Even if you only do squats, you still get almost as strong at leg extensions as someone who only does leg extensions—so squats aren’t useless for that movement.

If you want to get stronger at leg extensions, doing squats works just as well as doing leg extensions.

If you want to get stronger at squats, doing squats is way better than doing leg extensions—but if you want to get stronger at leg extensions, both exercises work about the same.

If you want to get stronger at squats, doing squats makes you way stronger than doing leg extensions.

Which leg exercise you choose matters: leg extensions grow the front thigh muscle more, squats grow the lower outer thigh more and make you stronger at squats—but both help you get stronger at leg extensions.

Whether you train one leg or both legs at once for 12 weeks, your legs get just as big and strong overall — the way you do the exercise doesn’t change how much muscle you build or how strong you become in general.