Types of muscle As in all vertebrates, horses have three types of muscle:
Skeletal muscle: this type of muscle contributes to movement and posture, and is consciously controlled (voluntary muscle). While some muscles attach solely to skin or cartilage (muscles of facial expression, etc.), contraction of skeletal muscles more commonly leads to the muscle pulling a tendon, which in turn pulls a bone, resulting in either flexion or extension of a joint. Skeletal muscles are usually arranged in antagonistic pairs in opposition to each other, with one flexing the joint (a flexor muscle) and the other extending it (extensor muscle). Smooth muscle: this type of muscle is controlled by the autonomic nervous system (making it an involuntary muscle type). Smooth muscle is involved in digestion and other organs such as the eye Cardiac: involuntary muscle that causes the heart to beat. This type of muscle has high numbers of mitochondria, allowing it to be fatigue-resistant.
Build of skeletal muscle Skeletal muscle is made up of several muscle bundles, which in turn are made up of muscle fibers. Muscle fibers have bundles of myofibrils, which are all parallel to one another, and are able to contract due to actin and myosin. Muscle is covered by a fibrous tissue called fascia, to which other muscles can attach, and muscles attach to bone via tendons.
Tendons of the lower leg Tendons attach muscles and bone, and are classified as flexors (flex a joint) or extensors (extend a joint). However, some tendons will flex multiple joints and extend another (the flexor tendons of the hind limb, for example, will flex the fetlock, pastern, and coffin joint, but extend the hock joint). In this case, they are classified according to whether they flex or extend the joints of the digit. The following tendons are the main tendons found in the lower leg. When they pass over a joint, they are protected in a tendon sheath, which contains synovial fluid as a lubricant.
Common digital extensor: the common digital extensor muscle becomes tendon in the bottom third of the radius and continues down the front of the leg. The tendon pulls upward to extend the carpal, pastern, and coffin joints. It is the major extensor tendon of the leg. However, unlike the flexor tendons, a horse with a damaged or non-functional "extensor unit" (i.e. tendon and musculature) is not lame, but rapidly learns to compensate by "flicking" the lower limb using the carpal or tarsal extensor units. Lateral digital extensor: the lateral digital extensor muscle becomes the lateral digital extensor tendon at the proximal portion of the metacarpus. The tendon continues down the front of the leg and inserts into the proximal portion of the first phalanx. Important in the treatment of stringhalt in the hind limb. Extends the carpal, pastern, and coffin joints Deep digital flexor: 3 tendons of the deep digital flexor muscle travel distally and join at the carpus, where they pass through the carpal canal, and travel distally along the back of the leg, finally inserting into the palmar side of the third phalanx. Below the knee/hock, the tendon is superficial to the suspensory ligament, but deep to the SDFT. Fairly commonly injured by horses doing fast work, the DDFT is round in cross section. Superficial digital flexor: Runs down the back of the leg, behind the carpus and cannon, branches below the fetlock and inserts into the distal side of the 1st phalanx and proximal side of the 2nd phalanx. Flexes the elbow, carpus and lower joints. Additionally, the superior check ligament inserts into this tendon from the caudal side of the radius. The SDFT is the most commonly injured tendon, and appears oval or flattened in cross section.
Main skeletal muscles of the horse
Muscles of the neck, shoulder, chest, and back
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