THE GROWTH OF PHYSICAL CHARACTERISTICS IN MALE AND FEMALE CHILDREN
Borms, J. (1986). The child and exercise: an overview. Journal of Sports Sciences, 4, 3-20.
" . . . there is an increasing awareness and concern on the part of parents and educationalists about the possible harmful effects on children who participate at a progressively younger age and with ever-increasing intensity in sports competitions designed by and for adults." (p. 3)
Exercise and Growth
Boys and girls differ in stature. Girls experience their adolescent growth spurt and peak height velocity on average about two years earlier than boys. The growth spurt of boys lasts longer and is somewhat more intensive than in girls. Subsequently, boys tend to catch up and then pass the growth period of girls.
In any random sample, there is a remarkable range in body sizes in both sexes.
Still tenable hypothesis: Epiphyseal growth may be stimulated by physical activity to an optimal length but excessive and prolonged pressure can retard linear growth.
There is no convincing evidence to support the view that regular and natural exercise promotes an increase in body size.
There have been no studies in children of effects of training on bone growth and its mineral contents although exercise does promote these factors (actually reverses demineralization) in adults.
Aerobic power increases with age during childhood in both sexes and is quite similar. Girls hardly differ from boys in the prepubertal period but, from the age of 14 years on their aerobic power is significantly lower by about 15%. The maximal aerobic performance capacity in girls reaches a plateau from 14 years onwards while in boys it increases up to the age of 18 years. Thus, even though the aerobic capacity is fully developed aerobic performance continues to improve. That is because other growth factors, such as larger levers, greater musculature, etc. are still developing and govern the effectiveness and mechanical efficiency of aerobic activities.
The potential effect of endurance training programs on VO2max is not consistently shown in studies involving children. Endurance training has been shown not to effect aerobic capacity before 11 years. After the age of 12, an improvement in VO2max has been shown in males, particularly swimmers. This suggests that there is an increased trainability of the heart and circulatory system around puberty in males. However, studies at the International Center for Aquatic Research in Colorado Springs have shown that swimmers' aerobic capacity reaches its ceiling level at the time of onset of the adolescent growth spurt.
It takes a lot of intense aerobic training to produce shifts in aerobic factors in children. The apparently high threshold of a stimulus for training effects on VO2max in children is probably related to their naturally active lives. The stresses induced by short-term training are probably small when placed in the context of the overall activities of children. VO2max improvements are similar to those reported for adults when the training volumes and intensities are very high.
VO2max training effects are larger in swimmers probably because it is an unnatural and specific activity (the starting point is much lower than those of other everyday activities).
Short-term training programs (such as in schools) probably should not even consider improving endurance in pre-pubertal children.
Boys and girls 7.6 to 10.3 years have shown a significant improvement in running performance (up to 18%) but without an obvious increase in VO2max. The improvements are probably due to motor coordination and running technique. This suggests that if VO2max is the only criterion for aerobic fitness it may be misleading.
Implication. In pre-pubertal children, the gains from endurance training will largely result from improvements in mechanical efficiency NOT a large change in physical aerobic power. Thus, for endurance improvements, an emphasis on the techniques of performance is more beneficial than the programming of assumed physiological stimulations of training.
There is no difference between children and non-trained adults relative to physical capacity at the anaerobic threshold.
In children, cardiovascular adaptation is efficient and similar to adults; muscle structure is identical to adults; and glycogen storage mechanics and values are similar to adults.
Unlike aerobic capacity, the anaerobic capacity of children expressed per Kg of body weight is much smaller than adults. It is lowest in children and increases progressively with age in both boys and girls.
Little to nothing is known about the trainability of anaerobic capacity in children.
The ratio of aerobic:anaerobic metabolism contribution to exercise differs between children and adults.
"Most researchers agree that a paced 3000-metre run . . . . is less strenuous for children than a vigorous 200-800-metre run . . . . instructions on running speed must be given, otherwise both a 3000-meter run and a 200-800-metre run may be equally strenuous since lactate accumulation depends, among other things, upon running intensity. Even longer distance runs (e.g., 30-min duration) for children are more justified than vigorous short sprints as they may also lead to the maintenance of optimal body composition . . . . it is not the duration but the intensity of the effort which could prove harmful." (p. 9)
Implication: Children are endurance animals and are best suited to adapt to aerobic exercises. Frequent and stressful stimulation of anaerobic metabolism will be particularly fatiguing and if overdone, could be harmful. Children will fatigue rapidly in anaerobic work when compared to their response to endurance work. The major content of swimming program for children should be "distance" work at a comfortable level (anaerobic threshold and lower) with an obvious concentration on skill, smoothness, and mechanical efficiency.
"In the prepubescent age, muscle weight is about 27% of the total body weight and the effect of training on muscle hypertrophy is small so that strength gains are perhaps more the result of an improvement in coordination . . . . After sexual maturation [the onset of the adolescent growth spurt], muscular development is influenced by androgenic hormones and the percentage of muscle weight then increases to over 40%." (p. 9)
Since the increase in testosterone production in adolescent children is markedly higher in boys than girls, boys will become stronger faster and to a higher degree.
Implication: If strength training is to be done with pre-pubescent children, exercises should involve submaximal resistance, such as one's own body weight, light dumbbells, or medicine balls. Sophisticated and restrictive weight exercises, particular on machines, are useless for strength-limited children. General, whole-body activities are more important and beneficial than the same exercises used for post-pubescent athletes.
" . . . a yearly increase in sprint velocity has been noted from age 5 years until age 16 years for boys, and until age 13 to 15 years for girls. The rate of development of speed seems to accelerate in two phases. A first phase occurs around 8 years of age, both in boys and girls . . . Probable reasons for this are the development of the nervous system and improved coordination of arm and leg muscles. A great variety of exercises involving the whole body should be offered to children to stimulate improvement of this ability. A second phase . . . occurs around 12 years of age for girls and between 12 and 15 years for boys . . . related to the increase in body size with age and the concomitant increase in muscular strength, power, and endurance . . . slightly higher performance levels for boys than for girls until the onset of adolescence when the differences favoring the boys becomes more marked." (p. 10)
There is a gradual increase in flexibility with age as measured on the sit-and-reach test. However, generalization is not clear because of the absence of studies and data that take into account growth spurts and anthropometrical size changes (e.g., longer arms produce a better sit-and-reach measure).
Coordination and Skill Learning
Most authors agree that the sensitive skill learning period is between 9 and 12 years. Very early training may produce learning of a less economical nature. Later starters would soon catch up. One must not confuse performance with skill. Early maturers will compensate, usually advantageously, for lack of skill with strength and leverage.
Implication: Up to the age of 8, children should enjoy a variety of stimulating activities to develop a general base of physical and movement aptitudes. From then on, more detailed instruction in particular skills can be entertained but against a background of general stimulation. It has been shown that, in general, children who specialize early will lack the "background" development of capacities for flexible maximum responses in the later years, and higher performance categories, of participation.
" . . . early maturation in boys is an advantage in some sports, but the opposite applies in girls . . . there is an apparent delay in maturity in sports where females who maintain preadolescent physique seem to have an advantage. An ordering of sports on a continuum from participants demonstrating early maturation through to late maturation might be as follows: alpine skiing, field events, swimming, synchronized swimming, track events, diving, figure skating, gymnastics . . . " (p. 12)
Successful female athletes display physical characteristics that favor good performances (more mesomorph, less ectomorph); successful young female athletes have similar somatotypes to older successful athletes.
" . . . There is a trend towards increase linearity in these athletes and this linear physique characterizes the physical attributes of late maturing girls." (Carter, J. E. (1981). Somatotypes of female athletes. In J. Borms, M. Hebbelinck, & A. Venerando, (Eds.), The female athlete. Basel: Karger).
Early maturing girls undergo a socialization process which does not motivate them any more to excel in physical exercise. On the other hand, late-maturing girls tend to be socialized into sports participation. Late-maturing girls are older chronologically when they attain menarche and have not yet experienced the social pressures regarding competitive athletics for girls and/or are more able to cope with the social pressures.
Children and Adults
The child differs in some aspects from the adult and is comparable in others. The training principles appropriate for both groups are generally different. That is because not only are developmental factors different, but so are the skill/experiences that are taken into each participation realm. Intensive training to acquire specialized sports skills at too early an age has more disadvantages than advantages. Early specialization is by definition achieved at the expense of developing a broader base of fundamental movement skills such as balance, agility, and coordination, and usually occurs at the expense of learning other sports.
Early specialization, in a sense, produces the physical equivalent of a specialist who has little competency outside of the specialty.
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