There are subsequently several areas of physical development that should be considered:
Physical development: height and weight
Growth rate in the first two years is considerable, with birth weight often doubling by 6 months and tripling by the end of the first year. However, since this rate of growth would be unsustainable, from this point to puberty it slows, averaging 2-3 inches in height and 6-7 pounds in weight each year (D.R. Shaffer, 1993).
Physical development: skeletal and muscular development
Skeletal development varies, with skull and hands maturing first and most structures in place by the age of 18. According to Tanner (1978) neonates are born with all the muscle cells they will ever have, with muscular development occurring in a cephalocaudal direction, that is, from top to toes.
Physical development: the brain and nervous system
The newborn's head is 70 per cent of adult size at birth, and then maturation and growth proceed in a cephalocaudal way, although this will be complemented by proximodistal development, where growth also extends from the centre of the body outwards. The brain is, however, only 25 per cent of its prospective adult weight, although by age 5 approximately 90 per cent weight has been achieved.
Neurons are vital for the development of the brain and nervous system. They develop through the process of mitosis, and are usually fully formed by birth, although as many as half of those produced early in life subsequently die (Janowsky & Finlay, 1986). The neurons also take on many functions in the early years due to their plasticity or capacity for change which will be shaped by environmental factors/experience. This was demonstrated by the findings of Riesen's (1950) study of chimps who were raised in the dark and it was found that the deprivation they had therefore experienced prevented neural and visual development, highlighting the role of experience in physical development. The nerve cells that nourish these neurons, insulate them and facilitate the transmission of neural impulses (glia), also develop during the prenatal period (myelinisation occurs) but then continue the process until at least 2 years of age (Tanner, 1978).
The brain stem and midbrain are mostly developed at birth and allow the child to display reflexive behaviours as basic biological functions. Subsequent development occurs in the cerebrum, most notably in the primary motor and primary sensory areas of the brain, such that motor development matures, as do senses such as sight, sound, smell and taste.
Recent theories would postulate that the left and right hemispheres of the brain assume different functions, such that the left engages in processes concerned with speech, hearing, verbal memory, decision-making and language while the right is more focused on visuo-spatial processing, sensation and emotion. Children will tend, however, to show lateral preferences.
Physical development: puberty
Physical development shows a significant change at puberty - defined as the point where sexual maturity is reached. In girls this is easier to define as it is marked by the first menstrual period (menarche) and increased breast size, while in boys it is more subtle and marked by growing penis and testicles, change in voice and, in both cases, growth of pubic hair. A growth spurt will also occur in both sexes, although this will generally be later in boys than girls and takes the form of increases in both height and weight.
Physical development is obviously a biological process of maturation, but it will also be influenced by factors such as nutrition, illness, emotional factors and stress, leading to failure to thrive.
> The key areas of physical development that undergo change involve those in height and weight, skeletal and muscular development, those to the brain and nervous system and the changes that occur at the time of puberty.
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