The Skeletal System provides the framework and structure for the body. It also provides protection for the vital organs. There are two other lesser known functions of the skeletal system. First, bones store calcium. Second, bones produce red blood cells. It's important to teach children that their bones are living tissue that have more uses than simply holding us together.
Bone tissue changes constantly, with bits of old bone being removed and replaced by new bone. A child has approximately 300 bones at birth. As the child grows, the bones begin to fuse together (ossify) leaving an adult with only 206 bones. Each specific child's skeleton will fuse and develop at an individual rate. So if someone asks how many bones an 8 year old has, the answer is, "between 206 and 300 bones". The bones stop ossifying at around 25 years old. At this age, growth is normally complete and the human body has reached its maximum height. The bone mass (or strength and density of bone tissue) in the skeleton reaches maximum density around age 30. This is referred to as Peak Bone Mass. This is important because the amount of stress and level of activity a child has will forever alter their body's structure and stability.
Genetic factors account for 60-75% of a child's capability, BUT physical activity that stresses bones (along with nutritional sufficiency, of course) has an effect on peak bone mass. Increasing peak bone mass in childhood reduces future fracture risk by 50%. Not only that, but bone mass acquired at the end of the growth period (age 16-25) appears to be the most important factor to offset bone loss occurring during adult life. In a study, it was calculated that an increase in peak bone mass of 10% would delay the onset of osteoporosis by 13 years! In comparison, a 10% increase at the age of menopause would only delay the onset of osteoporosis by 2 years. Peak bone mass could be the single most important factor for the prevention of osteoporosis later in life. Also note that before puberty, boys and girls acquire bone mass at similar rates. After puberty, however, boys tend to acquire greater bone mass.
Time of peak bone mass attainment. In adolescent females, gain in bone mass declines rapidly after menarche (the first occurrence of menstruation); no further statistical gains are observed 2 years later at least in sites such as the lumbar spine or femoral neck. In adolescent males, the gain in bone mass that is accelerated particularly from 13-17 years declines markedly thereafter, although it remains significant between 17-20 yrs in both lumbar spine bone mass and in midfemoral shaft bone mass; in contrast, no significant increase is observed for femoral neck bone mass. In subjects who reached puberty and grew less than 1 cm/yr, a significant bone mass gain persisted in males but not in females. This suggests the existence of an important sex difference in the magnitude and/or duration of the so-called "consolidation" phenomenon that contributes to peak bone mass. - Study performed by Jean-Philippe Bonjour, MD; Thierry Chevalley, MD; Serge Ferrari, MD; René Rizzoli, MD
So how do we build peak bone mass?
Weight bearing yoga poses generate compression forces which lead to bigger bones and greater bone mass that lasts for life. Running, jumping and dancing are essential for children. These stimulate bone mineral acquisition because of the resistance of gravity and ground reactive forces on the bones. We'll go into more detail on how bone is built later in the training. For now, note that resistance training for children helps them to put down calcium in the bones and produces a healthier blood fat/protein profile.
Similar to the Skeletal System, the Muscular System has multiple functions. The most obvious is it creates movement as we contract eccentrically and concentrically. In addition, the muscular system uses energy which generates heat in the system. It also maintains posture.
Muscle structure is determined at birth by genetic inheritance. And the number of muscle fibers is fixed at birth, but as they are used they grow in size. As muscles mature their ability to contract is more efficient. In other words, as muscle size increases so does strength.
Children recover more quickly than adults. They have less muscle mass so they generate less power in exercise and have less muscle fiber fatigue. They have better carbohydrate metabolism as well, so they generate less muscle activity by-products, such as lactic acid, and remove them faster. Muscles and tendons lengthen proportionally during maturation. So as the bones grow, so do the muscles. Thus the muscle-tendon stiffness and range are similar in children and adults. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass). Children have 28% muscle mass and reach similar muscle mass percentages to adults in their late teens.
Children's muscle tends to use more fat as fuel than carbohydrates. However, for the same amount of energy, approximately 10% more oxygen is needed, so it is a less efficient fuel. Children also tend to utilize glycogen or glucose (carbohydrates) more slowly the younger they are. Thus the younger the child, the less efficient they are aerobically. And the lengths of younger children's limbs are not completely in 'kinetic balance' with their muscles.
Children do not grow at an even rate, but in a series of stops and starts, by far the greatest of which is the growth spurt at puberty. The growth increase typically starts between ages 10 and 12 in girls, and between 12 and 14 in boys, although in both it may start even later, or occasionally earlier.
Growth results from the lengthening of bones regulated by the Growth Hormone (GH), also known as somatotropin, which is secreted by the pituitary gland. The bulk of secretion occurs in bursts with the largest during sleep. The majority of this growth occurs as growth of cartilage at the ends of the long bones (note, legs) which gradually ossifies to form hard bone. The legs and hips compose approximately half of adult human height.
In addition to increasing height in children, the Growth Hormone (GH) has many other effects on the body such as:
- Increasing calcium retention
- Strengthening and increasing the mineralization of bone
- Increasing muscle mass
- Stimulating the growth of all internal organs excluding the brain
- Stimulating the immune system
In addition to changes in height, adolescents also experience a significant increase in weight during growth spurts. The weight gained during adolescence constitutes nearly half of one's adult body weight. The accelerated growth in different body parts happens at different times. The first places to grow are the extremities... the head, hands and feet... followed by the arms and legs, then the torso and shoulders. This non-uniform growth is one reason why an adolescent body may seem out of proportion and "clumsy".
Development during puberty also affects circulatory and respiratory systems as heart and lungs increase in both size and capacity. These changes lead to increased strength and tolerance for exercise and decreased flexibility if a regular stretching routine is not maintained. Go go yoga!