View Full Version : Muscle efficiency

October 1st, 2003, 04:29 PM
Does anyone know how efficient human muscle tissue is as a means of converting stored energy into mechanical work? More precisely, how much stored energy (in terms of ATP, fatty acids or glycogen -- say, expressed in calories) must a muscle burn in order to perform one foot-pound of mechanical work? Of course this will depend on a lot of things (age, conditioning, fatigue level,etc.), but just a range or ballpark estimate would be useful.

(I am trying to estimate the energy expenditure of various activities using this approach.)

October 2nd, 2003, 01:57 PM
A study by Barclay and Weber on mice (rather than humans) found muscle efficiency to be between 13.5% and 13.9%. This is the percentage of the total energy expended by the muscle tissue that results in mechanical work (i.e., a force applied over distance). The rest of the energy is mostly dissipated as heat

Let's assume that human muscle is also about 14% efficient. Suppose I bench press 100 pounds, 10 reps x 3 sets, and my arm extension is 2 feet. That's 6,000 foot-pounds of work. Divide by 0.14 to get 42,857 foot-pounds, the energy cost to the muscle. Finally convert to Calories by multiplying by .000324, to get
a measly

13.89 Calories

from my bench press workout. (This doesn't count the energy expended when the weight is being lowered back to my chest -- not sure how to handle that.)

If I painstakingly calculate the effect of the rest of my weight training routine, I come up with about 75 Calories total -- which is equivalent to a small apple.

Rob Copeland
October 2nd, 2003, 04:59 PM
I have no idea what you are looking for here, but just a couple of observations.
1) you did not take into consideration the mechanical work involved in lowering the weights during each rep.
2) you did not consider the weight of each part of the arm, there is mechanical work involved in just raising your arms in a bench press motion.
3) the burning of calories in humans is not directly related to mechanical work. To prove this, put 800 pounds on the bench press and push with all of your might for a couple of minutes. No mechanical work has been accomplished with respect to the weight; however I would assume that calories were burned in your physical effort.

So cheer up! Your 30 reps may be worth an apple and a banana!

October 2nd, 2003, 06:56 PM
Excellent points, Rob. I already mentioned item (1), which I figure might contribute a couple of calories, but not another full 13.89. As for (2) and (3), I suspect that to some extent these effects are reflected in the 14% efficiency. I might not be able to improve on my calculation, however, without having a lot more detailed knowledge.

I've also tried a completely different approach to determining calorie costs. From January until June, I kept a daily log of calories consumed, my body weight, number of minutes swimming, cycling, or weight training. I then ran a least squares estimate (I did some averaging in advance so as to make the computations tractable -- we're talking huge matrices). The results showed that I burn 1950 Calories per day, plus 260 per half hour swimming, 150 per half hour cycling, and 130 per half hour weight training (above and beyond basal metabolism). I believe that the averaging smoothed out the sampling errors (e.g., swimming harder some days); the main confounding variable being changes in body composition (I'm gaining muscle, losing fat), which I do not know how to adjust for.

If you're wondering why I bothered with all this, it's my left-brained control-freak idea of fun.

Phil Arcuni
October 2nd, 2003, 08:20 PM
ex-runner, I am impressed. You are my kind of geek. You and lefty make me feel like part of a family!

October 4th, 2003, 07:15 AM
Dang! The next time people joke about my being quick to calculate relay splits, I should point them to your post. :D

I'm trying to remember back to high school biology. I think reptile muscle is a lot more efficient for work output, because they are cold blooded. Our system is designed to put out a lot of heat, to keep the body temperature up.

October 5th, 2003, 09:28 PM
Someone has determined a while ago, how many calories muscles burn, pound of muscle, per hour. also, how mahy calories muscles burn under certain activities.
If you want to know how many calories you need, research that number, and multiply by your lean body mass. That's your basic caloric need at rest. then adjust for the activities.
You can probably find that with some net searching...
I've seen and used it before.

Fat fdoesn't requre much in a way of spent calories to 'live', so that number is neglectable.

October 9th, 2003, 01:40 PM
Energy expenditure in sports activities is always fun to calculate...For a thorough understanding you can look at chapter 19 in Wilmore and Costill (Physiology in exercise and sport). They describe the energy cost of lots of activities…remember that the people they tested may not do the activities regularly and with regular activity these numbers will go down (increased economy).

On the direct answer to your question…when we teach this in biomechanics we use ~25% efficiency of metabolic to physical work. We also use 1/3 as the relationship between positive (concentric) to negative (eccentric) work. If you use the previous 6000 ft-lbs, you multiply by 4 to get 24000 then take 1/3 or 8000 and add for a total of 32000 ft-lbs. This is slightly lower than the other methods and using anywhere for 15-30% efficiency is probably good. In swimming, there is another aspect to be considered…the best swimmers are only ~85% efficient in the water.

October 16th, 2003, 02:49 PM
Thanks, Bryan, for the information and the book recommendation -- exactly what I was looking for.