The SAID principle is a fundamental concept in exercise physiology, exercise science, sports training, physical training, and physical rehabilitation that stands for Specific Adaptation to Imposed Demands, which involves understanding that the human body will adapt precisely to the specific type of stress, load, or stimulus placed upon the system.
Examples …
Heart generally improvesc
Bones generally strengthen under stress of load.
Tendons generally strengthens under stress of load.
Connective tissue generally strengthens under stress of load.
Muscles generally enlarge, strengthen, and become more speedily responsive with practice.
Nervous system adaptation generally improves improved activation of muscles and coordination.
For example, weight training improves the nervous system by adaptation from the brain to send faster, stronger, and more coordinated signals to the muscles. Early strength gains are often due to immediate nervous system adaptation that involves greater efficiency activating muscle fibers.