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Muscle groups

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Muscles can be grouped into functional assemblages, which are often also visually distinct. Throughout the regions of the body there are flexor groups, which bend the joints, and extensor groups, which straighten them. Such muscle masses usually lie on opposite sides of a bone. For example, the thigh muscles on the front of the femur together form the extensor group of the knee joint, whereas the hamstring muscles behind the femur form the flexor group of the same joint (the hamstring group also functions as a hip extensor). There are also muscle groups that abduct, or pull a limb away from the body, or adduct, pulling the limb toward the centerline of the body. Functionally opposing muscles, whether single or in groups, are called antagonists. When a body is active, one muscle group will usually be contracting and become defined on the surface, while the antagonist tends to be relaxed and less defined, or even sag.

Some muscles (and muscle groups) cross only one joint and perform a single action (brachialis). Other muscles cross two or more joints, and can, for example, flex one joint and extend the other (biceps brachii).

The next section, on individual muscles, shows each of the muscles that create or influence surface form. For each of these muscles, an illustration shows where it begins on the skeleton (usually at the less mobile bone), called the origin, and where it ends (usually at the more mobile bone), termed the insertion. The positions of these attachments are quite consistent across the various species. In general, the horse is illustrated, and it is usually very similar to the ox (with the major exception of the toes and the overall proportions). When the ox or the dog differ substantially, they too are illustrated. The feline (cat, lion) is very similar to the dog. Noteworthy differences and variations are shown or described.

The origin, insertion, action, and structure are listed for each muscle. Important differences in the other species are listed, but the common features are not repeated.

CLOSE AW

HUD AND NECK EXTENSORS

SPINE EXTENSORS

FOOT EXTENSORS/ DIGIT FLEXORS

WRIST AND DIGIT EXTENSORS

HEAD AND NECK FLEXORS

OPENS IAW

ELBOW FLEXORS

SHOULDER FLEXOR/ ELBOW EXTENSORS

CLOSE AW

HUD AND NECK EXTENSORS

HEAD AND NECK FLEXORS

EXTENSORS/ KNEE FLEXORS

WRIST AND DIGIT EXTENSORS

OPENS IAW

SPINE EXTENSORS

ELBOW FLEXORS

SHOULDER FLEXOR/ ELBOW EXTENSORS

FOOT EXTENSORS/ DIGIT FLEXORS

MAJOR ANTAGONISTIC MUSCLE GROUPS

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