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.

When people train with their muscles bunched up, the more their muscle fibers angle upward, the longer they can keep pushing hard before getting tired.

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

For people who regularly lift weights, training until they can't complete another repetition leads to the same increase in quadriceps muscle size as training that stops 1-2 repetitions before failure, even though the failure condition causes more short-term muscle fatigue.

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 strength exercises with your muscle stretched out for 8 weeks can make the muscle fibers longer, but doing them with your muscle bunched up doesn’t change their length.