The business of voluntary muscle is to contract, to exert an appropriate amount of force under the direction of the central nervous system. The coordination of muscle contraction and relaxation permits not only a wide range of movement, but also the maintenance of position in defiance of gravity.

The temporary or permanent loss of the ability to exert appropriate force characterizes most muscle disease. That is not to say that the defect is always within muscle fibers, as abnormalities of the tendon or tendon sheath are often the culprits.

The temporary or permanent loss of muscle function has a variety of causes which differ significantly between species. In athletic and companion animals, trauma is by far the most common cause, especially in those constantly stressed to perform. Resultant mechanical injury can vary from muscle or tendon sprains to, in the most severe cases, complete separations of the muscle–tendon unit. This is in contrast to economic animals – when the most important diseases primarily affect the muscle fiber itself, and range from the nutritionally based myopathies associated with deficiencies of selenium and vitamin E to the clostridial infections.

This chapter therefore has two major themes: those abnormalities of muscle and tendon that are of a traumatic nature and those that include degenerative and inflammatory diseases of muscle.

The nature of muscle

Individual muscles are composed of bundles of muscle fibers within a connective-tissue framework. The predominantly collagenous connective tissue may be subdivided into three basic levels.