Equine Conformation, Soundness, and Movement

Tendons and the Suspensory Apparatus

• Deep Digital Flexor Tendon: This is an inner tendon that runs behind the carpal bones, travels around the fetlock joint, and crosses the navicular bone. It fastens specifically to the underside of the coffin bone.
• Superficial Flexor Tendon: This is an outer tendon that runs from behind the carpal bones and around the fetlock joint, eventually branching out to each side of the pastern.
• Common Digital Extensor Tendon: This tendon runs down the front of the leg and attaches to the top of the coffin bone.
• Sesamoid Bones: These consist of $2$ small bones located at the back of the fetlock joint. They function as a ‐pulley‐ through which the flexor tendons pass.
• Suspensory Apparatus: This system is essential for the horse's ability to move and bear its own weight, even while standing still. Because the horse places significant weight on this structure, injuries to the suspensory ligament are serious and typically slow to heal due to the limited blood supply inherent in ligaments.

The Stay Apparatus and Reciprocal System

• Definition and Function: The stay apparatus is a complex system of ligaments, tendons, and muscles designed to lock the major joints of the front and hind legs. This allows the horse to remain standing firmly in position even when relaxed.
• Suspensory Component: The suspensory apparatus is a part of the stay apparatus and is identical in both the front and hind legs.
• Stay Apparatus of the Forelimb: This includes muscles attaching the forelimb to the ribs and neck, muscles of the arm, elbow, and shoulder, the extensor and flexor muscles of the forearm and their tendons, and the suspensory apparatus of the lower leg.
• Stay Apparatus (Reciprocal System) of the Hind Limb: This includes the major muscles of the hindquarters (from hip to stifle, thigh muscles, gluteals or croup muscles, and hamstrings), ligaments of the stifle joint, and tendons and ligaments of the gaskin, hock, and suspensory apparatus.
• Reciprocal Joints: The hock and stifle are considered reciprocal joints, meaning they must bend or straighten simultaneously.
• Patella Locking Mechanism: The stifle joint is constructed so the patella (kneecap) can be lifted and locked over the end of the femur (thighbone). It is held in place by the ligaments of the stifle joint. This locking of the stifle also locks the hock, allowing the horse to stand relaxed. The biceps femoris and quadriceps femoris muscles are responsible for flexing the stifle and unlocking the patella.

Anatomy and Structures of the Foot

• Primary Functions: The foot supports the horse, absorbs shock, provides traction for secure footing, and acts as a pump to drive blood back up through the lower leg.
• Coffin Bone (Pedal Bone): The major bone of the foot that supports the weight of the horse.
• Navicular Bone: A small, wedge-shaped bone situated under the back of the coffin bone.
• Navicular Bursa: A fluid-filled sac that provides cushioning for the navicular bone and the deep flexor tendon.
• Deep Digital Flexor Tendon (Foot Attachment): This crosses the navicular bone to attach to the underside of the coffin bone.
• Digital Cushion: A spongy structure located above the frog that contains blood vessels. Pressure on this cushion helps pump blood up the leg during each step.
• Coronary Band: The outer band of tissue at the hairline where the hoof grows from.
• Corium: The deep tissue located beneath the coronary band that produces the horn.
• Wall: The hard outer shell of the hoof made of hairlike tubules known as ‐insensitive laminae.‐ It supports the majority of the horse's weight. The wall angles backward at the ends to form the bars, which aid in shock absorption and allow foot expansion.
• Sensitive Laminae: Tiny hairlike tubules growing from the surface of the coffin bone. These interlock with the insensitive laminae of the wall. They contain blood and nerve supplies. This interlocking suspends the coffin bone in a hard, protective casing that remains flexible.
• Periople: A thin, varnish-like outer layer of the hoof that retains moisture.
• Sole: The ground surface of the hoof inside the wall. The outer layer is insensitive, while the deep layer (sensitive sole) next to the coffin bone has blood and nerve supplies. Ideally, the sole should be arched or concave.
• Frog: A rubbery, wedge-shaped structure between the heels that absorbs shock and compresses the digital cushion to pump blood.
• Lateral Cartilages: Wing-shaped cartilages extending from the upper sides of the coffin bone to form the flexible bulbs of the heels, aiding in foot expansion.

Gaits, Movement, and the Phases of a Stride

• The Importance of Movement: Horses are athletes valued for their movement. Good movement is efficient, athletic, and easier to ride, while poor movement is uncomfortable, damaging, and inefficient.
• Definition of a Stride: A sequence within a gait where all $4$ legs complete one step.
• The Cycle of a Stride:
  • Swing phase: Starts when the foot leaves the ground, swings backward, flexes, and then swings forward.
  • Grounding (impact): The specific moment the hoof strikes the ground.
  • Support: The phase where the leg is bearing weight.
  • Thrust: Begins as the leg reaches a vertical position under weight and continues until the foot leaves the ground.

The Biomechanics of Horse Movement

• Power and Thrust: The hindquarter muscles provide the power for forward movement. Deep muscles of the back and spine stabilize the back and transmit this thrust.
• Balance and Shock Absorption: Neck muscles help the head and neck act as a balancer. Muscles of the neck, shoulder, arm, and forearm move forelegs and absorb shock.
• Engagement: The abdominal muscles and psoas group (running from the underside of the lumbar spine to the pelvis and femurs) draw the hindquarters forward. Engagement refers to the degree to which the hind leg reaches forward under the body; greater engagement equals greater power.
• Loin Flexion: Abdominal muscles flex the loin at the lumbar-sacral joint to bring hind legs forward.
• Thrust and Breakover: Croup and hindquarter muscles (gluteals and hamstrings) straighten the leg to push the body forward (thrust) until the leg extends and leaves the ground (breakover).
• Spinal Stabilization: Several layers of deep muscles connect the vertebrae of the croup, loin, back, and neck to create a ‐chain of muscles‐ from the hind leg to the poll.
• Nuchal Ligament (Cervical Ligament): Connects the $7$ cervical vertebrae to the withers and the dorsal ligament system. Lowering the head pulls on the withers and back, changing the horse's balance.
• Neck and Shoulder Action: Deep neck muscles stabilize the neck base and help the horse raise the forehand. Shoulder and arm muscles rotate the shoulder blades and flex/extend joints. The ‐shoulder sling‐ of muscles attaches the forelegs to the trunk to absorb shock and lift the ribs.

The Circle of Muscles

• Definition: The entire system of muscle groups working in harmony.
• Consequences of Disruption: Poor movement breaks this harmony, overstressing some groups while underusing others. This leads to inefficiency, reduced athletic ability, muscle soreness, poor development, and unsoundness.

Standards of Normal and Faulty Movement

• Good Normal Movement:
  • Sound: Free from lameness or disability.
  • Symmetrical: Front and hind legs move evenly in height, arc, and length.
  • Straight and true: Feet travel straight without deviating inward or outward.
  • Free: Legs swing freely from hip and shoulder.
  • Coordinated: Steady, even, and well-balanced.
• Faulty Movement:
  • Lame, sore, or ‐off.‐
  • Asymmetrical, uneven, or unlevel.
  • Abnormal arc of stride.
  • Crooked: Examples include winging in, paddling, plaiting, and interfering.
  • Stiff: Abnormally shortened strides.
  • Uncoordinated: Unbalanced or stumbling.

The Influence of Conformation on Movement

• Proportions:
  • Muscle Length: Long muscles move limbs farther. Length in the neck, shoulder, forearm, and croup/hip-to-hock distance favors longer strides.
  • Strength/Compactness: Shorter structures are usually stronger. Short, wide cannon bones and flexor tendons are stronger than long ones.
  • Back and Pasterns: Long backs offer springy gaits and scope but are weaker and injury-prone. Long pasterns favor injury and stress the flexor tendons.
  • Ideal Foreleg Proportions: Long shoulder, short arm, long forearm, short cannon, and medium pastern provide maximum efficiency and range.
  • Hock Placement: ‐Hocks well let down‐ (long hip to hock) signifies short, strong cannons and a powerful leg.
• Angles:
  • Hind Leg Lever: In an ideal hind leg, a vertical line from the point of the buttock passes through the back of the hock, cannon, and fetlock.
  • Sickle Hocks (Standing Under): Angle is too acute; hocks are too far under the body to push effectively, causing stress on back-of-hock structures.
  • Straight Hock: Angle is too open; efficient for swing but puts extra stress on the joint when carrying weight.
  • Shoulder Angle: Sloping shoulders offer a greater range of motion for long strides and jumping, and better shock absorption. Upright shoulders result in shorter, rougher strides.
  • Croup Angle: A long, slightly rounded croup allows for good engagement. Flat croups often lead to poor engagement (legs set too far back), while steep croups often lead to sickle hocks.
  • Pastern Angle: Should be medium for shock absorption without being so sloping that the fetlock strikes the ground.
  • Neck Angle: Low-set necks result in low head carriage and movement on the forehand. High-set ‐swan necks‐ encourage collection but can lead to a dropped back. An average angle is generally best for all-around balance.