Possible Effects of Excessive Physical Exercise on Prepubescent Children

The ill effects of a sedentary lifestyle on children are well documented, and is a heavyweight contributing factor to childhood diabetes and obesity today. More often, we hear calls to steer our kids away from their attachments to TVs and computers, and to involve them in more athletic activities. Hence, it is rarer to find a child who is too active than one who is inactive, and even more of an anomaly for a mother to be concerned that her child might be overexercising.

So, can too much of a good thing really be bad for you? Make no mistake, I am certainly of the view that a physically active lifestyle for children and adults alike, together with balanced nutrition, is the answer to a healthy body. I do not, for one single moment, believe that inactivity is an option for anyone capable of physical movement, unless immobility is your goal later in life! But as important as exercise is, I believe that it can indeed be hurtful when it becomes too much or too strenuous, leading to injuries, endocrine disruptions, eating disorders, and can be especially detrimental to the development in children.

Children are not mini me's of adults. In fact, they are very physiologically variant from adults. Their metabolism, hormonal responses work differently; their musculoskeletal systems are still undergoing shifts, formation and growth at different rates; hence, making them susceptible to physiological disturbances brought on by excessive physical stress. Of special concern to prepubescent girls is the effect on their endocrine system. Control of growth and development leading to, and at menarche, involve complex interaction of many factors, including physical and metabolic demands of intensive athletic training.

Again, I want to emphasize that this does not indicate that children are not well suited to physical movement. On the contrary, in 2008, a study by The National Institute of Arthritis and Musculoskeletal and Skin Diseases concluded that jumping exercises performed for a duration of one year in pre puberty, led to increased bone densities in children, and such effects remained with the children up to 8 years after (1). In another study completed in February 2011, researchers of the same institute showed that although genetics govern skeletal growth, physical activity during a child's development can improve skeletal health dramatically, and such benefits were found to be greatest in activities that involved loading weight and intense impact (2). Studies also proved that the benefits of regular activity for children extended beyond musculoskeletal development, and in fact could enhance their cardiovascular fitness and insulin sensitivity (4). As such, it is without doubt that physical stimulation is required for healthy growth, but the challenge lies in finding the right balance between exertion and rest.

Rates of growth and sexual development through puberty in children depend on the actions of hormones such as Growth Hormones (GH), Insulin-like Factors (IF), Steroid Sex Hormones (SSH). The same hormones also influence metabolic processes during physical activity. For young girls, "Adrenarche", signifies the beginning of secretion of Androgens and generally takes place in girls between the ages of 6-8 years of age. "Gonadarche" usually follows about 2 years later. It denotes activation of the ovaries and is governed by GH, IF and SSH. "Menarche" marks the onset of menstruation (3).

Hormones released during these stages control later development in puberty as well as other bodily strictures, and exercise causes dramatic changes in hormonal and metabolite levels. Hence, it is important to manage physical activity to the extent that it does not interfere with the child's endocrine and metabolic systems in adverse manners. Excessive training in adolescence can delay menarche in girls, bring about amenorrhea and the suppression of the body's immune system (4).

According to an article published in the American Journal of Clinical Nutrition in 2000 (5), delayed puberty is most prevalent in elite female athletes, most notably gymnasts, dancers and long distance runners. Bone formation is governed by a combination of nutrition, weight bearing activity, and secretion of SSHs. Since over 90% of adult bone mineral is accrued during pubertal years, delaying the onset of puberty might cause a shortfall in bone mineralization. Hypoestrogenism (under production of estrogen) as a consequence of delayed sexual maturation can also negate the benefits of weight bearing exercises on skeletal development. In a group of prepubescent runners, subjects with delayed periods were found to have lower bone density compared to the runners with regular period within the normal range.

Contrary to the undesirable effects of excessive activity, the same study found moderate exercise to favour cardiovascular health and body composition. However, careful attention should be drawn especially to sports such as wrestling, gymnastics, dancing. Children involved in these activities require close monitoring due to the emphasis on strict weight control and the high energy requirements. In a comparative experiment conducted over 2 years between a group of swimmers training 8 hours per week and a group of gymnasts training 22 hours per week, the gymnasts were found to have considerably decreased growth velocities (i.e. they were shorter and had less body mass), although it might be worthy to note that the data connecting intense athletic training and sexual growth in female gymnasts were not conclusive. 

For the purpose of interpretation, it will be useful to account for variables such as intensity of training, individual state of health, genetic disposition, socioeconomic, psychological and emotional factors, nutrition etc. Furthermore, perturbations in pubertal development were uncommon among young women engaged in recreational exercise or adolescents who trained less than 15 hours per week. Hence, it is necessary to distinguish elite athletic training from non elite sporting activities because training time and intensity are integral factors for consideration. It has been shown that olympic athletes have dramatically later menarche than high school, college or club level athletes.

Findings by The Committee on Sports Medicine and Fitness from the American Academy of Pediatrics in 2000 (6) recommended that it would be more beneficial to engage children in a variety of sports rather than focusing on just one sport. Not only will specialisation deny children of the benefits of varied activity and widened range of skills, it also adds unnecessary physical, physiological and psychological demands from intense training. Moreover, repetitive stress of doing the same movement over and over, may create detrimental results. However, this pertains to high intensity and sports specialization. 

The committee also reported that "although child athletes have superior cardiac functional capacity compared to non-athletes, data suggests that myocardial function can be depressed, at least transiently, after intense exercise". This raises some questions on very intense exercise on young children, but much closer investigation on the specifics of this area would be required before jumping to conclusions 

Other undesirable outcomes of excessive stress of physical activity in the report included tissue breakdown, overuse injuries such as tendonitis, apophysitis, fractures etc. In addition, injuries to joint surfaces and the spine during development were said to be particularly damaging to growing children because of the long term implications. Also of special concern were disruptions to the epiphyseal growth centre, which entails parts of the bone that are still growing. Soft tissues surrounding the physeal plate i.e. growth plate, grow at a faster rate, therefore external stress might disrupt the growth plate itself rather than the softer ligaments round it, resulting in stunted growth and/or deformed bones. It is also important to know that thermoregulatory responses in children are different from that of an adult. Children sweat less and create more heat in response to exercise, therefore care should be taken in temperature control while engaging young children in physical activities.

A 2008 paper published in the Journal of Applied Physiology (4), postulated that children  rely heavily on aerobic metabolism for energy and have low anaerobic capacities. They also have an immature glucose regulatory system compared to adults. The higher proportion of slow twitch, fatigue resistant (type 1) fibres in their musculature makeup might account for their reliance on aerobic metabolism, and at the same time, heighten their need for the oxidative metabolism of fat for energy. Together with the smaller endogenous carbohydrates reserves within their bodies, children are not suited to highly intensive, exhaustive physical activity.

A study on the effects of moderate exercise on adolescents, between a first group of 180 children of ages 10-14 years and a second group of 190 children between ages 15-19 years, showed that the the former group utililised considerably more fat than the second group. Moreover, that phenomenon lasted even when carbohydrates were fed to both groups before exercise, proving the point that aerobic metabolism and hence, fat oxidation was comparatively more pronounced in youth; and that their metabolic mechanism will shift towards a more anaerobic mode as children grow. As such, prepubescent children might be more adapted to start physical activities that are aerobic in nature and of a moderate pace, and gradually move on to more anaerobic activities that are of high intensity for brief periods, as they progress into puberty and beyond.

Similar conclusions were echoed by Researchers at the University of Rennes in France in 2000 (3), who also found children to respond unfavourably to exhaustive resistance training. Yet in contrast, they responded well to prolonged exercises of moderate intensity. Furthermore, the same study showed that the capacity to store muscle glycogen is much lower in children than in adults. Hence, in picking an athletic activity, it would be pertinent to consider this physiological constraint in young children. Any activity of moderate intensity (70% maximal oxygen uptake) that lasts over an hour, is unsuitable to be performed by children, as depletion of carbohydrate storage occurs much sooner.

Here you can find the key guidelines issued by U.S. Department of Health and Human Sciences on recommended level of physical activity : http://www.hhs.gov/news/press/2008pres/10/20081007a.html

Sources:

1. Simple Exercise in Children yields long-term Skeletal Benefit. (November, 2008). National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH. Retrieved October 30, 2011 from: http://www.niams.nih.gov/news_and_events/spotlight_on_research/2011/child_exercise_bone.asp
2. Childhood Exercise leads to Sustained Improvements in Bone Mass. (February, 2011). National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH. Retrieved October 30, 2011 from: http://www.niams.nih.gov/news_and_events/spotlight_on_research/2011/child_exercise_bone.asp
3. Boisseau Nathalie, Delamarche Paul. (2000). Metabolic and Hormonal Responses to Exercise in Children and Adolescents. Sports Medicine 2000 Dec; 30 (6). Pp 405-422.
4. Riddell Michael C. (2008). The Endocrine Response and Substrate Utilization during exercise in Children and Adolescents. Journal of Applied Physiology . Volume 105. August, 2008. doi:10.1152/japplphysiol.00031.2008. Pp 725-733.
5. Rogol Alan D, Clark Pamela A, Roemmich James N. (2000). Growth and Pubertal Development in Children and Adolescents: Effects of Diet and Physical Activity. American Journal of Clinical Nutrition. 2000. 72(supp). Pp 521S-8S.
6. Intensive Training and Sports Specialization in Young Athletes. Committee on Sports Medicine and Fitness. (2000).106 (1):154. American Academy of Pediatrics. ISSN 0031 4005.