The Leg Curl Lie

“Leg curls train your hamstrings.”

That’s what you hear everywhere.It’s repeated so often it’s treated like fact.

But it’s only half true—and that’s the problem.

What’s Really Happening

Leg curls are knee flexion. That’s it.

They train the back of the thigh to bend the knee, and in practice they tend to bias the biceps femoris—the outer, more visible portion of the posterior leg.

So yes, technically they involve the hamstrings.

But calling them “hamstring training” without qualification is like calling a bicep curl “full arm development.”

It’s incomplete.

What They Don’t Do

They do not meaningfully train:

• Hip extension

• Loaded lengthening at the hip

• The function that actually carries over to sprinting, hinging, and real strength

That’s where:

• RDLs

• Stiff-leg deadlifts

come in.

These movements load the hamstrings where it matters—at the hip, under stretch—shifting emphasis toward the semitendinosus and semimembranosus, not just the lateral fibres.

Why This Matters

This is why people can:

• Stack plates on a leg curl

• Yet have weak deadlifts, poor sprint mechanics, and underdeveloped posterior chains

Because they’ve trained a joint action, not a complete muscle function.

The Reality

Let’s say it properly:

Leg curls bias the biceps femoris through knee flexion.Hip hinges train the hamstrings as hip extensors, with greater emphasis on the semitendinosus and semimembranosus.

Same muscle group.Completely different stimulus.

Bottom Line

If all you do is leg curls, you’re not “training your hamstrings.”

You’re training part of their function—and calling it the whole story.