From the lower end of the ribs to the upper edge of the hip bone, a region the width of four fingers that hollows softly. In a slender woman the ribs show here; in a fuller woman it draws a gentle curve. Waki-bara (the flank) is the core region that physically forms the waist taper (kubire), strongly determining body impression. Not a direct sexual sign, but as the “substance of the taper” it bears on the sexual context through both sight and touch.

Waki-bara is the colloquial name for the region of the lateral trunk from the lower ribs to the upper iliac region. Written “脇腹” or “横腹”, it is called the lateral abdomen in anatomy. The oblique muscles (external and internal) and the transverse abdominis run in layers, and the contour varies with the accumulation of fat tissue. As a sex-typed sign, it sits at the constructional centre of the waist taper.

Anatomy and function

The flank is the region of the trunk’s side opened between the lower ribs and upper iliac bone, unprotected by bone. The rectus abdominis lies in front and the erector spinae behind, with the external oblique covering the surface at the side. The internal oblique and transverse abdominis form layers in the deep part, governing lateral flexion, rotation, and abdominal-pressure regulation.

Fat distribution varies greatly by sex and individual. In men, visceral fat accumulates toward the front and upper abdomen, with a relatively thin flank. In women, subcutaneous fat accumulates thickly on the waist and lower flank, generating the hourglass form. Pregnancy, childbirth, and aging shift fat distribution, and the flank’s contour changes with it.

Innervation comes from the lateral and ventral branches of the intercostal nerves, supplying touch to the surface skin. The flank, with thin skin, is known for producing a strong sensory response (the tickle reflex) on light contact, an evolutionary defence response that instantly detects external contact on the side region protecting the viscera.

Relation to the waist taper

The waist taper is visualised as the region where the width of the flank narrows front-to-back. In a woman with the hourglass form, the flank hollows from the lower ribs to the upper iliac region, lowering the waist-to-hip ratio (WHR). WHR has long been discussed in evolutionary psychology and behavioural economics as a signal of beauty, occupying a central place in aesthetic evaluation of the female body.

The line of the flank is not simply “the thinner the better”, however. Too slender, the ribs show and give a bony impression; too full, the hollow disappears and produces a straight-sided figure. Between the extremes, a line where fat sits just enough for the ribs to show faintly and swells gently toward the waist is held to be a “beautiful flank” visually.

The flank in sexual life

The flank is not a genital, but it serves several functions in the sexual setting.

First, as a visual sign. In sideways poses, the lateral recumbent position, and the rear-entry position, the line of the flank becomes the centre of the frame. The sign of the hourglass form is recognised more strongly from the side and the oblique than from the front. In adult photography, poses with the woman twisting her waist or lying with one knee raised are standardised to emphasise the line of the flank.

Second, as a tactile sign. The flank, with thin sensitive skin, is a contact surface through which the partner’s body temperature, breathing, and the tension of the abdominal muscles are directly conveyed. Running fingers along the flank and gripping it with both hands are central techniques of embrace and caress.

Third, as a source of the tickle reflex. Light touch on the flank produces an involuntary bodily response. This functions as a tool for easing tension in the sexual setting, and in BDSM contexts is sometimes used as material for limited restraint play.

Fourth, as a grip point in rear-entry positions. When a man draws a woman toward him in the rear-entry position or doggy position (inu-kake), gripping the flank is the basic hand placement: above the hip bone, with the thumb meeting the ribs and the other four fingers wrapping the waist, a natural point of force.

The flank in body types

In the chubby (pocchari) type, fat lies thickly on the flank, and the ribs can be confirmed only by touch. A soft, graspable flank forms the tactile core of the chubby preference.

In the slender type, the flank is thin and the ribs may show. In extreme cases this passes beyond slimness into a bony impression, though a moderate show of rib functions as a sign of healthy slimness.

In the intermediate type, fat just enough for the ribs to show faintly coexists with a gentle swell toward the waist, the most frequently imaged ideal figure of the present day.

Related Terms

• Waist taper (kubire)
• Waist (koshi)
• Back (senaka)
• Body contours (dekoboko)