A horses chest conformation is important to the horses ability to carry weight, so a horse that doesn't do well with draft work may be fine in harness or with a light rider.

The Narrow Breasted Horse Chest

With the horse standing square, the width between the front legs is relatively narrow. However, this can be skewed by how far apart feet are placed at rest. A narrow breast often represents general thickness and development of shoulder.

A narrow breast is usually seen in Gaited horses, Saddlebreds, Paso Finos, and Tennessee Walkers

Narrowness may be from turned-in elbows which can cause toes to turn out, making the horse appear narrow.

Narrowness in the chest may be from immaturity, poor body condition, inadequate nutrition, or under-developed breast muscles from a long time in pasture and lack of consistent work. The horse usually has undeveloped shoulder and neck muscles.

The horse may tend to plait, and is more likely to interfere, especially at the trot
The horse is best for pleasure riding, driving in harness, and trail riding

The Pigeon Breasted Horse Chest

The front legs come too far back under the body, giving a bulky appearance to the breast as viewed from the side.

The front legs lie behind a line drawn from the withers to the ground, setting the horse under himself.

The Pigeon Breasted Horse Chest is often associated with a long shoulder blade that drops the point of shoulder somewhat low with the arm bone relatively horizontal, setting the elbow more to the rear.

Relatively uncommon and mostly seen in Quarter Horses with big, bulky muscles

Bulky breast muscles and legs set under the body decrease the efficiency of stride and swing of shoulders, hastening fatigue. It may interfere with the front legs, forcing them to move to the side rather than directly under the horse. It causes a rolling gait that slows the horses speed, especially at the gallop.

It should interfere little in the spiriting sports that need rapid acceleration. The inverted V of the pectorals are important for quick turns, dodges, and spins needed by stock horses.

This conformation quality is most useful in Quarter Horse racing, barrel racing, roping, and stock horse sports where low front end crouches help the horse makes quick turns.