The shoulders and front legs of the horse are not attached to the rest of the body through a bone because they don't have a clavicle (humans do and it attaches the shoulder to the axial -body- skeleton), so it works like an independent suspension system. Instead the horse's axial skeleton is suspended from the shoulder and legs by the pectoral and the ventralis muscles, these 3 muscle groups are also known as the Thoracic Sling. These muscles allow the body to rotate and elevate without disturbing the front legs, and with limits visa-versa. Any contraction holding pattern in these hard working muscle groups can show up as restriction in the shoulder (restriction of the shoulder also gets caused by ill fitting saddles and off balanced riders) or rotation in the body reflecting trough the spine affecting the rear of the horse! More often the horse will develop a compensation in one of the front limbs. The front legs hang from the body and neck by the brachiocephalicus, rhomboids, lattisimus dorsi and cervical serratus and trapezius muscles. If you feel your horse is too much on the forehand, unbalanced, restricted in the shoulders, uneven weight distribution on the front legs, your horse is suffering from tight muscles and time to have their muscles checked and loosened up.