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MATURATION OF THE NEUROMUSCULAR SYSTEM
Dendritic proliferation, multiplication of synaptic bonds and axon myelinization characterize the development of nerve tissue during growth and maturation. While dendritic proliferation occurs during embryo growth, the more gradual myelinization takes place during infancy and childhood. Cortical myelinization occurs during the first months and years of life; the myelinization of motoneuron axons (which command remote muscular activities) continues into pre-puberty.
The multiplication of synaptic bonds and the establishment of “nerve circuits” confer the command system´s great “plasticity”. Both are highly dependent on the quantity and quality of neuromotor stimuli encountered by the child, who is equipped for developing fine neoromotor skills from a very early age. Fine motor skills requiring rapid, highly precise and specialized movements can only reach their full effectiveness when synaptic maturation, nerve fiber myelinization, neuromuscular bonds and coordination are at a fully mature stage, around the age of 6 or 7 (box 1).
The role that parents, day care and nursery school workers play in the acquiring and mastering of basic motor skills appears fundamental.
Other factors that come into play include genotype and every child´s specific maturity level, which can explain the commonly observed differences between individuals.
Age six, when the child enters first grade, is also the age of preparation for the future development of motor skills: the child becomes increasingly more capable of conceptualizing, memorizing, anticipating and retroactively controlling his or her movements. As soon as the “gross motor skills program” is established, the sole psychomotor learning must gradually make room for multiple types of learning.
If a child is not successful in one type of learning, it is probable that his or her nerve and/or muscle structures did not yet attain the necessary level of maturity. In this case, it is useless to start too early, but once the favorable period has passed, it will be more difficult for the child to learn.
Multiple learning and the practice of many different physical activities between the ages of 6 and 11 provide children with harmonious growth and provide a choice for their future activities. At this level, adults should help ensure that exposure is as pertinent as possible (box 2). It would be a shame to restrict this great “neuromotor plasticity” to early specialization.
Even in view of developing a single physical or athletic activity, it is advisable to introduce children to the widest possible range of learning situations. As several studies have shown, a child motivated to pursue a physical activity or sport will probably have more of a chance to avoid sedentariness and all its harmful consequences in adulthood!
OBESE CHILDREN
To date, there is no epidemiological data for obese children linking the presence of metabolic syndrome to cardiovascular risk.
However, by using the definition of NCEP ATP III adapted to pediatrics, the prevalence of metabolic syndrome in American teenagers was observed to be nearing 5%. In cases of obesity, it rose to 20%, and with severe obesity even 50%. By using the same definition and adapting it to French reference values, the prevalence of metabolic syndrome was nearly 14% among two hundred children aged 11.8 ± 2.2, monitored for severe obesity (Z-score of the body mass index above + 3 standard deviations) at the Armand-Trousseau Hospital. In this group, the most frequently encountered anomalies were android distribution of fat mass, measured by biphotonic absorptiometry, and HDL-cholesterol below the 5th percentile (figure 1).
The variations of the prevelance of metabolic syndrome from one country to another reveal not only the necessity to establish standards and a definition for children in line with vascular risk, but also the probable role of other factors (genetic and/or environmental) in adult populations.
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GROWTH AND PHYSICAL ACTIVITY
Starting at the age of 6, participation in physical activities contributes to proper skeletal, muscle and joint development and strength.
Through the muscle tension exerted on the bones, reasonably performed exercise increases bone thickness, density and resistance, without any effect on growth in length.
However, excess load on the bone, cartilage, ligament and tendon systems, when insufficiently adapted to intense traction and pressure, may be harmful to the child’s health. Extreme sports and the intense, repeated training that they require are likely to generate psychological competition stress and recurrent micro-trauma.
Similarly to emotional and family-life deficiencies, these may be responsible for a temporary growth blockage in weight and height, due to upset neurohormonal regulations in the hypothalamus and the hypophysis.
MUSCLE SYSTEM AND PHYSICAL ACTIVITY
The essential stages of muscle fiber differentiation and distribution take place from the gestation period to two years after birth. Varying according to the type of physical activity, the biochemical fiber characteristics evolve mainly to increase their oxidative power. Muscle mass accounts for 25% of total weight at birth and nearly 40% in adults. Most muscle growth occurs during puberty and is promoted by physical activity.
Growth is accompanied by an increase in the number of myofibrils, myofilaments and sarcomeres, which lead to the elongation of muscles. These developments may be accelerated by regular physical activity.
MOTOR SKILL DEVELOPMENT
Motor skills develop chiefly in the first eighteen years of life, although in girls their development tends to stabilize around puberty. Strength, power and rapidity increase in proportion to muscle mass, which is itself under the twofold dependence of hormonal concentrations (mainly growth hormone in boys and girls and testosterone in boys) and activity level.
The development of these qualities quickens during the post-puberty period.
Suppleness, on the other hand, is one of the natural qualities of children before puberty.
The essential stages of muscle fiber differentiation and distribution take place from the gestation period to two years after birth. Varying according to the type of physical activity, the biochemical fiber characteristics evolve mainly to increase their oxidative power. Muscle mass accounts for 25% of total weight at birth and nearly 40% in adults. Most muscle growth occurs during puberty and is promoted by physical activity.
Growth is accompanied by an increase in the number of myofibrils, myofilaments and sarcomeres, which lead to the elongation of muscles. These developments may be accelerated by regular physical activity.
MOTOR SKILL DEVELOPMENT
Motor skills develop chiefly in the first eighteen years of life, although in girls their development tends to stabilize around puberty. Strength, power and rapidity increase in proportion to muscle mass, which is itself under the twofold dependence of hormonal concentrations (mainly growth hormone in boys and girls and testosterone in boys) and activity level.
The development of these qualities quickens during the post-puberty period.
Suppleness, on the other hand, is one of the natural qualities of children before puberty.
- Suppleness
From early childhood on, children have a high level of suppleness resulting from low muscle mass and muscle tone and from high elasticity in the ligaments and muscles. With the exception of children who practice gymnastics, dance and all other motor activities with high corporal expression, “intense” joint training does not seem necessary before the age of 9 to 10. Under all circumstances, suppleness training in children must be supervised by competent physical educators, properly trained in and informed on the limitations to be observed. It is, on the other hand, especially recommended to teach children stretching techniques as early as possible. They will be useful to them throughout their lives because the range of motion tends to decrease rapidly after puberty and must be regularly maintained. - Gesture speed
Gesture speed corresponds to the maximum number of cyclical or acyclical movements likely to be performed in a given time. In certain activities such as swimming, cycling and short-distance running, gesture speed leads to travel velocity. It is defined as the minimal time required to cover a given distance.
Gesture speed depends on the conjunction of three factors: nervous, under the control of the central nervous system (CNS); neuromuscular, at the junction of the command system (CNS) and the effector system (muscles); and the quality of the required muscles.
Before the age of 10, the level of gesture speed (lower in children than in adults) is highly linked to the maturity of the nervous system (axon myelinization), the lower concentration of acetylcholine at the neuromuscular junction, the lower speed of calcium release and drainage in the sarcoplasmic reticulum, and finally to the coordination capacity of the muscles in use.
Although limited by hereditary factors, speed can be developed before and during puberty by exercise and all forms of play. It is hence perfectly justified to consider speed-increasing activities very early on (around the age of 6) because speed narrowly depends on and reinforces nerve coordination and motor skill development. The most obvious improvement in movement frequency and speed can be observed at the earliest school age. Later, the increase in muscle mass, lever heights and the extent of biomechanical movement, explains the improved gesture speed. - Muscular strength
Motor power rises gradually over the course of the growth process depending on the increase in body mass. Before puberty, maximum strength in boys and girls remains relatively similar.
On average, the increase in girls´ strength culminates during the maximum growth years (11.5 to 12.5 years) and the boys´ a year after peak growth (14.5 to 15.5 years). Later, maximum strength stabilizes around the age of 18 in girls and between the ages of 20 and 30 in boys.
Improved nerve activation and increased muscle mass (hypertrophy) are the main explanation for the increase in strength. Before puberty, improvement concerns mainly nerve activation.
Other less important mechanisms, including improved elastic energy release, intensified excitation-contraction coupling, and improvement in strength transmission to different bone levers, are also involved.
This rise in strength has an impact on the capacity for motor skill performance in fitness activities and in the prevention of injuries during such activities. By consequence, should we consider muscle development exercises for children before puberty, or not?
At the end of experimentally controlled body-building programs, many recent research projects have demonstrated that a very significant increase in strength was obtained without prejudice to the pre-pubescent child.
On the condition of respecting certain precautions, and undergoing a very thorough prior medical exam, as part of a properly conducted muscle-development program, pre-pubescent children are capable of increasing their muscle strength in the same proportions as adults.
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BOX 4: EXTRAPOLATION OF THE AVERAGE ENERGY EXPENDITURE OF VARIOUS PHYSICAL AND ATHLETIC ACTIVITIES PRACTICED BY CHILDREN AGED 6 TO 12.
Conclusion
Games and physical exercise must be encouraged within and outside school activities, because school alone cannot provide children with the activity necessary for their development.
Together, the “substantial increase in energy expenditure” recommended by French public health authorities, and the multiplication of sensory-motor experiences necessary for psychomotor and even cognitive development of children argue in favor of at least one hour of physical activity a day, executed with the most variable intensity possible.
References
- Binder M.: Quel sport pour quell enfant ?, Éditions Marabout, Paris, 2005.
- McArdle W.D., Katch F. Katch V.: Physiologie de l’activité physique : énergie, nutrition et performance, Éditions Vigot (3e éd.), Paris, 1987; 510–14.
- Molnar D., Livingstone B.: Physical activity and relation to overweight and obesity in children and adolescents, Eur J Pediatr, 2000; 159 [suppl 1]: S45–S55.
- Physical Activity and Health: A report from the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Center for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, 1996.
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