Doing two different hamstring exercises—Nordic curls and stiff-leg deadlifts—makes you stronger at both, even if your muscles don’t grow the same way, meaning muscle growth isn’t what’s making you stronger in the other exercise.

Both Nordic curls and stiff-leg deadlifts make your entire hamstring group bigger after 9 weeks of training—Nordic curls make them grow a bit more.

Even when your hamstrings get bigger from training, that doesn’t really predict how much stronger you’ll get at the other exercise—so bigger muscles don’t mean much more strength in this case.

Nordic curls make one part of your hamstring (semitendinosus) grow more, while stiff-leg deadlifts make another part (semimembranosus) grow more—so different exercises target different muscles in the same group.

Even if you're not pushing very hard, doing short muscle holds at different knee angles for eight weeks can make your hamstring muscles bigger and stronger, no matter if your knee is mostly bent or almost straight.

When untrained young men lift heavy weights three times a week for five months, they get much stronger—some exercises more than double their original strength.