There are several different ways to quantify the load the body is exposed to during physical exercise. These can be divided into measures of internal load and external load. Internal load refers to how hard the body is working, while external load refers to the work performed by the body. Examples of metrics that reflect the body`s internal load include heart rate, lactate, and oxygen uptake (VO2). Of these, VO2, unlike the other two, is also a measure of capacity (work capacity). Even though VO2 is a measure of work capacity, it is not in itself a measure of performance. To compare performance we need measures of external load. The best measure of external load is work performed per unit time — i.e., power (watts). Power is proportional to VO2 through the body`s efficiency. Put simply, this means that external power can be increased either by increasing internal capacity as measured by oxygen uptake, or by improving efficiency.
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