Foot Pounds to Calories

May 10, 2011 8:16 PM

There are plenty of charts that show calories burned from running. But is figuring calories burned lifting weights this simple?

I found a foot-pounds to calories conversion tool. If a bicep curls a 20 lb weight 21 inches (from mid thigh to chest), that's 20lbs times 1.8 feet or 45 foot-pounds. And the conversion tool says one repetition burns about 73 calories. Is that right?

posted by CollectiveMind to health & fitness (12 answers total)

Not 45 foot-pounds, 36 foot pounds, which converts to 11.6 calories. Fat finger on the calculator. Still, is that process correct?
posted by CollectiveMind at 8:20 PM on May 10, 2011

Does the tool you're using take into account your weight and muscle mass?
posted by decathecting at 8:22 PM on May 10, 2011

Daily Burn tells me that one rep of a 20lb curl burns 1 calorie.
posted by birdherder at 8:24 PM on May 10, 2011

From relative scale, it looks like it might well be correct - however, it's talking about thermochemical calories, which are 1/1000th of the kind of calorie you see on nutrition labels.

The Wikipedia article has info and the conversion to joules, and the foot-pounds article also converts to joules, so you can check the work.
posted by restless_nomad at 8:26 PM on May 10, 2011

Two things. First, food calories are measured in kilocalories, which is one thousand calories. So 36 foot pounds is about 0.1 food calories! Second, your body is about 25% efficient in doing work, so you burn 4 calories for every 1 calories of work performed. So 36 foot pounds burns about 0.4 food calories.

This is why weight lifting is good for building muscle, but bad for burning calories.
posted by notme at 8:27 PM on May 10, 2011 [1 favorite]

Ack! That should be 36 ft-lbs = 0.01 kcal, and so lifting 36 lbs 1 foot will burn about 0.04 food calories.
posted by notme at 8:28 PM on May 10, 2011

But the more muscle you have, the more calories you burn for the rest of the day, not just in the gym.

From the Mayo Clinic linked above

The bodies of people who are larger or have more muscle burn up more calories, even at rest.
posted by Ideefixe at 8:39 PM on May 10, 2011

To be clear, my point was that the act of lifting the weight burns surprisingly few calories, especially compared to an activity like running. It does build muscle, though, which is obviously a good thing for many reasons, including a higher resting metabolic rate.
posted by notme at 8:52 PM on May 10, 2011

In the larger sense, this measurement is pretty worthless, because there's a lot more going on both metabolically and physically than just moving a given mass a given number of inches. For example, in a bicep curl, you will no doubt rotate your wrist some greater or lesser degree. Your back muscles will contract to keep you balanced. Your abs will contract to prevent your back muscles from tipping you over backwards. Your legs are keeping you upright, if you're standing. Your hand and forearm muscles are contracted to keep the weight in your hand. All these things take energy and aren't involved in the simple foot-pounds to calories conversion.

tl;dr: The conversion you cite is mathematically sound, if somewhat confusing, but of no use deciding whether or not you've earned that cheeseburger.
posted by restless_nomad at 8:59 PM on May 10, 2011

But is figuring calories burned lifting weights this simple?

Absolutely not. Weight-lifting is anaerobic exercise: done without oxygen. Unlike aerobic exercise, the mechanism for energy conversion is entirely different. Read up on anaerobic metabolism.
posted by Civil_Disobedient at 11:06 PM on May 10, 2011

All these things take energy and aren't involved in the simple foot-pounds to calories conversion.

There's also the recovery period. It takes energy to repair the body.
posted by Civil_Disobedient at 11:10 PM on May 10, 2011

It also depends completely on your calorie intake. You can go to the gym and burn 1000 calories, but then eat a giant sandwich and be net positive for the day.

To lose weight, you need to ignore the daily calorie metric completely. Instead, titrate your calorie intake based on what you will be doing with that food. Eat enough food to fuel your body for the activities you will partake in until the next meal. Then, at the end of the week, see if you have gained or lost weight.

If you are doing a significant amount of muscle building, you probably want to shoot for breaking even. Because while your weight isn't changing, the composition of your body is shifting from fat to muscle. And because you want your body to have the fuel it needs to actually build that muscle. Keep this up for a while, and you'll probably find that your clothes fit better and that, after a couple of months, your weight will go down without changing anything.

Because you will have changed your "metabolism". Which is really just your muscle mass consuming calories simply by existing.
posted by gjc at 6:07 AM on May 11, 2011