Exercise is critical for children’s health, but there may be some problems in the way we measure a child’s exertion. Experts say measuring the maximum heart rate for children the same way we measure it for adults puts children at risk for negative effects — like breathlessness and dizziness.
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The common formula used to calculate adult maximum heart rate – 220 minus age – doesn’t apply for kids, says pediatric cardiologist Kenneth Zahka, MD.
“It’s very common that children’s maximum heart rates, on average, are lower than what you get from the ‘220 minus your age’ equation,” Dr. Zahka says. “That creates a maximum heart rate that’s too high for children under age 18.”
Why is a child’s maximum heart rate different?
Maximum heart rate is the highest heart rate a healthy person can achieve without experiencing severe problems related to exercise stress. Once you determine your maximum rate, doctors recommend you exercise in your “target heart rate zone,” typically at 60% to 80% of your maximum heart rate for a healthy adult.
Children, however, generally have a higher resting heart rate than adults because they have a smaller heart size, decreased stroke volume, and decreased blood volume. These characteristics throw off the formula and make it inaccurate for children, Dr. Zahka says.
Typically, children ages 6 to 18 tend to have lower maximum heart rates than the formula would indicate, often measuring as low as 185. However, maximums for children can climb as high as 215, he says. The actual heart rate varies by child and is likely genetically determined.
Your doctor can pinpoint your child’s actual maximum heart rate with a formal exercise test, if necessary.
Why does the difference matter?
Knowing that some children have lower maximum heart rates than others could change how schools conduct certain fitness tests, Dr. Zahka says.
Currently, for example, some fitness tests require students to reach 70% of their maximum heart rate.
For a 10-year-old, under the common formula, that means a target heart rate zone of 170 for exercise. But, if a child is genetically predisposed for a maximum heart rate of 180, he should aim for 70% of that, which is only slightly more than 140.
Gender also is a factor for exercise at certain ages, Dr. Zahka says.
Data shows that, at a young age, there’s very little difference in maximum heart rates between boys and girls. That doesn’t change much in adolescence, but it does impact exercise capacity. Between ages 10 and 18, a boy’s ability to exercise harder can increase up to 20%.
What happens when a child pushes too hard?
Pushing your heart rate up too high is dangerous, Dr. Zahka says. It can cause dizziness and shortness of breath. Exercising at or beyond the maximum heart rate for too long can also cause a burning sensation in the muscles due to a buildup of lactic acid.
A child who has these symptoms does not necessarily have an underlying heart or lung problem, Dr. Zahka says. But signs like these show that he is working beyond his abilities that day under those conditions.
Overall, children are typically good self-regulators — unlike adults, who will sometimes push themselves too hard and raise their heart rates dangerously high, Dr. Zahka says.
“Most children under most circumstances will limit themselves appropriately with exercise,” he says. “They’ll slow down or stop when they need to. The important thing is that they know to listen to their bodies.”