What is the training concept holding that the demands placed on a body system must be increased systematically and progressively over time to cause physiologic adaptation?

The training concept you're referring to is known as the "Principle of Progressive Overload." This principle asserts that in order to achieve physiological adaptations and improvements in strength, endurance, or other fitness parameters, the demands placed on the body must be gradually increased. This can be achieved by adjusting variables such as intensity, duration, frequency, or type of exercise. By consistently challenging the body, it adapts to the stress, leading to enhanced performance and fitness levels.