Tip: Dump the Seated Leg Extension, Do This Instead

The leg exercises you're doing have a glaring hole. Yes, the squats, lunges, and especially the leg extension are leaving out one very visible muscle, the rectus femoris (RF).

The RF is the big muscle that can be seen in the middle of the quads. Unlike the rest of the quads which only have one function – extending the knees – the RF has two more responsibilities: hip flexion (anytime the hips move the upper and lower body closer together) and anterior pelvic tilt (when the top of the pelvis tilts forward).

The fact that the RF has two extra tasks to perform means it has to be trained differently than the rest of the quads. To properly train the rectus femoris, your best option is the standing leg extension.

Standing Leg Extension

1. The key to performing this exercise properly is finding something to hold on to. As soon as the weights get a little heavy, balance will become an issue. If you don't happen to have a gigantic beam in front of your cable machine, then grab a person (preferably someone you know).
2. In order to get the weight into position, pull the cable all the way out and step on it. After you've got your foot on the cable, you'll have the freedom to wrap the attachment around your ankle.
3. Place the cable pin about equal with your butt. Any higher or lower and the range of motion will be shortened.

Reps and Sets

The RF is a fast twitch powerhouse, meaning that it expends energy relatively quickly. Keep the amount of reps between 4-8 and the amount of sets per week between 3-6.

Why does having two extra functions mean the RF has to be trained differently?

There's a concept called Length-Tension Relationship. Muscles that pull on two joints are called "bi-articulate." Because they attach to multiple joints, bi-articulate muscles have multiple jobs, but they aren't very good at multitasking.

Think about it like this: You CAN watch sports and listen to your girlfriend tell you about her day at the same time, but you can't do either thing especially well. You need to pick one: listen to your girlfriend or watch sports. You need to do the same thing for your bi-articulate muscles – pick one task.

Remember that the rectus femoris is responsible for hip flexion and knee extension. Most leg exercises involve hip flexion, including squats and lunges. If your hips are already flexed, then the RF can't properly do its job of extending your knees.

At least with squats and lunges, your hips extend as you lift the weight. Because the hips are in a fixed flexed position, the seated leg extension is worse than the squat and lunges for the RF. The natural solution is to do a leg extension while your hips are already extended, or while you're standing straight up.

What about the sissy squat? Isn't that technically a standing leg extension?

Yes, the classic sissy squat is a leg extension where the hips are extended. But for multiple reasons, the standing leg extension is the better option. The standing leg extension is more easily loaded, which means it's easier to make the exercise lighter or heavier. Adding or removing resistance from the sissy squat is not so easy.

The range of motion for the sissy squat is a bit longer if you touch your hamstrings to your calves. But the sissy squat is also much harder on the knees due to the forward tracking of the knee and the heels rising off the ground, which is necessary if you do a full sissy squat.

References

1. Garrett, W. E., Califf, J. C., & Bassett, F. H. (1984). Histochemical correlates of hamstring injuries. The American Journal of Sports Medicine, 12(2), 98-103. [PubMed]
2. Jennekens, F. G., Tomlinson, B. E., & Walton, J. N. (1971). Data on the distribution of fibre types in five human limb muscles. An autopsy study. Journal of the Neurological Sciences, 14(3), 245. [PubMed]
3. Johnson, M., Polgar, J., Weightman, D., & Appleton, D. (1973).Data on the distribution of fibre types in thirty-six human muscles: an autopsy study. Journal of the Neurological Sciences, 18(1), 111-129. [PubMed]
4. Kenney, W. Larry; Wilmore, Jack; Costill, David (2011-11-18). Physiology of Sport and Exercise, Fifth Edition. Human Kinetics. Kindle Edition.