Let's say I'm riding a stationary bike with a power meter. It measures my power as 200W and I workout for 10 minutes. This is a total work of 120 J or 28.7 Cal. Now is that how many calories I've "burned", period? Does heart rate, age, sex, weight, temperature, etc affect this?
Now is that how many calories I've "burned", period?
The short answer to your question is No. Other factors do play a part in how many calories you burn during exercise. Since calculating the exact amount is somewhat difficult, researchers have published tables of energy expenditure for common activities. The energy expenditure is typically expressed in METS for some of the tables. The tables are not an exact measurement of the expenditure for physical activity, but, rather a way to classify physical activity by METS. In the Compendium of physical activities: classification of energy costs of human physical activities (Department of Epidemiology and Biostatistics, Department of Exercise Science, School of Public Health, University of South Carolina), the researchers indicated that the amount of energy expended for weight bearing exercises was…
“…higher among heavier individuals than indicated by the Compendium’s MET intensities. For these individuals, use of the MET intensities in the Compendium would underestimate the actual energy cost of weight bearing activity. The opposite pattern would be observed for non-weight-bearing activities.”
Additionally, they stated that…
“Similar observations may apply to individuals who differ in age, cardiorespiratory fitness levels, and mechanical efficiency and when activities are performed in varied geographic and environmental conditions.”
So, the take away from this type of research would indicate that there are other factors that come into play when determining the energy expenditure of an activity. These include…
“…differences in body mass, adiposity, age, sex, efficiency of movement, geographic and environmental conditions.”
Calories burned are directly correlated to heart rate, which on its own, is directly correlated to a bunch of different factors as the ones you mentioned (age, sex, weight, physical shape of the individual).
To illustrate this, imagine a young, female and very small marathon runner, jogging for 10 minutes at a very slow pace around the block. She isn't going to be very tired and will barely make a sweat. Now think of a very obese, older man who hasn't done any exercise in decades trying to do the same thing. He will burn way more calories than her, since (1) he's much bigger and the energy required to move his whole body from point A to point B is higher and (2) he's so out of shape that the littlest exercise will get his heart racing.