Brawn refers to well-developed, functional muscle mass — the physical power produced by the muscular system rather than skeletal size alone. In horses, brawn is most valued and most visible in the hindquarters, the gaskin (lower thigh), the forearm, and the shoulder, where the largest locomotor muscles originate and insert. A horse described as brawny is one whose musculature is prominent, well-defined, and appropriate to its build — not simply heavy, but powerful through the engine.
The hindquarters are the primary power source for propulsion. The gluteal complex, the hamstring group (biceps femoris, semitendinosus, semimembranosus), and the quadriceps drive extension of the hip and stifle in every gait. A horse that lacks brawn in this region — flat, narrow, or weak in the croup and gaskin — lacks drive. Draft breeds are bred explicitly for maximum brawn: the Belgian, Clydesdale, and Percheron carry massive gluteal and forearm mass to move dead weight from a standing pull. Sport horses require brawn distributed differently: the withers and back muscles must sustain collection and carry a rider’s weight without fatigue.
Brawn is built through conditioning — progressive work that creates hypertrophy of the target muscle groups — and supported by adequate protein in the diet. A horse in poor nutritional state loses muscle before it loses fat; a visibly “tucked up” appearance with muscle wasting over the topline and hindquarters signals protein insufficiency or systemic disease. The body condition score system evaluates fat cover, not muscle — an independent variable from brawn; a horse can score a 5 on the BCS scale while still being poorly muscled. Brawn and condition are independent variables.
Further Reading: The Wikipedia article on muscle hypertrophy explains the physiology of conditioning-driven muscle growth; the Merck Veterinary Manual’s section on muscle disorders in horses covers the pathological end of the spectrum including exertional rhabdomyolysis and muscle wasting.