Bodybuilders can’t rely solely on squats to build the legs. True, the squat involves some hamstring action, some calf involvement, some sartorius, etc., but legs need more variety to pull them into real shape.
– Bob Paris
Research substitutes for the leg extension, and you’ll find endless recommendations for compound free weight options like squats, lunges, and step-ups.
These exercises do work numerous muscles, and the resisted leg extension is an unnatural movement that can stress the knees. Nonetheless, from a bodybuilding perspective, this completely ignores its unique quad-building value.
Traditional front thigh exercises focus on the single-joint vasti, which includes the vastus medialis, vastus lateralis, and vastus intermedius. They overload these muscles at a stable and strong position for overall active tension at the midrange, and therefore are rightfully known as mass builders. Other muscles like the glutes, hamstrings, and lower back get involved too.
Nonetheless, when hip extension occurs simultaneously with knee extension, the biarticulate rectus femoris muscle, attached to both the knee and the hip, will activate minimally. It only facilitates smooth movement since it barely changes length.
A pure knee extension, or countercurrent hip flexion, is required to activate the rectus femoris, especially the lower portion near the knee. This impressive two-headed, V-shaped muscle etches fine detail into the upper front thigh.
Due to the flexed hip position while seated on a leg extension, even despite a seat adjustment, the rectus femoris lacks strength as it slackens. Along with minimal range of motion for nearly all machines at the knee, it lacks the tension possible through these leg extension alternatives.
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Many attempt an old-school barbell hack squat, with heels elevated on a block in open space and the weight behind you, but I find this limits the range of motion while not necessarily preventing hip extension.
You can use a hack squat machine at a gym, but even at home you can create your own hack squat setup by performing it next to a wall with limited friction.
Done properly, the wall prevents most hip extension, keeping you upright. By placing the shoulder blades against the wall and your feet as close to it as possible, and keeping the hips slightly forward, you ensure a purer knee extension.
A 2-inch wooden block may be helpful to elevate your heels, though you should allow them to rise naturally as you descend.
Place a barbell across the cruces of your hips in front or even on the upper quads. This compression won’t affect the contraction, and this setup relies on far less weight than a machine hack squat. Smaller diameter plates for the maximum, like 25s, may be needed to prevent them from hitting the wall.
This maximal knee bend allows the vasti to stretch too. Any stretch can lead to more distal muscle growth near the knee since sarcomeres in-series are added. Growth near the knee draws the eye distally for an aesthetic physique.
This leg extension alternative has you extend the hips utmost as you descend, with the knees going beyond the toes, which really stretches the rectus femoris.
The more you drive the hips forward to extend the hips at the bottom, the greater the stretch for the rectus femoris. As explained, this leads to more distal growth via overstretch. Active tension eventually shifts to this new longer length.
Due to this extreme position, you’ll require hip flexion to ascend, involving unique motor units for the proximal or upper fibers of the rectus femoris.
This hip flexion, combined with knee extension, likely makes the sissy squat the most complete exercise for the rectus femoris, though not necessarily the best. A hack squat done properly, with greater stability, combined with hip flexion like on a sit-up for the upper portion of the rectus femoris, is better than a sissy squat alone.
Vince Gironda added a special movement called a burlesque bump to achieve a full range of motion. I’m unsure this is needed as his variation invites hip extension for a specific phase of the movement. This limits rectus femoris involvement.
Learn proper sissy squat form here.
Flexing During Exercises?
In his excellent bodybuilding guide Beyond Built, Bob Paris suggests flexing throughout the range of motion on all exercises. He argues, for example, that even raising the toes off the platform for a leg press to facilitate this will bring out separation of the front thigh.
Many bodybuilders feel the leg extension helps develops the teardrop-shaped vastus medialis, especially with the toes pointing out for a valgus position. It may activate more near both the stretched and shortened endpoints for knee extension due to better internal leverage, activation, or longer sarcomere lengths.
A solution is to flex in the shortened position for any quadriceps exercise, and perhaps even throughout the whole range of motion. This is a bodybuilding trick of the trade that may lead to more development by using manual contractions for the range of motion less overloaded by external weight.
Bodybuilding Alternatives for Leg Extensions
Steve Reeves possessed a well-developed rectus femoris yet rarely did leg extensions. He focused on hack squats, often using a special belt apparatus while elevating the heels. He included front squats and parallel-depth back squats on a block, with lunges occasionally, to develop his vasti and other major lower body muscles.
As a warning, hack and sissy squats are far tougher on the knees than common exercises. The pure knee bend, with greater challenge as you descend, increases shear over compression force. It’s worse than leg extensions, so make sure to warm up with lightweight reps and a limited range of motion.
Nonetheless, conventional squats alone cannot be the answer to full quad development. Consider the hack squat, the sissy squat, and flexing throughout the range of motion as viable leg extension alternatives.