Which exercise method involves alternating periods of intense exertion and active rest?

Interval training is defined by its structure, which consists of alternating periods of intense exertion and active rest. This method is designed to improve cardiovascular endurance, increase speed, and enhance aerobic and anaerobic performance by pushing the body through varying intensity levels. During the intense phases, the body works hard, allowing it to build strength and efficiency. The active rest periods further help in recovery while maintaining an elevated heart rate, maximizing calorie burn, and improving metabolic conditioning.

In contrast, continuous training involves steady-state exercise at a moderate intensity without rest intervals, which focuses more on endurance than on varied exertion. Strength training emphasizes building muscle through resistance and typically does not involve alternating intensities. Flexibility training focuses on improving the range of motion in joints and does not involve exertion or active rest like interval training. Therefore, the key characteristic making interval training the correct answer is its unique alternating exertion approach that targets both aerobic and anaerobic systems effectively.