The screen on the elliptical machine says you traveled four miles and burned 300 calories. Your heart is racing, and you're soaked in sweat...

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ST. LOUIS — The screen on the elliptical machine says you traveled four miles and burned 300 calories. Your heart is racing, and you’re soaked in sweat. Nice workout, true. Lots of calories burned? Well, maybe.

Experts say you can be sure of the workout if you’re out of breath and sweating, but the calorie burn is another question.

The bells and whistles on those fancy machines that light up the gym might be impressive, but using them in a health or weight-loss plan takes more common sense than blind trust, experts say.

“What consumers need to keep in mind is that the readouts are meant to be an estimate of energy expenditure,” said Mark Reinking, assistant professor of physical therapy at St. Louis University. “There are good data from lots of research over the years of about how much energy it costs to do certain activities.

“So when I walk up to the treadmill, I input my age and weight. It will give me an estimate based on my age, my weight, the speed that the treadmill is going and give me an estimate of the energy I’m expending.

“How is this useful? It gives me a ballpark of calories.”

What to trust


Here’s what the experts say about digital readouts on exercise equipment

Time: The time on a machine is pretty accurate. It’s just a stopwatch, and the digital readouts are as sharp as any electronic timing equipment.

Distance: On a treadmill, distance generally is accurate. It’s a measure of the revolutions of the belt over time. On elliptical trainers, stationary bicycles, stair climbers and other cardiovascular machines, the distance is an estimate from a formula worked out by the manufacturer. One exception: Some fitness centers have stair climbers that work and look like escalators. They measure the number of floors. That distance is accurate.

Speed: This is just a measurement of how fast you are going, based on miles per hour.

Calories: This is an estimate. Some are better than others, but they’re all calculations, not an exact science. Give the machine your weight and age, and the computer estimates from a formula or pre-programmed table what the calorie burn will be for a certain speed and distance. Experts say that’s pretty much the case for any caloric measurement, unless you’re willing to spend several thousand dollars for a complicated stress test that most doctors reserve for people with heart and lung problems.

Watts: This is simply a readout that tells how much energy you are expending. You’re actually generating enough power to run a light bulb. The harder the workout, and the more power you generate, the brighter the light bulb. To get a consistent workout, set the watts on a machine, and the machine will adjust the resistance, speed and incline to make sure you’re always putting out that amount of power.

Heart rate: This is an important measurement, because the faster your heartbeat, the more energy you’re using. The handgrips on the machines that measure heart rate are OK, but they can be off as much as 10 beats.

Treadmills might have the most problem with the accuracy of handgrip sensors, because the impact of walking creates a lot of “electrical noise,” according to one expert. Stationary bikes and elliptical machines have less impact and therefore are more accurate.

All of the experts said you should get a heart-rate monitor of your own — one that wraps around your chest and transmits its information to the machine or to a wrist-band receiver. You can get them for less than $100. But even those have flaws, they say. The more body fat you have, the less accurate they might be.

That’s where the trust ends and the common sense begins, he says.

“If I’m going to be on the same piece of equipment for the same period of time, the research tells us these (readouts) are relatively reliable,” he said. “It may not tell me exactly how many calories I’ve burned, but I can use it as a way to benchmark my progress.

“So if today, it said I burned 300 calories, and in a month (I’m working harder and) I burn 400, I’m making progress. But that’s contingent on the same piece of equipment. If I switch equipment, I might get a different readout.”

Even the best machines can’t tell specifics about who’s walking on a treadmill — their muscle mass, conditioning, resting metabolism and other variables. For example, says Reinking, a person who’s 5-foot-5 and 150 pounds with 10 percent body fat is going to burn more calories than a person who’s 5-5 and 150 pounds with 30 percent body fat.

Digital readouts have been on exercise equipment for nearly 30 years, said Bob Starr, product manager for True, a manufacturing company that makes high-end cardiovascular equipment.

“Originally, the readouts were on stationary bikes to give feedback to the rider,” Starr said, “much like if you’re a runner and you carry a stopwatch to time yourself.”

Time marched on, however, and the readouts began to give lots of other information: heart rate, calories burned, distance traveled, watts generated and so forth.

Starr said that when it comes to the accuracy of the readouts, you get what you pay for — the better the machine, the more accurate its innards.

Todd Cade, an endocrinologist and physical therapist with Washington University School of Medicine, said what’s important is that someone is working out, not whether the readouts are accurate.

Frankly, to find out how many calories you burn in any workout is all math, not science.

The importance of calorie burn is a no-brainer, says Cade:

• More calories in, fewer calories burned equals weight gain; fewer calories in and more calories burned equals weight loss.

• The average person uses 1,600 to 2,200 calories a day just being alive. Eat more than that and you gain, eat less than that and you lose even without exercise. Lean muscle mass burns more calories, so the more muscle you have, the higher your resting calorie burn.

• Add exercise, and you burn more calories.

The preoccupation with calories has to do with the goal of losing weight, Cade said. So, if you’re going to monitor something, “First, you need to monitor what you’re taking in,” he said. “Reading labels, watching serving sizes and estimating how many calories you’re eating. …

“Research has looked at the effect of keeping your calories the same and exercising, and reducing your calories and not exercising. The ones who reduced their calories lost more weight than those who just exercised.”

Still, large people who exercise are healthier than large people who don’t, he said.

When it comes to measuring how much energy you burn on a machine, all that’s available are the tables and formulas that health professionals use.

So is there a simple method to measure calorie burn without buying expensive equipment? “Pretty much not,” Cade said. “You pretty much have to rely on the formulas.” That’s called “indirect calorimetry,” measuring calorie burn from tables and formulas.

Outside of that, “You want to watch what you’re eating and try to establish an energy balance,” Cade said.