Human body shape is a complex phenomenon with sophisticated detail and function. The general shape or figure of a person is defined mainly by the molding of skeletal structures, as well as the distribution of muscles and fat. Skeletal structure grows and changes only up to the point at which a human reaches adulthood and remains essentially the same for the rest of their life. Growth is usually completed between the ages of 13 and 18, at which time the epiphyseal plates of long bones close, allowing no further growth (see Human skeleton). Many aspects of body shape vary with gender and the female body shape especially has a complicated cultural history. The science of measuring and assessing body shape is called anthropometry.

Physiology

During puberty, differentiation of the male and female body occurs for the purpose of reproduction. In adult humans, muscle mass may change due to exercise, and fat distribution may change due to hormone fluctuations. Inherited genes play a large part in the development of body shape.

Facial features

Due to the action of testosterone, males may develop these facial-bone features during puberty: Because females have around 1/15 (6.67%) the amount of testosterone of a male, the testosterone-dependent features do not develop to the same extent, and female faces are generally more similar to those of pre-pubertal children.

Skeletal structure

Skeletal structure frames the overall shape of the body and does not alter much after maturity. Males are, on average, taller, but body shape may be analyzed after normalizing with respect to height. The length of each bone is constant, but the joint angle will change as the bone moves.

Female traits

Widening of the hip bones occurs as part of the female pubertal process, and estrogens (the predominant sex hormones in females) cause a widening of the pelvis as a part of sexual differentiation. Hence females generally have wider hips, permitting childbirth. Because the female pelvis is flatter, more rounded and proportionally larger, the head of the fetus may pass during childbirth. The sacrum in females is shorter and wider, and also directed more toward the rear (see image). This sometimes affects their walking style, resulting in hip sway. The upper limb in females have an outward angulation (carrying angle) at elbow level to accommodate the wider pelvis. After puberty, hips are generally wider than shoulders. However, not all females adhere to this stereotypical pattern of secondary sex characteristics. Males and females generally have the same hormones, but blood concentrations and site sensitivity differs between males and females. Males produce primarily testosterone with small amounts of estrogen and progesterone, while women produce primarily estrogen and progesterone and small amounts of testosterone.

Male traits

Widening of the shoulders occurs as part of the male pubertal process. Expansion of the ribcage is caused by the effects of testosterone during puberty.

Fat distribution, muscles and tissues

Body shape is affected by body fat distribution, which is correlated to current levels of sex hormones. Unlike bone structure, muscles and fat distribution may change from time to time, depending on food habits, exercises and hormone levels.

Fat distribution

Estrogen causes fat to be stored in the buttocks, thighs, and hips in females. When females reach menopause and the estrogen produced by ovaries declines, fat migrates from their buttocks, hips and thighs to their waists. Later fat is stored in the belly, similar to males. Thus females generally have relatively narrow waists and large buttocks, and this along with wide hips make for a wider hip section and a lower waist–hip ratio compared to males. Estrogen increases fat storage in the body, which results in more fat stored in the female body. Body fat percentage guidelines are higher for females, as this may serve as an energy reserve for pregnancy. Males generally deposit fat around waists and abdomens (producing an "apple shape"). Transgender men and those begin masculinizing hormone therapy see body fat redistributed within 3-6 months. Within 5 years, testosterone may cause gynoid fat to be significantly reduced. Inversely, transgender women, or those who begin feminizing hormone therapy, experience the formation of gynoid fat along with natural breast development.

Muscles

Testosterone helps build and maintain muscles through exercise. On average, men have around 5-20 times more testosterone than women and naturally and biologically males gain more muscle mass and size than women. However, women can also build muscle mass by increasing the testosterone level naturally. Prominent muscles of the body include the latissimus dorsi and trapezius in the back, pectoral muscles and rectus abdominis (abdomen) in the chest and stomach respectively, as well as biceps and triceps in the arms and gluteus maximus, quadriceps and hamstrings in the thighs.

Breasts

Females have breasts due to functional mammary glands, which develop in puberty from the influence of various hormones such as thyroxine, cortisol, progesterone, estrogen, insulin, prolactin, and human growth hormone. Mammary glands do not contain muscle tissue. The shape of female breasts is affected by age, genetic factors, and body weight. Women's breasts tend to grow larger after menopause, due to increase in fatty deposits caused by decreasing levels of estrogen. The loss of elasticity from connective tissue associated with menopause also causes sagging.

Weight

Being overweight or underweight affects the human body's shape as well as posture and walking style. This is measured using Body Mass Index (BMI). Depending on the BMI, a body may be referred to as underweight, normal, overweight, or obese. A person with a BMI below 18.5 is classed as underweight, between 18.5 and 24.9 is ideal, above 24.9 is overweight and a BMI of 30 or higher is defined as obese.

Body posture and gait

Body shape has effects on body posture and gait, and has a major role in physical attraction. This is because a body's shape implies an individual's hormone levels during puberty, which implies fertility, and it also indicates current levels of sex hormones. A pleasing shape also implies good health and fitness of the body. Posture also affects body shape as different postures significantly alter body measurements, which thus can alter a body's shape.

Impact on health

According to the Heart and Stroke Foundation of Canada, those people with a larger waist (apple shaped) have higher health risks than those who carry excess weight on the hips and thighs (pear shaped). People with apple shaped bodies who carry excess weight are at greater risk of high blood pressure, Type 2 diabetes and high cholesterol. The United Kingdom's National Institute for Health and Care Excellence advises that a person's waist-to-height ratio (WtHR) should not exceed 0.5, and that this rule applies to everyone from the age of five and is irrespective of gender, ethnicity or BMI.

Fitness and exercise

Different forms of exercises are practiced for the fitness of the body and also for health. It is a common belief that targeted exercise reduces fat in specific parts of the body —for example, that exercising muscles around the belly reduces fat in the belly. This, however, is now proven to be a misconception; these exercises may change body shape by improving muscle tone but any fat reduction is not specific to the locale. Spot reduction exercises are not useful unless you plan proper exercise regime to lose overall calories. But exercising reduces fat throughout the body, and where fat is stored depends on hormones. Liposuction is surgery commonly used in developed societies to remove fat from the body.

Social and cultural ideals

The general body shapes of female and male bodies both have significant social and cultural symbolism. Physical attractiveness is closely associated with traits that are considered typical of either sex. The body mass index (BMI), waist-to-hip ratio, and especially waist-to-chest ratio in men have been shown in studies to rank as overall more desirable to women. To be deemed to have an "athletic built"/build is usually a reference to wide shoulders, a muscular upper body and well-developed upper-arm muscles which are all traits closely associated with masculinity, similarly to other specifics of the male sex, like beards. These traits are seen more sexually attractive to women and also associated with higher intelligence, good leadership qualities and better health.

Terminology

Classifications of female body sizes are mainly based on the circumference of the bust–waist–hip (BWH), as in 90-60-90 (centimeters) or 36–24–36 (inches) respectively. In this case, the waist–hip ratio is 60/90 or 24/36 = 0.67. Many terms or classifications are used to describe body shape types: