A technical breakdown of the futumomo, its main variations, and what to watch out for in each case.
Traditional form: hip flexion
Traditionally, a futumomo is built around hip flexion.
The tied person sits on the floor, the rope is tied to one ankle, and they pull the foot as close to their butt as possible.
Let’s call this version “hip-flexion futumomo.”
When both legs are tied like this, the result looks like the letter M, leaving the crotch fully exposed.
Naturally, this pose comes straight from porn, where it’s used to get up-close shots of the genitals—usually female.
What is “hip flexion”?
From a biomechanical point of view, hip flexion brings the thigh closer to the torso, decreasing the angle between the pelvis and the femur.
Visually, the knee will sit in front of the hip.
Humans have a pretty generous range of motion here, often going past 100°.
Factors to keep in mind
This range can be affected by tight hamstrings, lax psoas muscles (they link the spine to the leg), natural joint mobility, pelvic tilt (tilting backward reduces range), limb length, or neuromuscular control (fatigue and stress matter a lot).
Some of these can be adjusted on the fly, especially posture-related ones. Others—tight muscles and neuromuscular control—improve with regular practice. But anything rooted in anatomy or overall physical condition is out of our hands and should be checked by a professional.
When you bring the leg into hip flexion, remember that roughly every extra 10° exponentially increases compression in the groin, which can irritate nerves and ligaments.
Past 100–120°, the head of the femur starts to slide slightly out of place.
Remember the nightmare of stabilizing the shoulder?
The hip is basically the shoulder’s cousin—much more stable by design. But if you manage to destabilize something that’s already stable, you’re pretty much creating problems from scratch.
Even without reaching extremes, once you go beyond someone’s active range of motion, the muscles tighten up, which can affect the lower back, psoas, pelvic floor… leading to nerve compression or muscle spasms.
If the pelvis tilts into lumbar lordosis, nerves get pinched, causing tingling and numbness along the inner thigh.
Bottom line: stay within the person’s mobility limits. Don’t force that foot all the way to their butt just because you’ve seen it online.
Watch the knee
If you try to crank the leg from the lower section—around the ankle—instead of controlling motion from the hip, and you push for heel-to-glutes contact anyway, once the hip hits its limit, the knee will take over the remaining movement, shifting the hip into passive range (you lose muscular control).
On top of all that, the knee gets overloaded, increasing pressure between the kneecap and femur and stretching the side ligaments.
That chain reaction usually creates pelvic compensation and dramatically increases strain and injury risk—even without adding load.
Anatomical differences
Again, don’t blindly copy what you see in photos.
Western anatomy differs from Eastern, and individuals vary a lot.
Thigh thickness, muscle mass, calf size, leg-to-torso ratios, and the shape of the hip joint itself all influence how far someone can flex the hip
Reinterpreted form: hip extension
There’s another way to build a futumomo—one that relies on hip extension in internal rotation.
It’s less “in-your-face” visually and, instead of exposing anything, it triggers indirect genital stimulation through muscle engagement.
It doesn’t aim to glue the heel to the butt—though sometimes it happens—but to reach max extension.
It’s especially useful in positions like giaku ebi.
What is hip extension?
Hip extension moves the thigh backward, increasing the angle between the pelvis and femur.
It’s the exact opposite of hip flexion. And here, the knee sits behind the hip.
The gluteus maximus and hamstrings do most of the work, but it’s worth noting the range is much smaller than in flexion.
Even though the theoretical limit is around 10°, most people—especially older, sedentary folks—struggle to reach 5°.
Note: people with pelvic surgeries may feel discomfort because tissue tension and scar tissue can produce unpleasant sensations. If that happens, switch to something else.
The achievable range depends on ligament mobility, but mostly on someone’s ability to keep the required muscles actively engaged, because even small ranges demand a lot.
Watch the lower back
Avoid compensating with lumbar extension, arching the spine just to squeeze out more movement.
If you do, the hip joint drops into passive range, turning off the very muscles you’re trying to activate—and the “stimulating” part disappears.
To prevent this, core control is everything.
Shear forces on the knee
In both hip flexion and extension, keep a close eye on lateral shear at the knee—when the femur shifts sideways relative to the tibia.
Also watch for knees collapsing inward or bowing outward.
Any of these situations overload the joint, especially the side ligaments, menisci, and the kneecap–femur interface.
When that happens, engaging the glutes to stabilize the hip becomes harder—or impossible—and the kinetic chain breaks.
Leg kinetic chain: glutes → femur → tibia → foot.
This also applies to seiza-style positions, where people often keep their tibias parallel to the femur.
Sure, it looks kawaii, but it’s highly counterproductive.