When guys sit down and squeeze their quads as hard as they can, their hamstrings resist more than when they're lying flat — probably because the hips are bent.

When guys get stronger from doing isometric leg exercises, their hamstring muscles don’t start working differently alongside their quads—so the way their hamstrings fire doesn’t predict whether they’ll get stronger or not.

When healthy guys push with their legs, they're just as strong sitting or lying down—even though one position stretches the muscle more. That means muscle strength isn’t just about how stretched the muscle is.