The seated leg extension is one of the better quadriceps isolation movements you can do. Though they’re commonly seen in the gym, that doesn’t mean you’re getting the most out of the exercise. At the same time, changing up how you do them can change up exactly which muscles you target.
It might seem silly to ask “what muscles do leg extensions work?”, as the simple answer is to say the quadriceps. However, you need to realize that foot positioning can be critical. Altering what direction your toes point can actually affect where in the muscle emphasis is put.
Pointing the toes inward generally shifts the emphasis to the outer quadriceps. On the flip side, pointing the toes outward keeps the emphasis on the inner quads. However, most lifters will get the most out of keeping the toes pointed straight, which will work the quadriceps overall as a whole.
Now that you know the solution to the question, “what muscles to leg extensions work?”, you need to keep cognizant of what muscles you don’t want to work. Though the hip flexors and / or groin aren’t generally as strong as the quadriceps, they sometimes can be still brought into play when doing leg extensions. This usually happens when you have your machine setup incorrectly or you’re trying to use weight that’s too heavy.
There are a couple ways to minimize this. The first is to ensure that you don’t want to lower the bottom portion of the leg extension such that the angle in your knee decreases below 90 degrees. Doing so could not only strain the knee, but it’ll often involve the hip flexors.
Continuing along those lines, make sure your range of motion is kept on point. One way to ensure ROM is to keep your rep speed moderate. Going too far at one end or the other is much more likely when you’re moving too fast and start to lose control. Keeping a decent pace will not only keep proper tension on the muscle and help you maintain proper ROM, but will help you keep from inadvertently involving the muscles of the groin.
Since leg extensions are an isolation exercise, they’re best used later in your workout. Emphasis should be first be on compound movements like squats and the leg press, then you can finish your quads off with the leg extension machine at or near the end of your session. 3-4 sets of 10-12 reps works well, though once in a while doing sets of 15+ reps as a burnout can act as a great finisher.
The leg extension is one of the best finishing and isolation movements for the thighs that you can do. Perform it after your squats and leg presses for moderate to higher reps with a moderate cadence. Ensure you don’t extend the range of motion too far one way or the other to keep the knee healthy, and feel free to shift where in the leg emphasis is placed by altering foot position. Do all this and you’ll be improving your leg’s muscular development in no time.