1. Your muscles need to constantly work to keep you standing or sitting. Otherwise, you’d just fall down like a ragdoll.
2. Your body is constantly producing minute movements (pumping blood, breathing, moving your digestive system).
3. You’ve never perfectly balanced. Your center of mass is always at least a tiny bit off your center of balance, even by just a tiny amount, so gravity tends to flip you sideways, so your muscles need to correct against that.
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