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So I was looking at the work= force x distance equation and was just wondering if for example I normally do 100 dead lifts at 100 pounds. Would I get the same calories burned if I did 50 dead lifts at 200 pounds. According to the work formula I am doing the same amount of work. If this is true how do you know our muscles use the same energy at double the weight and half the reps? How can we prove that our muscles are burning the same energy with half the reps at double the weight. Do we just accept it because it satisfies the work formula?

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marked as duplicate by sammy gerbil, stafusa, glS, Kyle Kanos, rob Aug 10 '18 at 19:42

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    $\begingroup$ You don't "just accept it" because it is wrong. The energy burned by your muscles is not equal to the mechanical work done. Try standing still while holding a 100 pound weight motionless above your head for 30 minutes - you are doing no mechanical work, but your muscles are definitely "burning energy!" $\endgroup$ – alephzero Aug 4 '18 at 16:20
  • $\begingroup$ Closely related: physics.stackexchange.com/q/1984 and links therein. $\endgroup$ – dmckee Aug 4 '18 at 16:26
  • $\begingroup$ This question is in the area of exercise physiology, not physics. You have no information regarding the efficiency of your muscles vs. the load on those muscles. That efficiency will definitely be well below 100%, and will no doubt be influenced by the load. $\endgroup$ – David White Aug 4 '18 at 17:50
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    $\begingroup$ Possible duplicate of How to calculate calorie expidenture from lifting a weight? $\endgroup$ – sammy gerbil Aug 6 '18 at 7:59
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The work energy formula applies to isolated systems, where all energy is accounted for and there are no external forces meddling with the energy budget.

When you double the weight you lift and lift it half as many times, you do the same work against gravity for that weight. But that's not the only weight you are lifting. Your body has mass that moves up and down. You must do work to lift your body too.

Gravitational potential energy is not the only energy type to account for either. The conversion of chemical potential energy to mechanical energy in your muscles is not perfect: when you burn 100 food Calories, you don't get 100 Calories of mechanical energy. Your body also uses energy to do other tasks while you are lifting: maintain it's temperature, push blood around, etc.

Your muscles don't burn the same energy in each case. But you do the same work on the barbell against gravity in each case.

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  • $\begingroup$ This isn't wrong, but it misses the huge issue of biological efficiency as it applies to this problem. $\endgroup$ – dmckee Aug 4 '18 at 16:27

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