Squats vs. Leg Extensions For Quad Growth: New Study

Key Points:

  • A new study compared quad muscle growth from five weeks of squat vs. leg extension training.
  • Leg extensions were found to provide superior muscle growth in the rectus femoris compared to squat training.
  • Vastus lateralis tended to grow more in the distal parts (25% and 50% of femur length) from squat training but grew more proximally (75% of femur length) from leg extensions.

***

The quadriceps is one of the largest muscles in your body. It is one of your most important muscles for moving around or in sports, and you cannot build a set of strong and impressive legs without them.

Your quadriceps has four different muscle heads:

  • Vastus lateralis
  • Vastus medialis
  • Vastus intermedius
  • Rectus femoris

The three vastus (or vasti) muscles originate high on your thigh bone (your femur), but the rectus femoris originates on the front of your iliac bone (your pelvis). Then, all four muscles join together into the quadriceps tendon that passes over your patella and inserts in your tibia.

Quadriceps muscle anatomy
The quadriceps muscle. Note that vastus intermedius is hidden underneath the rectus femoris.

The vastus muscles thus pass over only one joint (the knee), but the rectus femoris passes over two (the hip and knee). This means that your vastus muscles are trained in pretty much any exercise where you extend your knee, but for your rectus femoris, things aren’t always that simple.

In classic leg exercises like the squat or leg press, you extend your knee while simultaneously extending your hip. In this case, your vastus muscles are shortening, but your rectus femoris muscle length doesn’t really change, and it can’t contribute much to the work – because it is shortening in one end (the knee) but lengthening in the other (the hip). For the rectus femoris to be worked, you’d need something like the leg extension, where your hip is fixated during the exercise, or some kind of kicking exercise where your hip is being flexed during knee extension, not extended.

Taken together, this probably means that if you wanted to train your rectus femoris and thus build your quadriceps as big and effectively as possible, you can’t rely on squatting-type exercises alone.

Up until now, we’ve mostly had EMG studies and biomechanical reasoning to support everything I wrote above. And also a study or two showing that the rectus femoris doesn’t grow very much from squatting.1 2 34

That is: up until now. A new study has compared squats with leg extensions and measured how much the vastus lateralis and the rectus femoris grows from each.5

Squats or Leg Extensions for Five Weeks

In this new study, twenty-seven young men with strength training experience completed five weeks of either smith-machine squat or leg extension training.

Yes, they used the infamous smith machine, *gasp*! However, in terms of quad muscle growth, squatting with a smith machine or a free barbell probably doesn’t make much, if any, difference.

They trained their respective exercise three times per week, doing four sets of twelve reps to muscular failure each workout. The squats were done until their thighs were at least parallel to the ground, and the leg extensions used full ROM as provided by the machines used. When they could do more than twelve reps per set, they increased the weight. They took a three to five-minute rest between sets.

Before and after the five weeks of training, the researchers used ultrasound to measure the cross-sectional area of the participants’ rectus femoris and vastus lateralis at three different sites along their thigh bone: 25% (closest to the knee), 50% (middle), and 75% (closest to the hip) of femur length.

They also performed counter-movement jump (CMJ) tests before and after the training.

Results: Different Exercises, Different Muscle Growth

When the researchers retook the measurements after five weeks of training, they found several differences in muscle growth.

The leg extension group saw statistically significant growth in their rectus femoris at all three measurement sites, while the squat group didn’t see statistically significant growth at any site.

Rectus femoris growth after squats vs leg extensions
Increase in muscle cross-sectional area at three different measurement sites in the rectus femoris: at 25% (closest to the knee), 50%, and 75% (closest to the hip) of femur length. The asterisk (*) signifies a statistically significant increase (p<0.05) compared to pre-training.

The only group that saw a significant increase in their vastus lateralis was the squat training group, and only at the 50% (midway) site. However, just eyeballing the results, it seems like the vastus lateralis of the squat group generally increased more distally (towards the knee). In contrast, the vastus lateralis of the leg extension group increased more proximally (towards the hip).

Vastus lateralis growth after squats vs leg extensions
Increase in muscle cross-sectional area at three different measurement sites in the vastus lateralis: at 25% (closest to the knee), 50%, and 75% (closest to the hip) of femur length. The asterisk (*) signifies a statistically significant increase (p<0.05) compared to pre-training.

If we just calculate a simple mean value for the three measurement sites in both muscles, we get:

Quadriceps growth from squats vs leg extensions
Mean quadriceps growth from squats vs leg extensions.

The researchers also measured thickness in the vastus medialis, but sadly, less than half of the images were clear enough to use and they scrapped it from their analysis.

Finally, counter-movement jump height increased by 3.9% in the squat group, but not at all (0.3%) in the leg extension group.

Take-Away & Practical Application

The main takeaway from this study is fairly straightforward: squats don’t seem to do much for growing your rectus femoris, but leg extensions do.

When it comes to the largest of the four quadriceps muscles, the vastus lateralis, the squat group was the only group that saw statistically significant muscle growth. In terms of absolute values, however, the squat and leg extension group seemed to increase their vastus lateralis cross-sectional area to an equal degree, albeit in different regions: the squat group increased their vastus lateralis CSA mostly at the 25 and 50% measurement sites, while the leg extension group saw the biggest growth at 75% of femur length (closest to the hip).

Notably, the muscle growth in the rectus femoris was also the largest at the 75% mark for the leg extension group. This study hints at the possibility that the leg extension is good for developing your quadriceps from the midway and up to your hip, and the squat is good for developing your quads from the midway and towards the knee. Or maybe that is just me reading in way too much in this.

In my opinion, the results from this study at least indicate that for optimal quad growth, you should probably incorporate both a squat-type exercise and a leg extension-type exercise into your training program. These two exercises seem to work different quadriceps muscles to different degrees, and might also work different regions (proximally or distally) of the same quadriceps head to different degrees.

When I say “squat-type exercise”, I mean any exercise where you extend the knee and hip simultaneously. Examples other than the squat are:

For leg extensions, the options are more limited. What you’re after is an exercise where you extend your knee without simultaneously extending your hip, as that decreases the rectus femoris activity. Perhaps something like the sissy squat or the reverse nordic curl would do.

I suggest you use the squat (or squat-type exercises) to lay the foundation of your leg training, as they not only build your quads but also your glutes and adductors (but not your hamstrings).6 Then, after squatting or whichever exercise you choose, you can top off your quad training with some leg extensions.

A benefit of leg extensions is that they are generally easier and more comfortable to do. You don’t use as much muscle mass as in the squat which lowers the general degree of effort, and the technique is really simple compared to free barbell squats. Therefore, leg extensions are great for quad isolation towards the end of your workout.

Alright, that’s it for today. Now go ahead and click one of the articles below to learn even more about squats, life, the Universe, and everything.

References

  1. Eur J Appl Physiol. 2016 May;116(5):1031-41. Unique activation of the quadriceps femoris during single- and multi-joint exercises.
  2. J Strength Cond Res. 2014 Nov;28(11):3085-92. Changes in exercises are more effective than in loading schemes to improve muscle strength.
  3. Am J Physiol. 1995 Sep;269(3 Pt 2):R536-43. Resistance exercise-induced fluid shifts: change in active muscle size and plasma volume.
  4. Eur J Appl Physiol. 2019 Jun 22. doi: 10.1007/s00421-019-04181-y. Epub 2019 Jun 22. Effects of squat training with different depths on lower limb muscle volumes.
  5. J Sports Sci. 2021 Oct;39(20):2298-2304. The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial.
  6. Eur J Appl Physiol. 2019 Sep;119(9):1933-1942. Effects of squat training with different depths on lower limb muscle volumes.
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Daniel Richter

Daniel has a decade of experience in powerlifting, is a certified personal trainer, and has a Master of Science degree in engineering. Besides competing in powerlifting himself, he coaches both beginners and international-level lifters. Daniel lives in Lund, Sweden with his wife and three kids. On StrengthLog, Daniel geeks out about all things related to his lifelong passion of muscle and strength.