Beware of Fitness Instructors That Tell You You’re Torching 800 to 1,000 Calories.

They may not be telling  you the whole truth.

I received an e-mail today with an article attached and a question about calorie expenditure during exercise.  The article listed a variety of workouts and stated how many calories would be burned  per hour for each one.  Here’s what it said:

  • Kickboxing – 800
  • Spinning – 700
  • Zumba – 500
  • Jump Rope – 780
  • Step Aerobics – 600
  • Running – 650

Figuring out how many calories you burn during exercise is tricky because everyone is going to burn a different amount.  I couldn’t tell you which participant these estimates are based one, but I do know that I’ve never come close to burning 500 calories in Zumba.

Which one torched 1,000 calories in Spin class?

Which one torched 1,000 calories in Spin class?

Your Metabolic Fingerprint

The number of calories you burn in a normal day is determined by your Basal Metabolic Rate (BMR).  It is as individual as your fingerprint and is influenced by a number of factors including your body size, body composition, gender, and age.

Accurately determining your BMR can be done with a breath test.  There are also many on-line calculators that will give you an idea of what your BMR is, but remember, those are only estimates and if you’re trying to achieve an exact calorie-in-calorie-out balance you don’t want to rely on them.

How You Burn Calories When You Exercise

In a Spinning class with 25 people on 25 bikes each person will burn calories a little differently based on their BMR and the intensity that they are exercising.  The number of calories they burn in that hour is determined by how much oxygen they are using, in other words, their heart rate.

Your Maximum Heart Rate (MHR) is about 220 heartbeats per minute minus your age although, an older physically fit person can achieve a higher MHR than someone younger who is out-of-shape.

To improve cardiovascular endurance and burn the most calories, your target heart rate zone when exercising should be between 80 and 90% of maximum.  So, theoretically a 32 year old working at 80% MHR would be at 150 beats per minute.  But, let’s say everyone in the Spin class is 32 years of age.  Not everyone would be at 80% of their MHR if they counted their pulse and found their heart was beating at 150 BPM.

This is one reason why the Target Heart Rate Chart, that at one time was on the wall of every fitness class studio in the world, has been replaced by a chart that allows the participant to measure Perceived Exertion.  Perceived exertion asks you to determine, on a scale from one to ten, how hard are you working?  If you’re at 10 you may need to slow it down.  If you’re at two you need to pick up the pace if you plan on burning any calories to speak of.

Getting an estimate of your BMR, taking your heart rate during class or while you’re on the treadmill, and measuring perceived exertion are all worthwhile.  But it’s important to keep in mind that all of these are estimates.  To know exactly how many calories you burn during a 60 minute Turbo Kick class would require you to be hooked up to equipment that measures your VO2 max.  That doesn’t sound like much fun.

Less is More

The danger in over-estimating calorie expenditure – which is what I think some fitness instructors do to sell the class – is that you leave thinking you’ve created an 800 calorie deficit.  To some of us that means we have an 800 calorie hole that needs to be filled.  With food.  If, in fact, you’ve only burned 400 calories but eat 800, you’ve got a problem and a new question: “Why do I gain weight when I exercise?”

The old rules of exertion still apply.  Whatever it is you do, do it consistently and work hard enough at it that you can’t carry on a conversation with you neighbor, but not so hard that you’re completely exhausted when you’re done. Find that happy place where you leave the class thinking you had a good workout and are anxious to come back for more.

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