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Similarities and Differences Between Children and Adults in the Physiological Responses to Exercise

For the last few years, schools, community park and recreation districts, agency and independent fitness centers have been trying to implement the recommendations from both the 1996 report of the U.S. Surgeon General and the 1997 Guidelines for School and Community Programs to Promote Lifelong Physical Activity among young people. Amongst these suggestions is the recommendation that schools, community programs and the fitness industry cooperate to provide safe environments, role models, competent leadership and developmentally appropriate activities for children and adolescents. In addition, parents are encouraged to plan and participate in physical activities with their children. However, if you were playing with your child, would you be concerned if you noticed that your child seemed to be breathing harder and had a higher heart rate but wasn't sweating as much as you were?

Respiratory System

The primary role of the respiratory system is to provide oxygen and to eliminate carbon dioxide from the muscle cells. The amount of air exchanged per minute is called minute ventilation and is the product of the number of breaths (frequency) times the volume of each breath (tidal volume). Children and adolescents exhibit a higher frequency and lower tidal volume than adults at all intensities. (1,2) Because rapid breathing is readily noticed, it can be upsetting to a well-intentioned adult. The higher frequency and lower tidal volume is normal and no call for alarm. Actually, if you were to make adjustments for the body weight, children and adolescents breathe more air per minute per kilogram than adults do at the same 

sub-maximal intensity. These differences are offset by a smaller dead space. Dead space is the volume of air trapped in the conduction portion of the lungs that is not available for exchange at the alveoli. The alveoli provide the vital surface for gas exchange between lungs and the blood.. Alveolar tissue has the largest blood supply of any organ in the body.(3) This is similar to the portion of water in the pipes of your house. The smaller dead space results in a greater portion of the air inhaled by a child actually getting to the alveoli.(4)

A comparison is often made between the amount of air that is processed (minute ventilation) and the amount of oxygen used (VO2) to produce energy aerobically. This comparison is called ventilatory equivalent. Children and adolescents have higher ventilatory equivalents than adults do and the difference if inversely related to the age of the child. (1,2) Therefore, the younger the child, the more air they must breathe in. Because of the higher ventilatory equivalent seen in children and adolescent's generally considered to be insufficient the youngster must expend additional energy to support respiration during exercise. (2) However, neither this insufficiency nor any of the other differences previously described for the respiratory system contraindicates physical activity for children or adolescents.

Cardiovascular System

The cardiovascular system is primarily a transport system composed of the heart, blood and blood vessels. Its job is to transport oxygen, nutrients in the form of carbohydrate, fat, protein and hormones such as insulin and epinephrine to reach the muscle cells. Carbon dioxide , heat and other by-products of energy metabolism are removed from the cellular level by the cardiovascular system. (5) The delivery of oxygen depends on the amount of blood pumped each minute from the heart which is called cardiac output (Cardiac output = stroke volume: the amount of blood pumped per ventricular contraction x heart beats per minute) and the amount of oxygen unloaded from the red blood cells, the arteriovenous oxygen difference. Children and adolescents exhibit a lower stoke volume and higher heart rate than adults do at all intensities of exercise. Because stroke volume is linked to ventricular size, the higher heart rate is probably an attempt to compensate for the smaller ventricular size of children and adolescents. It's normal for a child to have a higher heart rate than a parent if they performing the same exercises together.

Maximal heart rate is higher in children and adolescents than adults, but does not significantly change during the growing years of 7 to 15. This makes estimation of maximal heart rate by set equations such as 220-age inaccurate for children and adolescents until the late teenage years. 

It also means unless there are signs of stress or duress, there is no cause for concern for heart rate values greater than 200 bpm. Healthy individuals should be able to exercise for several minutes at maximal heart rates. In fact, because VO2 max (the greatest amount of oxygen that can be inhaled during aerobic exercise) is relative to the individual's body weight, VO2 max values are as high or higher than most adults. Heart rate will return to resting values quicker in children and adolescents than adults.(2)

Temperature control of the cardiovascular system is critical for the exercising participant and is more of a challenge for children and adolescents. Their surface area-to-mass ratio is larger than adults which allows for a greater heat exchange by convection and radiation. Convection is related to conduction which is the ability to gain or lose heat by the exchange of a solid, liquid or gas from one molecule. Because our bodies are usually warmer than the environment, the net exchange of radiant heat energy is through the air to solid, cooler objects in the environment. This does not require molecular contact with the warmer object. Radiation is how the sun warms the earth or in other words a person can remain warm by absorbing radiant heat from the sun or indirect sunlight reflected from snow, sand or water.

Children do not sweat as much during exercise as adults. The number of sweat glands is the same but the activation of the sweat glands happens at a higher temperature; less sweat is produced from each gland and the sweat rate per unit of surface area is lower. Children and adolescents also have a smaller plasma volume than adults from which to draw fluids for sweating.(6) In neutral climates the temperature-regulating capacity of children and adolescents is equal to that of adults. However, children and adolescents have a shorter tolerance time for exercise in extreme temperatures. (6) They can acclimate to the heat, but it takes them longer than adults. The tendency is to try to do too much too soon. It is recommended to postpone or recommend strenuous activity when heat and humidity are high and making sure plenty of fluids are ingested before, during and after exercise. Thirst is not an accurate guide for fluid need.

Well, now you know the answer to the question in paragraph one. It's perfectly normal for a child or adolescent to breath harder and have a higher heart rate than an adult, however, if the heat or humidity is excessive it may be prudent to postpone the exercise are lower the intensify. Don't forget to hydrate before during and after. Good luck!

1. Bar-Or. Pediatric Sports Medicine for the Practitioner: From Physiological Principles to Clinical Applications. New York: Springer-Verlag, 1983; pp 1-65

2. Rowland, T. W Development Exercise Physiology. Champaign, IL: Human Kinetics, 1996

3.McArdle, William D., Katch, Frank, I., Katch, Victor, L., Exercise Physiology, 2nd edition, pg. 192.

4. Bar-Or. Pediatric Sports Medicine for the Practitioner: From Physiological Principles to Clinical Applications. New York: Springer-Verlag, 1983; pp 1-65

5. Plowman, S.A. and D. L. Smith. Exercise Physiology for Health , fitness and Performance, Boston: Allyn

6. Bar-Or, O. children and physical performance in warm and cold environments In Advances in Pediatric Sport Sciences. Vol I: Biological Issues. R.A. Boileau (Editor), Champaign, Il: Human Kinetics Publishers, 1984; pp. 117-0130

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John Platero is the founder and owner of Future Fit, Inc. a successful multi-faceted fitness company that manages personal trainers, provides fitness products, services and educational services for the consumer.  He is also the Director of the N.C.C.P.T. (The National Council of Certified Personal Trainers) which has certified thousands of personal trainers at its intensive, two-day workshops held throughout the country.  For more information please visit http://www.nccpt.com