports have statures that equal or exceed the median for the normal population. Female basketball players, volleyball players, tennis players, rowers, and swimmers have been to shown to have mean statures above the 50th centile of the reference populations from 10 years onwards.3 However, gymnasts consistently present mean values below the 50th centile, with a secular trend for decreased stature: today's elite female gymnasts are, on average, shorter than the gymnasts of 20 years ago.4 Figure skaters also have shorter statures. Ballet dancers tend to have shorter statures during childhood and early adolescence, but catch up with non-dancers in late adolescence.3
On average, young female athletes from most sports have statures that equal or exceed the median for the normal population.
In general, female athletes tend to have body masses that equal or exceed the reference medians. Gymnasts, figure skaters, and ballet dancers consistently have lighter body mass. However, gymnasts and figure skaters have appropriate body mass for their height, whereas ballet dancers and distance runners have low body mass for their height. Although female athletes from a number of sports tend to be heavier than reference populations, they also, in general, have lower percentage body fat.
PHYSIQUE OF YOUNG ATHLETES
Successful early adolescent and adolescent athletes (about 12–18 years of age) tend to have, on average, somatotypes similar to adult athletes in their respective sports.5 Compared with adult female athletes, young female athletes tend to be less endomorphic, less mesomorphic, and more ectomorphic. The latter component reflects the role of growth in the transition from late adolescence into young adulthood. Physique is a significant contributor to success in many sports, and may be of particular importance in aesthetic sports such as gymnastics, figure skating, and diving, where performance scores may be influenced by how the judges perceive the athlete's physique.6
MATURATION OF YOUNG ATHLETES
Maturity differences among young female athletes are most apparent during the transition from childhood to adolescence, and particularly during the adolescent growth spurt. During childhood, the skeletal ages of gymnasts are average or on time for chronological age. As they enter adolescence, most are classified as average and late maturing, with few early maturing girls. In later adolescence, most gymnasts are classified as late maturing.3 Gymnasts and ballet dancers tend to attain menarche later than the normal population and girls in other sports.7 Early and average maturing girls are systematically represented less among gymnasts as girls pass from childhood through adolescence, probably reflecting the selection criteria of the sport, and perhaps the performance advantage of later maturing girls in gymnastics activities. Ballet dancers and distance runners show a similar maturity gradient in adolescence. In contrast, young female swimmers tend to have skeletal ages that are average or advanced in childhood and adolescence.3
STATURE AND REGULAR TRAINING
The smaller size of elite gymnasts is evident long before any systematic training starts8 and is in part familial. In our own studies we have found that gymnasts have parents who are shorter than average.9 There is also a size difference between those who persist in the sport and those who drop out.10 Female athletes in volleyball, diving, distance running, and basketball show rates of growth in height that, on average, closely approximate rates observed in non-athletic children,3 which are well within the range of normally expected variation among youth.11 Most recent studies have found no evidence to suggest training causes changes in anthropometric variables.12–,15 Available data also indicate no effect of sport training on the age at peak height velocity or the growth rate of height during the adolescent spurt.3 Data are insufficient to warrant that intensive training may delay the timing of the growth spurt and stunt the growth spurt in female gymnasts. Many confounding factors are not considered, especially the rigorous selection criteria for gymnastics, marginal diets, short parents, and so on. Female gymnasts, as a group, show the growth and maturation characteristics of short, normal, slow maturing children with short parents. Although we believe that training does not compromise adult stature, others suggest an opposite view. A short term longitudinal study in which the adult stature of gymnasts and swimmers were predicted concluded that gymnasts were failing to obtain full familial height.16 However, decreasing predicted adult height during puberty is a characteristic of slow or late maturation, confirmed by the late onset of menarche in these subjects.16 Other studies of gymnasts, over longer time periods, have also observed lags in adolescent growth but then report subsequent catch up growth.2, 10 Lindholm's group17 found that six of 21 Swedish gymnasts studied over a five year period ended up 3.5–7.5 cm shorter than their predicted adult stature. Final height was predicted from parental heights. Although this protocol gives a target adult height, one would expect 95% of daughters to fall within a 9 cm range of this value. Thus, the final heights of these Swedish gymnasts17 are, in fact, well within the range of variation expected with the stature prediction method used.
In contrast with height, body mass can be influenced by regular training for sport, resulting in changes in body composition. Reduced skinfold levels have been observed in growing female athletes and are dependent on continued, regular activity or training, or continued energy restriction. This situation often occurs in sports such as gymnastics, ballet, figure skating, and diving. However, it is difficult to separate specific effects of training on fat mass from expected changes that occur with normal growth and sexual maturation during adolescence.
The limited longitudinal data for girls active in sport compared with non-athletic girls indicate no effect of training on the timing and progress of secondary sexual characteristics (development of breast and pubic hair).18 The interval between ages at peak height velocity and menarche (1.2–1.5 years) for girls active in sport and non-active girls also does not differ, and is similar to that of non-athletic girls.12 Most discussions of the potential influence of training on sexual maturation have focused on the later mean age at menarche, often observed in female athletes.2, 3 Typically, training for sport was indicated as the factor responsible for this finding, with the inference that training “delayed” menarcheal onset. Unfortunately, most studies of athletes do not consider other confounding factors known to influence menarche.7 Thus, given the many factors known to influence menarche, sport training per se has yet to be proven beyond reasonable doubt to be the causative factor for later menarche in female athletes.
Concerns have centred on the suggestion that intensive training causes growth retardation and pubertal delay in female athletes, specifically gymnasts. Interestingly, male gymnasts also have consistently short statures and late maturation, but these trends are not attributed to intensive training.3, 9 From our review of the literature, training does not appear to affect growth and maturation. It is more likely that young athletes select themselves, or are selected by coaches and sport systems, into their specific sports. Therefore, in general, the differences observed in stature between athletes and non-athletes are mainly the result of nature rather than nurture. With regard to pubertal development, the evidence suggests that the tempo is slowed down in some sports, but it has not yet been possible to identify whether this is an effect of nature or nurture. To clearly show that intensive training is a factor, future studies must be longitudinal, and be able to partition constitutional factors and the other components of the sport environment of female athletes before causality can be established.