Which training principle suggests that certain adaptations occur only if the body is pushed beyond its comfort zones?

The principle of progressive overload is fundamental in understanding how the body adapts to training. This principle asserts that in order to achieve certain adaptations—such as increased strength, endurance, or muscle size—one must continuously increase the demands placed on the body beyond its current capabilities. By pushing the body beyond its comfort zone, whether through increasing weights, repetitions, or intensity of exercises, the body undergoes stress that leads to physiological adaptations. These adaptations occur as the body responds to the demands placed upon it, ultimately leading to improved performance and greater resilience to the same stressors in the future. This concept supports the idea that merely maintaining a consistent routine without challenge will not yield significant improvements in fitness levels.