We get what we train for.
If stresses, strains and stimuli are placed upon the body, it'll generally find ways to respond or adapt. The SAID Principle states “the demands placed on the body dictate the type of adaptation that will occur”. So the type of stimulus applied during exercise will determine the physiological response and adaptation. For example, lifting heavy weights will improve motor unit recruitment (how the brain uses muscle fibers) and stimulate muscle growth resulting in strength gains.
Another example is how doing cardio-respiratory or aerobic work will improve the efficiency of how the heart pumps blood and efficiency of how the cells use oxygen. Proper application of the SAID principle can therefore lead to more effective training for your goals! (This is from the National Strength and Conditioning Association, US)
Another description: It means that when the body is placed under some form of stress, it starts to make adaptations that will allow the body to get better at withstanding that specific form of stress in the future. The adaptation process does not occur by any one mechanism - it is a general tendency of the body which is played out in innumerable separate mechanisms. While it is almost impossible to understand and account for all these separate mechanisms in devising a training program, it is easy to remember the general SAID principle - it means that the body is always trying to get better at exactly what you practice. Be careful what you choose!
This is from Bettermovement.org
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