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The departments of Pediatric Physical Therapy and Exercise Physiology and the Pediatric Orthopedics of Wilhelmina Children's Hospital, University Medical Center Utrecht, The Netherlands, published in 2007 a study that collected data on the response to exercise and muscle strength of healthy children and adolescents and compared it with their peers with achondroplasia.

The study subjects were 7 boys and 10 girls with achondroplasia with a mean age of 11.8 years that performed a maximal treadmill exercise. The research team concluded at that point:

"The exercise capacity of the subjects with achondroplasia was significantly reduced compared with age and sexmatched reference values for the general population. Their reduced exercise capacity cannot be explained by their smaller height because no significant correlation was found between these two variables.

In fact, these children showed a significantly increased exercise capacity(...), which is a result of a higher muscle mass for a given height compared with healthy subjects. (...). The increased ventilatory equivalent for oxygen uptake showed that subjects with achondroplasia have to ventilate more (higher breathing frequency) for the uptake of 1 liter of oxygen compared with age- and sex-matched reference values. This might be caused by the reduced vital capacity."

Vital capacity is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume)..

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"A lower tidal volume is thus compensated with a higher breathing frequency and a higher ventilation of alveolar death space and thus a lower ventilatory efficiency. Moreover, the reduced oxygen pulse of the patients showed that they have a higher heart rate for a given oxygen uptake compared with age- and sex-matched reference values. This implies that they have a reduced cardiac stroke volume during exercise as a result of their smaller thoracic volume.

Children with achondroplasia showed reduced muscle strength in almost all muscle groups compared with age- and sex-matched reference values.

Lower muscle strength may be caused by a decrease in muscle mass, by reduced neuromuscular coordination or by altered biomechanics. Because of the comparison with age- and sex-matched reference values, a lower muscle mass for their age could be an explanation for these results.  Another possible explanation for reduced muscle strength could be that their muscle tone is not optimal because of the relatively short bones in combination with relatively normal length of the muscles and other tissues in the extremities, causing relative muscle hypotonia and decreased muscle strength.  

The physical activity record showed that the achondroplastic children had a lower energy expenditure compared with the normative values for healthy children Moreover, the calculated energy expenditure was also lower compared with other studies from our country.

A  mean energy expenditure in healthy children is 8 megaJoules per day for girls and 9 for boys. The subjects with achondroplasia had a mean energy expenditure of 5.3 mega-Joules per day. However, because of their unique physique, the validity of the physical activity record is uncertain. (1megajoule=239 kcal)

One of the challenges we faced during this study was the choice of an appropriate comparison group. (...) Clinicians should review the energy balance of subjects with achondroplasia regularly because obesity is prevalent in this patient group. An appropriate activity program with acceptable physical activities should be developed to increase energy expenditure and improve exercise capacity."


  1. Takken, T., et al., Cardiopulmonary exercise capacity, muscle strength, and physical activity in children and adolescents with achondroplasia. J Pediatr, 2007. 150(1): p. 26-30.
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