Equine Muscle Anatomy Part 1 - the Superficial Muscles on a Horse

The horse is built for movement, from the up and down motion of a trot, to a flowing canter. Movement requires muscles and there are many more muscles in a horse than in a human. Three types of muscle can be found in a horse: skeletal muscles, cardiac muscles which are in the heart and smooth muscle which helps to construct automatic systems such as the digestive system.

Equine Muscle Anatomy - the Superficial Muscles on a Horse

Skeletal muscles attach to bone. When these muscles are contracted, the muscle pulls on a tendon which connects to the bone. The skeletal muscles are usually in opposite pairs - one to flex a joint while the other extends it. This means while one muscle relaxes, the other will contract so that the joint operates properly. This muscle or muscles and the tendon or tendons attached are called a flexor unit, if the joint is caused flexion and extensor unit if the joint is caused extension. Caudal muscles are towards the tail, 

Skeletal muscle consists of a number of muscle bundles made of muscle fibres. These fibres have bundles of myofibrils, which run parallel to each other. They are enabled to contract because of actin and myosin. The muscle itself is covered by fascia, a fibrous tissue which enables other muscles to attach.

Neck, shoulder, chest and back superficial muscles of the Horse

The brachiocephalicus begins at the base of the skull, behind the jaw (3rd - 4th cervical vertebrae) inserts below the shoulder at the humerus. This enables the horse to move its head from side to side and pull the scapula forward, raising it and swinging the foreleg forward. If the reins are held too strongly, then free forward movement is checked. 

The sternocephalicus begins from the cartilage of the sternum and inserts into the caudal side of the mandible. This muscle helps to lower the head.

The splenius begins from the 3rd - 5th thoracic vertebrae, the dorsal scapular ligament and the nuchal ligament. The muscle inserts on the nuchal crest, the atlas and the 3rd - 5th cervical vertebrae. It makes up the top line if it is well-developed and turns and extends the neck. 

The rhomboid originates from the nuchal and supraspinous ligaments, inserting on the medial scapular cartilage, under the trapezius. This muscle helps to raise the shoulder towards the head, raising the neck upward.

The deltoid, which originates in the caudal border and spine of the scapula and inserts in the deltoid tuberosity of the humerous. The aponeurosis of the deltoid is partially fused to the infraspinatus in the equine. The muscle helps to flex the shoulder.

The teres major begins at the dorsal part of the scapula and inserts in the teres major tuberosity, midway down the humerous. This muscle flexes the shoulder and supports adduction.

The sternomandibular connects the jowl (caudal side of the mandible) to the sternum cartilage, enabling movement of the head and neck.

The rhomboid originates from the nuchal and supraspinous ligaments, inserting on the medial scapular cartilage, under the trapezius. This muscle helps to raise the shoulder towards the head, raising the neck upward.

The trapezius begins at the dorsal side of the neck, near the poll and inserts on the spine of the scapula. This muscle includes the trapezius cervicalis (originating along the cervical vertebrae) and trapezius thoracis (thoracic vertebrae). It lifts the shoulder and forehand and if well-developed, the horse is working in a good outline. The nuchal ligament runs from the poll to the withers and helps the neck muscles support the head. The deltoid originates at the scapula and inserts into the humerus. It flexes the shoulder joint, but can load the shoulder if over-developed. 

The Latissimus dorsi begins at the lower thoracic vertebrae and is inserted into the humerus. It supports the back, pulling body of the horse forward when the foreleg is placed on the ground and flexes the shoulder (scapulohumeral joint). It works in opposition to the brachiocephalicus.

Serratus dorsalis cranial and caudal originate on the lumbar dorsal fascia, inserting into the 5-12t rib or the 10-18 rib. The caudal muscle inserts near the tail. These muscles help to move the front leg forwards and backwards as they work together, in opposition.

Tensor fasciae latae originates from the tuber coxae, inserting into the lateral patellar ligament, the tibia and the broad aponeurosis of the patella. This muscle helps to flex the hip and extend the knee.

Gluteus superficialis  is a muscle which originates from the gluteal fascia and tuber coxae and inserts into the femur. The muscle lies under the tensor fasciae later muscle, helping to flex the hip and allowing it to abduct.

Biceps femoris begins from a number of places, including the lateral sacral ligaments, coccygeal fascia and gluteal fascia, the interimscular septum between the biceps femoris and semitendinosus. The muscle is inserted into the caudal side of the femur, as well as the lateral patellar ligaments, cranial side of the patella and calcanea tuber. The muscle enables the leg to extend for kicking, rearing and movement. It also allows for abduction of the leg.

Semitendinosus begins at the transverse processes of first and second coccyxeal vertebrae and the ventral side of tuber ischium. The muscle inserts into the tibia and the tuber clans. This muscle is able to flex the knee joint, inwardly rotate the leg and extends the tarsus and the hip.

The triceps brachii branches into three: originating from the cardal side of the scapula and into the lateral and carpal side of the olecranon, from the humerus, inserting into the lateral side of the olecranon and from the medial side of the humerus, inserting into the medial and cranial side of the olecranon. This muscle extends the elbow, helping it to stay in position.

Horse Muscle Anatomy

Equine Musculature Anatomy Poster exclusive to AnatomyStuff.

Lower Leg Tendons of the Horse

The muscles are attached to bone through tendons. The tendons can be classed as flexors, which flex the joint, or extensors, which extend it. In a horse, however, some of the tendons can flex multiple joints and extend others. An example of this is when the flexor tendons of the hind leg can flex the fetlock, pastern and coffin joint, but also extend the hock joint. The tendons are named according to whether the joints they attach to flex or extend.

When tendons go over a joint, a tendon sheath protects them. It has synovial fluid to help lubricate.

The main equine tendons found, are the common digital extensor, the lateral digital extensor, the deep digital flexor and the superficial digital flexor. The common digital extensor muscle is the major extensor tendon in the leg. It changes to tendon in the bottom 1/3 of the radius, continuing down the front of the leg. The carpalpastern and coffin joints are extended by the tendon pulling upwards. If a horse has damaged its extensor musculature (or tendon), it can learn to compensate by flicking the foot, using its carpal or tarsal extensor units rather than showing as lame.

The lateral digital extensor muscle extends into the tendon of the same name at the proximal part of the metacarpus. The tendon travels down the front of the leg, inserting into the proximal part of the first phalanx. This muscle extends the carpalpastern and coffin joints.

The deep digital flexor has 3 tendons which extend distally, joining at the carpus through the carpal canal. They then travel along the back of the leg and insert into the third phalanx. When the tendon travels below the knee/hock, it is superficial to the suspensory ligament although deep to the superficial digital flexor tendon. This tendon can be injured by horses running fast.

The superficial digital flexor runs behind the carpus and cannon, down the back of the leg. It then splits below the fetlock, inserting into the distal side of the first phalanx, and the proximal side of the second phalanx. This muscle flexes the elbow, carpus and lower joints. The superior check ligament inserts into this tendon from the caudal side of the radius. The superficial digital flexor is the tendon that is most often injured.

Equine Muscle Anatomy Part 2 - the Deep Muscles on a Horse

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