Which principle in physical activity claims that in order to progress and improve our fitness we have to fat our bodies under additional stress?

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Which principle in physical activity claims that in order to progress and improve our fitness we have to put our body under additional stress?

What principle should be followed to improve physical fitness?

What principle of exercise states that the only way to improve fitness is to increase one variable such as resistance or repetitions as your fitness level improves?

Is the principle of exercise that states that in order to improve your level of fitness you should gradually increase the frequency intensity and duration of your workouts?