Squats or Leg Extensions?
(Positions of Flexion)
Steve Holman (1994)
Question: If muscle isolation is so important for growth, as so many experts claim, doesn't that make the leg extension a superior quadriceps exercise to the squat?
You can't really compare leg extensions and squats, because they affect the quads differently. It's like asking whether E or C is the superior vitamin -- you need both of them.
Squats, a mid-range movement in Positions of Flexion training --
-- [The mid-range position movement works the target muscle with synergy, or help from other muscle groups, so you can train it with maximum poundage. A mid-range movement doesn't take the target muscle to full stretch or complete contraction, however. Example: Squats for the quadriceps.
The stretch position movement puts the target muscle into an overextended state so you can take advantage of the pre-stretch reflex. With a quick twitch at the point of stretch you can involve more muscle fibers due to better neurological stimulation. Example: Sissy Squats for the quadriceps.
The contracted position exercise trains the target muscle with resistance at the point of complete contraction as well as through the full range of motion.] --
So, squats, a mid-range movement, train your quads with synergy, or help from other muscles including your glutes and erectors, which allows you to use heavy poundages. Leg extensions, on the other hand, are a contracted position movement for the quads; that is, your quads are in position to contract completely, and there's resistance at that contracted point in the range of motion. While it's true that leg extensions isolate the front thigh muscles to a great degree, they are a single joint exercise, and so you can't use as much weight as you can on a multi-joint movement like the squat. Nevertheless, leg extensions are necessary for more complete quad development.
Jerry Robinson, author of The Optimum Workout columns, discussed single joint movements and the fact that "development seems to occur at the end of the muscle that's closer to the movement." This means that leg extensions tend to have the potential to give you more size down near your knees, which may be due to the peak contraction effect, or top end resistance, that they deliver and squats lack. If you've ever read thigh training articles from the '60s and '70s, you may have noticed that the champs from that era often said that leg extensions gave them more definition in their quads. This was a somewhat inaccurate observation, however, and may well have been a reference to what was actually low-quad development. This buildup in the quads near the knees may have made it appear as if their definition was increasing, but, in fact, it was probably just added mass from the leg extensions making the teardrop shapes more pronounced.
Squats don't have a peak contraction effect, but because of muscle teamwork most people can eventually build up to tremendous poundages on them. This makes them perfect for packing on more overall thigh mass.
To get back to the original question, however, if you're asking about which one of these exercises you should choose if you can only do one, the answer depends on your development. If you're a beginner to advanced intermediate, stick with squats so that you'll continue to put on overall quad size. If you're more advanced, leg extensions are a must during certain phases of your training.
If you can afford to do more than one quad exercise and you're a sufficiently advanced trainee, then you should train your quads in all three positions of flexion for the most complete development possible in your front thigh muscles. This means that you should incorporate squats for the mid-range position movement, leg extensions for the contracted position movement, and sissy squats for the stretch position movement.
I've already touched on the mid-range and contracted positions with squats and leg extensions, respectively. The stretch position is also important because it allows you to use the myotatic (pre-stretch) reflex to involve more muscle fibers. This reflex is a myosynaptic reflex, which provides automatic regulation of skeletal muscle length. With a quick "twitch" at the bottom of a stretch position movement like sissy squats, you can get a more powerful contraction in the target muscles. Baseball players often use the pre-stretch reflex before they swing, flicking the bat back before moving it forward to get more power. The same principle is why stretch-position movements allow for such a strong contraction.
Here's an example of how to implement all three positions of flexion in a quad routine:
*One second positive and three second negative, slower cadence and more controlled turnaround in the stretch position exercise. Take all sets to at least positive failure.
Mid-range position: Squats, 2 x 8-10 reps
Stretch position: Sissy Squats, 2 x 8-10
Contracted position: Leg Extensions, 2 x 8-10
Remember, because you're working your quads completely and thoroughly with this routine and you are going to at least positive failure on all your work sets, it won't take many sets to completely fatigue your quads. Two sets per position is all that's required.