After doing leg exercises with your knee bent all the way back, your muscles are weaker not just at that angle—but even more so when you try to use them with your knee only slightly bent, as if your muscle fibers got stretched out and lost their normal power.

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.

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.

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.

When people train with their muscles stretched out, the longer their muscle fibers get, the more quickly they tire out during repeated efforts.

Lifting weights until you can't do another rep leaves you much more tired during and after the workout than stopping a few reps short — even if both methods build muscle equally.