Final answer:
The FITT principle used when beginning exercise at a comfortable level and gradually increasing the intensity is called 'Progression'. Progression involves the gradual increase in any of the FITT components, such as frequency or time, thereby increasing the challenge of workouts over time.
Step-by-step explanation:
Recalling the FITT principles in the context of your question, the principle used when you start exercising at a comfortable level and gradually increase the challenges is called Progression. The FITT principle, namely Frequency, Intensity, Time, and Type, is often referred to fitness training. However, the Progression principle specifically deals with the gradual increase in any of the FITT components, thereby increasing the challenge and making your workouts more rigorous over time. For example, if you start running once a week (a comfortable frequency), then you may increase this to twice a week after a month (a progression). This principle ensures our bodies adapt to exercise gradually preventing injuries and promoting healthier physique.
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