The centre of motion in sex is the hips. The hips govern penetration, the hips decide depth, the hips keep the rhythm. In every position, the hips are not merely a body part but the grammatical subject of the sexual act itself. Koshi (Japanese: 腰) is anatomically the lumbar region of the spine, yet in Japanese usage it forms a distinctive vocabulary that crosses between bodily motion and psychological state, in expressions such as “put the hips in,” “swing the hips,” and “the hips give way.”

Koshi is the colloquial name for the central torso along the lumbar spine, from the lower ribs to the upper sacrum. Anatomy calls it the lumbar region, the area of the first through fifth lumbar vertebrae. As the junction with the pelvis and the connection to the hips and thighs, it is the centre of the body’s motion.

Anatomy and function

The skeleton of the lumbar region is the five lumbar vertebrae, with vertebral bodies larger than the cervical or thoracic ones, supporting the upper body’s weight while securing a wide range of motion. The lumbar lordosis (forward curvature) is an anatomical feature formed when humans achieved upright bipedal gait; women have stronger lordosis than men, so the arch of the lower back and the projection of the hips are emphasised. In the muscular layer, the erector spinae, quadratus lumborum, and obliques surround the lumbar region, governing flexion, extension, lateral bending, and rotation of the trunk. The gluteus maximus and medius continue at the pelvic junction, providing the driving force for hip motion.

Hip motion in intercourse is described in three directions: front-to-back (thrusting and withdrawing), up-and-down (sinking and thrusting up), and rotation (circling and undulation). The ancient Indian Kāmasūtra classifies these movements by names such as citraka, mathita, and nirghāta.

The hips in sexuality

The hips play a central role across every aspect of sexuality. First, as the agent of motion: the angle, depth, speed, and rhythm of penetration are all controlled by hip motion. The man’s hips carry thrusting and withdrawing; the woman’s hips carry reception, response, and the lead in the cowgirl position. “Swinging the hips” and “using the hips” mean the core of active motion in intercourse, while “the hips give way” means the receiving side’s climactic response. Second, as a visual sign: the continuity of waist curve, hips, lower back, and thighs forms the most sexually charged region of the female body. Third, as a contact surface: gripping the hips, putting an arm around the lower back, and pulling the hips close are basic contact in embrace, foreplay, and the rear-entry position, where a man gripping a woman’s hips with both hands and pulling her toward him is coded as the standard posture.

Culture and idiom

In Japanese, koshi carries a distinctive set of idioms crossing body part and psychological state: “settle the hips” (to calm down), “low hips” (humble), “hip-less” (cowardly), “hip-broken” (half-hearted). These reflect a body culture in which the hips have been seen as the centre. The body theories of martial arts, tea ceremony, and Noh all centre on the hips: kendo’s “put the hips in,” sumo’s “split the hips,” and Noh’s concentration of energy below the navel all build technique from the hips outward. The centrality of the hips in sexuality lies on the extension of this body culture.

The hips as a sexual sign

The hips show a large sex difference. The female pelvis is widely splayed for childbirth, making hip width equal to or greater than shoulder width; the male pelvis is narrow, with shoulder width far exceeding hip width. This difference can be read from the hip line even when clothed. The culture of clothing that emphasises the hip line (bodikon, tight skirts, high-waisted trousers) is directly linked to the signification of female sex. The undulation and motion of the hips are read across cultures as signals of sexual appeal: the side-to-side sway of the hips in walking, the rhythm of the hips in dance, and the motion of the hips in intercourse all function as bodily expressions linked to sexuality.

Derived usages

“Put the hips in” means to penetrate deeply, or to engage seriously; “swing the hips” means hip motion in intercourse, or courtship; “the hips give way” means losing strength at climax, or being half-hearted; “hip work” is the core element of sexual technique.

Related terms

• Waist curve
• Hips
• Beautiful hips
• Cowgirl position
• Rear-entry position