Final answer:
The measure of energy expenditure to mass during physical activity is referenced in terms of metabolic rate, particularly using oxygen consumption rates to quantify energy use.
Step-by-step explanation:
The measure of the ratio of a person's energy expenditure to their mass during physical activity is usually referred to in terms of metabolic rate. One may discuss this in terms of the basal metabolic rate (BMR), which provides an estimate of the energy required by a person at rest, in a neutral temperature environment, to maintain basic bodily functions. This energy consumption is directly proportional to oxygen consumption, with about 20 kJ of energy being produced for every liter of oxygen consumed.
For measuring the metabolic rate during physical activity, oxygen consumption rates can offer insights since the digestive process is fundamentally about oxidizing food, and thus energy consumption can be quantified by measuring oxygen use. Energy expenditure during various activities can be determined by the amount of oxygen consumed, which is indicative of the total power expended in sustaining those activities. Therefore, the metabolic rate during physical activity would be higher compared to BMR due to increased demands on the skeletal muscles and heart.