Energy consumption when lifting weights

• Sep 4th 2011, 04:25 AM
Energy consumption when lifting weights
Hello.

This is probably quite basic, but it's been a bit more than 20 years since I studied math/physics, so that's why I turn to you guys. I am trying to figure out how to calculate the energy consumption during weight lifting.

I guess there's five calculations;
1. Lifting a weight in a straight line
2. Lowering a weight in a straight line
3. Lifting a weight with a circular motion
4. Lowering a weight with a circular motion
5. Holding a weight in a fixed position

I think I got the first one right.

Using these definitions,
• Mass (m) = 90 kg
• Distance (h) = 0.48 m
• Gravity multiplier (g) = 9.82
• Joule (J) to calorie (cal) multiplier = 4.2

I get this;

\$\displaystyle J = mgh = 90 x 0.48 x 9.82 = 424.224\$
\$\displaystyle cal = J/4.2 = 424.224/4.2 \approx 101\$

(However, I am uncertain of this as it sounds quite a lot to use 101 calories to lift 90 kg 0.48 m, compared to, say, about 500 calories by running for 20 minutes.)

For the second item in the list, I guess time needs to be added as a variable, as it's "slowing down the g-speed" (if dropping, the item would fall by 9.82 m/s2, right?).

Number three and four will probably be some variants of one and two and I'm not even sure if they're relevant, but I suspect they may be, since the lifting/lowering don't happen along a straight line.

The last one, number five, should, imho, be the same as number one, but since the weight ain't moving neither up or down, the distance will be 0 and as such the calculation fails. This tells me I don't understand this one at all.

I'm sorry for turning to you with such (probably simple) problems and with such limited pre-knowledge, but in my attempts to find any formulas for this on the web, I've failed for about six months now. If anyone could help me out a bit, I'd be a very happy panda. (Happy)
• Sep 4th 2011, 06:52 AM
CaptainBlack
Re: Energy consumption when lifting weights
Quote:

Hello.

This is probably quite basic, but it's been a bit more than 20 years since I studied math/physics, so that's why I turn to you guys. I am trying to figure out how to calculate the energy consumption during weight lifting.

I guess there's five calculations;
1. Lifting a weight in a straight line
2. Lowering a weight in a straight line
3. Lifting a weight with a circular motion
4. Lowering a weight with a circular motion
5. Holding a weight in a fixed position

I think I got the first one right.

Using these definitions,
• Mass (m) = 90 kg
• Distance (h) = 0.48 m
• Gravity multiplier (g) = 9.82
• Joule (J) to calorie (cal) multiplier = 4.2

I get this;

\$\displaystyle J = mgh = 90 x 0.48 x 9.82 = 424.224\$
\$\displaystyle cal = J/4.2 = 424.224/4.2 \approx 101\$

(However, I am uncertain of this as it sounds quite a lot to use 101 calories to lift 90 kg 0.48 m, compared to, say, about 500 calories by running for 20 minutes.)

Food "calories" are kilo-calories.

CB
• Sep 4th 2011, 04:28 PM
HallsofIvy
Re: Energy consumption when lifting weights
The "work done" will be "mgh" but your muscles and joints are not perfectly "efficient". Even holding a weight a "constant height", your muscles "slip" a little and then have to move the weight back up.