When you push hard with your knee bent all the way back, your thigh muscles have to work much harder internally than when your knee is only slightly bent—even if you’re pushing with the same force on the machine.

Doing strength exercises with your knee bent further back makes your muscles weaker and harder to activate for up to two days afterward—even when you’re testing them at different knee angles—compared to doing the same exercise with your knee less bent.

If you do leg exercises with your knee only slightly bent, your muscles stay strong at that angle—even right after the workout—but if you do them with your knee fully bent, your muscles get weaker at that angle and also at other angles.

Lifting weights when your muscles are stretched out more (like at the bottom of a squat or a full bicep curl) tends to make them grow bigger than lifting with your muscles mostly shortened, as long as there's still resistance pushing back when they're stretched.

Doing 8 weeks of strength training with your muscle either stretched or bunched up doesn’t make you much better at resisting fatigue — the improvement is very small and not clearly real.