For quad growth
For quad growth, the two are close. Leg extension rates slightly higher: more of the work on the quads.
Smith machine squat gives more stretch.
Both are good choices, and they train the quads differently. Using both across your week covers more of the muscle than either alone: in studies, varied exercises grew parts of a muscle that one exercise missed. (Fonseca et al., 2014; Costa et al., 2021)
The research score comes from these, scored by hand from the studies. Lifter scores appear once an exercise has 10 ratings. How scores work
Leg Extension. The only exercise that fully loads the rectus femoris: it grew all three of its regions in one trial, where Smith squats didn’t. Lean back to stretch it more.
Smith Machine Squat. A squat on rails, so you can place your feet forward and push the quads hard without balancing a bar.
For quad growth, the two are close. Leg extension rates slightly higher: more of the work on the quads. Smith machine squat gives more stretch. Both are good choices, and they train the quads differently. Using both across your week covers more of the muscle than either alone: in studies, varied exercises grew parts of a muscle that one exercise missed.
About the same: both are easy to learn.
Leg extension needs a machine, so for most people it’s a gym exercise. Smith machine squat needs a Smith machine, so for most people it’s a gym exercise.
About the same: both rate Strong for joint and recovery cost in our research rubric.