How much energy do muscles consume to hold an object stationary above the ground?

[Accelerando]

While trying to write a character sheet for a game I wanted to know how to find how much energy a human being would use in order to carry a weight on, say, their back. Since there appears to be no work done if I regard the human as a solid mass, how can I calculate how much energy the muscles are spending to keep the body in this position? Google yielded no informative results.

[*Aqua*]

http://en.wikipedia.org/wiki/Lever

A standing human with a backpack is a lever from a physical view point. If you fill the backpack the center of mass will move a bit in its direction. To not falling over we lean a bit forward to shift the center back.

But whatever you calculate remember that a 1 kg backpack will always apply ~10 N force on the human.

Edited March 11, 2014 by *Aqua*

[peadar1987]

But work done (energy expended) is force multiplied by distance.

In theory, no work is done at all. In reality, all of the constant, small corrective movements you make in order not to fall over are doing work. I would guess that they are almost impossible to analyse mathematically, you'd have to do it experimentally.

This article: http://www.bbc.co.uk/news/magazine-24532996 isn't peer-reviewed or anything, but it suggests about 0.7 kilocalories an hour extra are burned when you're standing up. That's 2940J, so to stand up instead of sit down expends about 0.816W of power.

[DonLorenzo]

I imagine in mechanical terms a human is more like a liquid than a solid. You expend work to keep your shape (standing upright and balancing). If you lock your joints and lean against a wall or something you resemble a solid more

[RuBisCO]

Human muscles must consume energy just to remain contracted (with exception of smooth muscle which can "lock") if we assume human human skeletal muscles must spend the same amount of energy to remain contracted as to contract, then the determine the energy to hold a kilogram above the around is nothing more then the force of that kilogram (~10 newtons as stated above) per second. Now comes the tricky part, convert newtons to joules or watts.

[Sirrobert]

On the back, very little, asuming it's a properly designed backpack.

A good backpack transfers all the weight to the hips, and through that on the legs.

Thus, a good backpack simply ads a few kilograms to the weight the legs already have to carry (about 65-80kg)

[Seret]

This is a biomechanics problem, you can't really simplify it down to modelling a human as a static object. The human body is not statically determinate, so it's is a dynamics problem even when you're standing still.

Most the weight will be carried by the bones, true enough, but the skeletal muscles will have to be making constant adjustments, and for a load carried on the back that will be made worse by having balance affected. Obviously this gets even more complicated when you start moving, as you're now dealing weight shifting in all sorts of directions.

But the muscles aren't the only story, as the whole system has a lot of overheads. Forget trying to calculate it, go for actual experimental data that shows increases in metabolic rate, like this one. For backpacks of 0% of bodyweight, 15%, and 30% VO2max while walking briskly was measured as 30%, 36% and 41% respectively.