The phrase “big boned” is a common way people explain why they have a larger body size or experience difficulty with weight loss. This colloquial term suggests that the size of the skeleton is responsible for a significant portion of an individual’s overall body mass. While the idea of a massive skeletal structure dictating body weight is largely a myth, researchers do address the underlying concept of skeletal variability through specific measurements.
How Scientists Define Body Frame Size
The scientific community recognizes that skeletal size, often referred to as body frame size, varies among individuals. Frame size is an anthropometric measure that quantifies the dimensions of a person’s supportive bone structure. Researchers employ standardized methods to objectively categorize an individual as having a small, medium, or large frame.
One common technique involves measuring the circumference of the wrist in relation to a person’s height. The wrist is measured just below the prominent bone of the forearm, where minimal soft tissue exists. These measurements are then used in a ratio calculation to determine the frame size category, using different cutoff values established for men and women.
Another reliable method is measuring the elbow breadth. This is accomplished using specialized calipers to measure the distance between the two bony protrusions on either side of the elbow joint when the arm is held at a 90-degree angle. Since the elbow joint has very little overlaying fat or muscle, this measurement provides an accurate assessment of the underlying bone structure’s width. Categorization is determined by comparing the measured elbow breadth against established population percentiles.
Bone Weight Versus Total Body Mass
Despite measurable variations in frame size, the actual weight difference between a “small-boned” and a “large-boned” person is negligible in the context of total body mass. The human skeleton typically accounts for a small and consistent percentage of an adult’s overall weight. On average, bone mass comprises approximately 14% of total body weight in a healthy adult, ranging from about 12% to 17%.
The entire adult skeleton generally weighs between 4 to 6 kilograms, or roughly 9 to 13 pounds. Even a substantial difference in frame size, such as the difference between the 15th and 85th percentiles, only translates to a few pounds of actual bone weight. This small difference in skeletal mass cannot account for the significant weight variations seen across the population.
The vast majority of weight differences are instead attributed to the volume and density of muscle and adipose tissue. Muscle tissue is dense and weighs substantially more per unit volume than fat tissue, but both contribute far more to overall body mass than the skeleton. It is this soft tissue, not the supportive bone structure, that primarily determines an individual’s weight.
Genetics and Predisposition to Body Shape
If skeletal size is not the main determinant of body weight, the answer lies in the individualized nature of body composition, which is heavily influenced by genetics and hormonal factors. Genetic inheritance determines a person’s predisposition to muscle development and patterns of fat storage. These inherited characteristics establish a blueprint for overall body shape and size.
Researchers often use the concept of somatotypes to categorize these inherited body compositions, describing individuals along a spectrum of three broad types. An ectomorph is characterized by a lean build and a fast metabolism, while a mesomorph tends toward a naturally muscular and athletic physique. The endomorph somatotype is associated with a rounder shape and a greater predisposition to store body fat.
These body types are driven by genetic and hormonal influences on muscle mass and fat distribution, not just bone size. While the initial frame size provides the foundation, the body’s eventual shape and size are determined by the complex interplay of inherited tendencies for muscle growth, fat storage, and metabolic rate. This genetic framework explains why individuals with similar frame sizes can have vastly different body weights and shapes.