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.

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 men who don't usually exercise start lifting weights, the angle of their muscle fibers changes by about 9% in 12 weeks. It doesn't matter if they lift heavy or light weights, or if they push until they can't do any more reps or not.

Doing strength exercises with your muscle bunched up for 8 weeks can make the muscle fibers angle more sharply, which might help them generate more force.

If new guys lift weights for 15 weeks using only part of the movement—focusing on the shorter muscle position—they build more muscle size in that specific area and get stronger at certain joint angles, according to this claim.