# Energy use by muscles, actual work done by muscles and more

Lately, I've started exercising in the gym and outside. I've also started to look at the details of food I eat.

Food usually has a label saying the amount of energy is inside it. For example, some chocolate says it has 400 kilocalorie for 100 grams.

I have some questions about that:

1. Does the label say the amount of total chemical energy exists inside the food, which will be released if burned, or the energy available to the human body after digested, and because digesting requires energy, the food contains actually MORE energy then written?

2. When I'm riding on a static bike, it has a screen which counts the kcal I burn. It has a thing connected to my arm, measuring my heart beat. After about an hour of riding, it said I used about 700 kcal. Is it the amount of work I did, i.e if there was a battery connected to the bicycle, I'd generate 700 kcal of electricity, OR, the amount of energy my body LOST, because muscle efficiency is far from 100% (so, I lost 700 kcal, but generated only 150 kcal of electricity, for example)

3. When doing a physical activity, the heart beat and breathing rate increases. Lets say I'm riding a bicycle or running. Whats the ratio between energy consumed by the muscle to energy consumed by the lungs or heart?

• Looking at the answers for question one it's hard to think of any other meaningful way to do it other than recording the energy released when burnt, any other measure would hardly be standardised between people. Commented Jun 2, 2012 at 23:40

1. I speak only for the U.S. regulations: the calorie labels on wrappers refer to the energy released when burned. Sometimes these are inaccurate. Many dieticians recommend calculating the calories based on weights of protein, carbohydrates and fats in the serving: 4 kcal in each gram of protein and carbohydrate and 9 kcal in each gram of fat in your food. Multiply carbs, proteins and fats by the appropriate value and add them up.

2. The reading on the bike does not take into account your pulse. Just the raw work done turning the pedals. Some very smart bikes do take pulse into account, but the often inaccurate heart monitors lead to inaccurate calculations of total energy.

3. The heart and diaphragm (muscle primarily responsible for breathing) do use more energy when exercising than when you are sitting still. But that increase is small relative to the increase in the primary muscle sets involved in the exercise. Your resting heart rate is probably around 70 bpm, your exercising heart rate is probably around 160-180 bpm. So it has increased its rate and energy consumption by about 3 times. Your skeletal muscles maintain a basal metabolic rate during inactivity (one purpose of this is your body needs the heat from slightly tensed muscles to thermoregulate itself) - but during exertion, their metabolic rate increases enormously. Exercise increases their energetic demands by whole orders of magnitude.

The actual value of the (energy used by heart) / (energy used by skeletal muscles during exercise) depends on what exercise you do and how efficiently you do it. The change in the ratio between inactivity and activity depends on how much muscle mass you have (more muscle uses more energy during inactivity) and how strenuously you exercise. But the point is that the increased energetic demands by skeletal muscles far outpace the 3x increase in energetic demands by the heart and diaphragm - so the latter accounts for pretty much all total energy consumption at even moderate activity levels.

• to be clear one of the major sources of error in calories in food is fiber and other such material that will burn but not be digested. which is why many countries require carbs to show fiber content.
– John
Commented Jul 16, 2018 at 16:24

1) It depends on each country and your minister. In Finland, the thing is done so that it says the energy stored in food, which will be released if burned.

2) It is the energy pushed to the bike when the bike does not take into account your pulse. If the bike is smart one, it takes into account your pulse rate, then it tries to estimate the amount of energy released by your body. This is done differently among different manufacturers of those watches.

3) By the way heart is a bunch of muscles, a myogenic muscular organ! So consider instead heart and some part of lungs. Very broad problem otherwise.