Which growth pattern explains why an infant’s head at birth is one-third the size of its body?

From IAAF "Introduction to Coaching Theory"

Children are not just smaller versions of adults. They have very particular needs and capabilities. One of the major issues in children's sport is a lack of knowledge on the part of coaches and parents about how children grow and develop. This ignorance places unrealistic expectations on the child and often causes them to give up the sport.
    Good coaches know and understand the many changes that take place from child to adult and structure their coaching to best suit the needs of the young athlete. In the following descriptions in this chapter it is assumed that children are receiving good levels of nutrition. Restricted nutrition and sickness will both affect the way in which a child grows and develops. This should be taken into account by the coach when deciding the needs of the child.
    There are clear stages that children pass through from birth to adult. These stages are the same for boys and girls, but girls generally mature before boys. This is clearly shown in the diagram below.

Physical Development

    Physical growth is obviously important to performance. We will start by looking at how the body changes during development. There are important changes in body size and proportions. These changes affect the way children can perform different skills and activities.

●     Rapid growth in infancy and early childhood

●     Slow, steady growth in middle childhood

●     Rapid growth during puberty

●     Gradual slowing down of growth in adolescence until adult height is reached

Both sexes are of a comparable shape and size during infancy and childhood.

Patterns of Growth - Changes in Proportions
    The physical proportions of the body at birth are very different from those of the adult. Some body parts grow more than others during development to reach the final adult proportions. The illustration shows the relative size of body parts at different ages.

    The head is proportionally large and the legs proportionally short during childhood. At birth the head is one quarter of the length of the body compared with about one sixth in the adult. The legs are about one third the length of the body at birth and one half in the adult. Because the body proportions change this means that not all of the body segments grow by the same amount.

Changes in the size and shape of the body are caused by different segments growing at different times.

These changes in body proportions will have a great influence on how skills will be performed. For example, changes in the relative size of the head in childhood affects the balance of the body during movement and the relative shortness of the legs in the very young limits running ability. At the beginning of puberty children have long arms and legs. They are better suited for running but the rapid growth may make them appear to be clumsy and to have difficulty in coordination.

Growth Spurts
    When the rate of growth increases rapidly it is called a growth spurt. The most important growth spurt is the one which occurs at puberty. This spurt produces a rapid increase in both weight and height
    The peak of this growth spurt occurs at about age 12 for girls and age 14 for boys. Before this growth spurt there are no important differences between boys and girls in weight and height. During growth spurts most of the child's energy is used for growing. Children will be easily tired and may not be able to keep up their usual volume or intensity of training. Light training will stimulate bodily growth if the child has enough energy.

Differences Between Boys and Girls
    The growth spurt and puberty occur at different ages for girls and boys. Girls usually start and finish the stages of puberty and adolescence earlier than boys. The characteristic differences between boys and girls occur at puberty in response to changes in hormones produced by the body. Typically, this results in broader shoulders and little change in hip width in boys and broader hips and little change in shoulder width in girls. These changes affect the way boys and girls move.
    Wider hips in the girls result in the thighs being angled more inwards which changes their running action. This may be very frustrating and difficult for the athlete to understand. Knowledgeable coaches prepare their female athletes before the changes at puberty. There may be a period for the athlete when there is little or no improvement in running performance. Once the running action has been adapted to the new body shape progress can be made. This period of adjustment can take up to two years. Patience and encouragement from the coach during -this time will be of most benefit to the young woman.
    The sexual development which happens at puberty can bring physical difficulties for adolescent children, as well as causing them mental and emotional preoccupation. Coaches need to be particularly understanding with girls when menstruation begins. This may, but should not, inhibit their participation in physical activity.
    Menstruation is a normal biological process that commences with the sexual development at puberty. Male coaches in particular should understand what is happening to the bodies of the female athletes they coach. At puberty girls start to produce mature eggs in their ovaries. They will notice this because each month they will lose a small amount of blood through the vagina. This menstruation is also known as a period and generally lasts for about five days. The illustration shows the changes that occur in the ovaries and womb during the menstrual cycle.

    An athlete should note any irregularities in the timing of her menstrual cycle and, as with any physiological irregularity, seek medical advice if necessary. A female's weight fluctuates naturally during her menstrual cycle and this may mean differences in the range of O.5Kg to 3Kg. Good communication between the coach, athlete and parents can help reduce anxiety over what is a natural, biological change in the body.

Early and Late Developers
    Each child develops at their own rate and some children develop earlier and some later than the average. For both boys and girls the age at the peak of the major growth spurt frequently occurs up to two years before or after the average age.

    There can easily be differences of four years in development between children of the same age. Thinking about growth stages and developmental age rather than age alone is perhaps one of the most important considerations when coaching young athletes. Early success may be due entirely to relative size and strength at the time. As other children catch up the early maturers may be left behind. On the other hand, the late developers are frequently overlooked if they are judged only on their performances.
 

Structure of the Body

    The changes in size and proportion are the easily observed signs of development. They are the result inside the body of changes to the skeleton. The skeleton of a child is mostly cartilage, which is softer than bone and can bend. The process by which cartilage becomes bone begins very early in life in special growth areas in the bones. These special growth areas are called growth plates.
    These growth areas in the bone are the weakest part of the bone. They can be easily injured by a sudden force or a repeated force. Mild forces can stimulate bone growth, but excessive forces can cause damage and have serious long term effects.
    Rebounding, repeated vigorous throwing and the use of weights should be avoided during periods of rapid growth. Once the body stops growing the growth regions become bone and are no longer weak areas.

Children and Exercise

    Children do not tolerate exercise as well as adults. They are much less aware of their real limits. Children do not breathe as slowly or as deeply as adults. The average six year old child breathes in 38 litres of air to get one litre of oxygen. The average 18 year old needs only to breathe 28 litres of air to get one litre of oxygen. This means that the younger the athlete the harder their bodies must work to provide the oxygen their muscles need.
    The body has three energy systems. Two of these are anaerobic, without oxygen, and the other is the aerobic system, using oxygen. Before adolescence children get a higher proportion of their energy from the aerobic system than adults do. In general, children are better at steady, extended exercises. Physical changes during and after puberty will improve their anaerobic abilities. The amount of this improvement will help decide what event or distance is best for a young athlete.

Implications for the coach

●     Think about growth stages rather than ages

●     Think how changes in physical proportions will affect performance

●     Help children understand the changes taking place in their bodies

●     Set standards of performance according to developmental age not chronological age

●     Group children according to physical development, using height and weight as a guide

●     Encourage skill learning for all your athletes, late developers could be very successful later

●     Don't use exercises which place excessive force on bone growth regions during periods of maximum growth

●     A void weights before adolescence

Recommended Resources UK

• Running: Biomechanics and Exercise Physiology in Practice
• Physiology of Sport and Exercise
• Essentials of Exercise Physiology
• Exercise Physiology: Nutrition, Energy and Human Performance

Recommended Resources USA

• Running: Biomechanics and Exercise Physiology in Practice
• Physiology of Sport and Exercise, Fourth Edition
• Essentials of Exercise Physiology
• Exercise Physiology, International Edition: Nutrition, Energy, and Human Performance

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