Pumping up pulse rates

Pregnant women who count their heartbeats as a way to measure how hard they are exercising can wind up exercising harder than they expect, researchers say.

The finding should help pregnant women more accurately target how hard they are working out.

The heartbeat count is a standard gauge of exercise intensity, from pulse checks during aerobics classes to pulse monitors on treadmills and exercise bikes. The checks are based on heart rates rising as cells demand more oxygen. Oxygen demand goes up as metabolism rises to meet the higher energy demands of exercise.

Gretchen Griggs can feel her heart rate rise while she's exercising, though she tries to keep her pulse rate at a normal range for her age, according to the charts posted in the aerobics room at Universal Health and Fitness Center. Griggs is expecting a baby in September.

She regularly attends the Fit 2 Be Due exercise class that is designed specifically for women in the second and third trimesters of their pregnancy. Instructor Shannon Sievers tries to keep to a low-impact aerobics routine with an emphasis on hip and leg strength as well as exercises in breathing and stretching.

And while Griggs checks her pulse at least twice during the hour-long class, the typical heart rate targets for exercise are based on rates for people who are not pregnant. And researchers say those rates don't accurately measure the metabolic demands of a woman who has the additional burden of meeting the blood and energy needs of pregnancy.

At a given heart rate, she is actually doing more work during pregnancy, said researcher James M. Pivarnik of Michigan State University.

Pivarnik and his colleagues studied 52 women around their fifth and eighth months of pregnancy and around the third month after giving birth.

The researchers measured the women's heart rates and compared the rates with the standard scientific measurement of how much energy people burn in aerobic exercise.

In the fifth month, women who stuck with their pre-pregnancy heart rates would be doing about 5 percent harder work than their pulse rates would indicate. In the eighth month, a woman could be averaging 17 percent more work, Pivarnik said.

Griggs said she doesn't feel like she's exerting herself any more than she was before getting pregnant. She was working out at least three times a week.

Her doctor advised her to continue the exercise. It means having a healthier baby, she said.

Griggs likes the low-impact aerobics with its leg exercises, toning and stretching. Most of the movements are just repitition, she said.

"There's not a lot of change but we want to get her heart rate up," Sievers said. She tries to keep that rate between 120 to 140 for the women in the class. Each participant must have a release from their doctor.

Pregnant exercisers can use the heart rate information to decide how hard they want to work out, Pivarnik said. If a woman wants to keep the same level of intensity, she should realize that her pulse rate will be faster than it was before she was pregnant. If she wants to keep the same heart rate, then the exercise intensity would be less.