Eric Seifert Shows Jujimufu Two Effective Ranges of Motion for Leg Extensions

The backward tilt of the seat on the leg extension machine might be hindering your quad gains.

Most people might be doing their leg extensions wrong. Well, not wrong, necessarily, but not optimally. On Aug. 29, 2022, Jon “Jujimufu” Call took to his YouTube channel for part three of his exercise series with Core Muscle Activation Inc. founder Eric Seifert. Their first two episodes in the series covered triceps extensions and stiff-leg deadlifts.

During episode three, Seifert explained to Jujimufu the two effective ranges of motion for the leg extension for bodybuilders and the cues to execute each movement with optimized form. The two ranges are the high end when the legs are fully extended, and the other works the quad in the fully lengthened position (i.e., when the knee is at maximum flexion). Check out their training session below:

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Seifert directed Jujimufu’s attention to the seat’s position on the leg extension machine. More specifically, he draws attention to how most leg extension machines have slightly tilted back seats. While the seat may be positioned for comfort, Seifert suggests that most people who perform leg extensions don’t consider that tilt.

When performing leg extensions, extending the legs to parallel with the seat is beyond parallel to the floor due to the seat’s backward tilt. Seifert often sees people just extend their legs until they’re parallel to the floor, leaving the most critical range of motion entirely off the table.

Seifert recommends that Jujimufu split the leg extension into two different sets into two ranges of motion. The first is fully extended legs while pulling into the chair so Jujimufu’s butt and lower back remain connected to the seat. Maintaining that contact with the chair removes the “fulcrum” effect that undermines the range of motion when performing each rep.

Once Jujimufu extended his legs to that fully contracted position, Seifert directed Jujimufu to “crack the knees,” meaning a slight bend, before returning to the fully contracted position. This small range of motion keeps more tension at peak contraction.


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The second range of motion works the quad in the fully lengthened position but also on a different plane of motion. With Jujimufu’s knees in full flexion, Seifert has him perform an extension that looks like half the range of motion Jujimufu could move. However, rather than moving the weight up, Seifert directs Jujimufu to extend his legs forward. 

Rather than try to move the weight vertically, Seifert aimed to have the weight move horizontally. Jujimufu’s natural tendency was to allow his legs to rotate outward — he has big legs, after all — but Seifert had Jujimufu maintain his legs in a neutral position. Jujimufu used the cue of imagining there is a ball between his knees that he can’t let fall.

Maintaining that tension in the neutral position and focusing on extending the weight forward put significant emphasis on Jujimufu’s quads which seemed to generate a burn. Seifert explained that the human body is optimized to find more efficient movements to perform more reps. Still, when it comes to bodybuilding, the rewards are garnered by keeping the tension on the target muscle being worked, even if it is a less efficient way to move the weight. 

Featured image: @core_muscle_activation on Instagram