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How Much Exercise Is Enough for Adolescents to Achieve Cardiorespiratory Fitness?

June 14, 2022

I always encourage my patients to exercise, but when I am asked “How much should I exercise?” or “What kind of exercise is best?”, I don’t really have an evidence-based answer for them. My suggestions to start with 5-10 minutes daily and gradually increase to 20-30 minutes a day are based on adult studies, not pediatric studies.

I was therefore interested to see an article by Samuel Burden and colleagues from the University of Oxford and Oxford Brookes University in the United Kingdom, entitled, “Intensity and Duration of Physical Activity and Cardiorespiratory Fitness” (10.1542/peds.2021-056003). It is being early released by Pediatrics this week.

The authors studied the activity levels of 339 13- and 14-year-old students at 2 British schools. Students wore accelerometers (similar to the monitors that are in fitness trackers) to measure the amount and intensity of physical activity (PA). PA was categorized as light, moderate, and vigorous.

In their physical education classes, students ran the 20 meter shuttle test, which is a gold standard measure of cardiorespiratory fitness (CRF). Students ran back and forth on a 20 meter course within a certain amount of time. Initially, they had to run at a minimum pace of 8 kilometers/hour, then at 9 kilometers/hour, and the speed requirement increased by 0.5 kilometers/hour for each subsequent increment. The number of 20 meter increments completed was used as the measure of CRF.

I would recommend that you read the paper, because it contains a lot of interesting detail. It is not surprising that there was a dose response of vigorous PA and CRF, meaning that CRF improved with increasing daily amounts of vigorous PA, but the CRF benefit plateaued at 20 minutes/day of vigorous PA. Light and moderate PA was not associated with improvements in CRF. 

The World Health Organization recommends 60 minutes of moderate-to-vigorous physical activity. However, my experience is consistent with the point that Dr. Michele LaBotz and Dr. Sarah Hoffman from Tufts University make in an invited commentary, that this is not a realistic expectation for many children and adolescents who have school activities, homework, and sometimes jobs (10.1542/peds.2022-057026). They also provide tips about how to tell whether PA is vigorous or not- when you start to sweat, your face gets red, and/or you feel short of breath and can’t talk during the activity. Importantly, Drs. LaBotz and Hoffman point out that PA has benefits beyond just CRF, and some of these benefits may be gained with PA that is less than vigorous.

I’ve found that patients and their parents appreciate very specific guidelines. This paper and the accompanying commentary allow me to provide more specific guidelines for physical activity.

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