How Accurate is My Calculation of Calories Burned During Weight Lifting?

In summary, a forum member is questioning their calculation of calories burned during weightlifting exercises. They are using conservation of energy principles and accounting for only the upward movement, resulting in a calculation of ~1600 calories burned. Other forum members suggest checking for a cal/kcal confusion and using a more accurate calculator from a reputable website. They also mention that the human body is inefficient and the relationship between work done and energy expended is complex. The original forum member notes that they are using light weights and another member suggests using heavier weights and recommends a website for better estimates.
  • #1
AretePhile
4
0
Hello to the Forum.

I just started doing some very light weight lifting and decided to do a rough calculation of how many calories I might be burning during each of my workouts, however the figure I arrive at seems too high by comparison with the numbers one regularly see bandied about. What am I doing wrong?

My calculation goes as follows:

I'll use conservation of energy and only account for the upward movement (when you're letting the weights down again in the second half of each repetition you're also exerting force in the direction opposite to movement so as not to let them fall at the full acceleration due to gravity).

I just started so I'm using a pair of 9 lbs dumbbells. This gives us ~4Kg for each dumbbell, 8Kg for the two, and 78.4N. Let's say the average length that you extend each dumbbell upwards during a single repetition is 0.3m. This gives us 23.52J spent in each repetition according to conservation of energy principles, mgh=W.

Now, I do 12 repetitions of each of 8 different exercises, and repeat the whole cycle 3 times. This gives us 23.52(12)(8)(3) = 6774J, which converted to calories result in ~1600 calories, not taking into account the work done during the downward movement of the weights nor leg work, which is perhaps more strenuous.

By all accounts though, 1600 cal burned during a single workout session seems a lot, even more so for such light weights. Is my physics wrong?
 
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  • #2
With this type of calculation it is usually a cal/kcal confusion. You should check for that, then you are probably off by a factor of 1000.
 
  • #3
0xDEADBEEF said:
With this type of calculation it is usually a cal/kcal confusion. You should check for that, then you are probably off by a factor of 1000.

Thank you for your reply. In that case we would be talking about 1.6 calories, which again is much too little.
 
  • #4
Your calculation tells you that you are doing 1.6kcal of equivalent useful work. Meanwhile, your body is consuming loads more energy in just performing that exercise - around four times as much.
Look at this link for some representative values of work done and food consumed. It requires an awful lot of mechanical work to burn off a few pies!
 
  • #5
I've often wondered about this myself. Weightlifting is usually not considered a good way to burn calories, but I've never done the math. One suggestion: 9Lbs is far to light, even for a beginner. Unless you have some physical disability.

"There are no gains without pains." ~ Benjamin Franklin: the man who "discovered" electricity.
 
  • #6
sophiecentaur said:
Your calculation tells you that you are doing 1.6kcal of equivalent useful work. Meanwhile, your body is consuming loads more energy in just performing that exercise - around four times as much.
Look at this link for some representative values of work done and food consumed. It requires an awful lot of mechanical work to burn off a few pies!

Still, the range I usually see mentioned is around 220-250 "calories", so we're still off by a factor of ~40.
 
  • #7
Hetware said:
I've often wondered about this myself. Weightlifting is usually not considered a good way to burn calories, but I've never done the math. One suggestion: 9Lbs is far to light, even for a beginner. Unless you have some physical disability.

I've never done exercise and these dumbbells is what I had available, so while I learn good form and put together a set of exercises it'll have do.
 
  • #8
AretePhile said:
Still, the range I usually see mentioned is around 220-250 "calories", so we're still off by a factor of ~40.

That's 'per hour', I believe. This mild bit of exercise doesn't take an hour, surely. I would certainly die of boredom, doing that for a whole hour.
 
  • #9
AretePhile said:
I've never done exercise and these dumbbells is what I had available, so while I learn good form and put together a set of exercises it'll have do.
I highly recommend this site:

http://www.exrx.net/

They have a section with some calculators that will give you some better estimates than what you can get by the procedure you are attempting. The problem is that the human body is extremely inefficient and the inefficiency is highly variable. So the relationship between work done and energy expended is non trivial. Running a marathon on a level track does approximately 0 work, but burns lots of calories.
 

1. How does physics play a role in weight lifting?

The physics of weight lifting is essential to understand in order to maximize performance and prevent injury. Newton’s laws of motion, specifically the second law, explain how force and acceleration are related. In weight lifting, the force exerted on the barbell determines how much weight can be lifted and the acceleration of the barbell. Additionally, the concept of work and energy can be applied to weight lifting, as lifting weights requires the transfer of energy from the muscles to the barbell.

2. What is the relationship between weight and mass in weight lifting?

Contrary to popular belief, weight and mass are not interchangeable terms. Weight is a measure of the force exerted on an object due to gravity, while mass is a measure of the amount of matter an object contains. In weight lifting, the weights on the barbell determine the force that must be overcome by the muscles, while the mass of the barbell, combined with the weights, determines the total amount of weight being lifted.

3. How does the center of mass affect weight lifting?

The center of mass is the point at which the mass of an object is evenly distributed. In weight lifting, the center of mass is an important factor to consider as it affects the stability and balance of the lifter and the barbell. The lifter must maintain their center of mass over their base of support to prevent the barbell from tipping over. Additionally, moving the center of mass closer or further away from the body can impact the difficulty of a lift.

4. How does the concept of leverage apply to weight lifting?

Leverage refers to the use of a lever to amplify the force applied to an object. In weight lifting, this concept can be seen in the use of different types of equipment, such as weightlifting belts, lifting straps, and lifting shoes. These tools can alter the position of the body and the barbell, allowing for a more advantageous position and increased leverage, resulting in a more efficient lift.

5. What role does friction play in weight lifting?

Friction is the force that opposes motion between two surfaces in contact. In weight lifting, friction can have both positive and negative effects. On one hand, friction between the lifter’s hands and the barbell can improve grip and prevent the barbell from slipping. On the other hand, friction between the lifter’s feet and the ground can create resistance and make it more difficult to lift the weight. Understanding how to manipulate friction can help a lifter maximize their performance.

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