The amount of oxygen consumed, which can be directly measured using a metabolic cart, is the main factor in determing caloric expenditure.
So, why don’t we all just hyperventilate and take in massive amounts of oxygen?
Oxygen Consumption refers to the oxygen that is actually taken up by the tissues and used for metabolic purposes. So, to increase oxygen consumption, you have to increase metabolic activity.
Increasing the amount of muscle mass used will increase oxygen consumption. Technique is important in many instances. For example, a successful cyclist activates his glutes and hamstrings, as well as his quads, during each downstroke, whereas a novice uses mainly his quads. Distribution of work and power in the former case allows the successful cyclist to go at a higher work rate with less muscular stress at the same oxygen consumption.
Also of note is that not only the oxygen consumed during an activity/exercise should be considered, but also the oxygen consumed after the activity/exercise due to the activity/exercise itself. That is, when weight training or performing high-intensity intervals, one has an elevated oxygen consumption at rest–relative to basal conditions–that is known as Excess Post-Exercise Oxygen Consumption.