Final answer:
The Bruce Protocol is the most commonly used treadmill test protocol, designed to assess cardiovascular fitness by incrementally increasing treadmill difficulty.
Step-by-step explanation:
The most commonly used treadmill test protocol is the Bruce Protocol. This protocol is a standardized way to test the cardiovascular fitness of an individual by increasing the difficulty of the treadmill exercise incrementally. It involves stages, each usually lasting 3 minutes, where both the speed and the incline of the treadmill are increased. The Bruce Protocol is often used in clinical settings to assess patients for heart disease or to evaluate functional capacity in individuals.