1 · SAID principle · specific adaptation to imposed demand
The body adapts only to the demand you actually impose: squat and it adapts to squats, run and it adapts to running, sit and it adapts to sitting.SAID = Specific Adaptation to Imposed Demand. The body only adapts to demands you actually impose, not what you 'want' to be good at.This means:
Train squats → body adapts to squats (neural recruitment, quad/glute hypertrophy)Train distance running → body adapts to running (mitochondrial density, capillaries, fat oxidation)Sit all day → body adapts to sitting (sarcopenia, insulin sensitivity decline, hip flexor shortening)
Adaptation is passive, not active. The body doesn't 'get healthier' — it gets better at what you make it do. This is why 'general fitness' is hard: you can't simultaneously maximize strength, endurance, mobility, and power because these adaptations partially conflict at the molecular level ( vs ).