What does the term 'progressive overload' refer to in training?

Progressive overload is a fundamental concept in strength and conditioning that emphasizes the gradual increase of stress placed on the body during training. This approach is crucial for stimulating muscle adaptation and growth. When a person progressively increases the intensity and volume of their workouts—whether through lifting heavier weights, increasing repetitions, or enhancing the overall workload—they force their body to adapt to new levels of stress. This adaptation is what leads to improvements in strength, endurance, and overall fitness levels.

In contrast to the other options: lifting weights every day without variation does not inherently lead to better results and can sometimes lead to overtraining. Reducing workout duration could be counterproductive to building fitness, and maintaining the same workout routine indefinitely can lead to plateaus in performance and a lack of advancement in physical fitness. Therefore, the identified answer correctly captures the essence of how to effectively progress in a training program by continuously challenging the body in a structured manner.