Muscles are one of the most important components in the equine body. Without them, the horse would be unable to walk, chew and digest food, or even swish his tail. Muscles comprise the largest tissue mass in the horse’s body. There are various types of muscles performing a wide variety of duties, but basically they all function in the same general way–a period of contraction or shortening of muscle fibers, followed by a period of muscle relaxation or lengthening of muscle fibers.

In this article, we’ll take a look at how equine muscles function and are nourished, as well as examine some of the problems that have surfaced, such as hyperkalemic periodic paralysis (HYPP) and tying-up. As has been the case throughout this series, we will draw on a wide variety of sources, including textbooks. However, a couple of researchers deserve specific mention. Much of the information concerning description of muscle structure comes from a paper authored by Craig H. Wood, PhD, coordinator of Distance Learning at the University of Kentucky, and published in the Horse Industry Handbook. When we discuss HYPP, the work of Sharon Spier, DVM, Dipl ACVIM, PhD, associate professor at the University of California, Davis, becomes a source of information. No discussion on tying-up should be conducted without drawing on the research of Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the Equine Center at the University of Minnesota.

Types of Muscles

The three muscle classifications are smooth, cardiac, and skeletal. The first two are involuntary or automatic, which means they function as needed without having to be called into action for a specific need. Skeletal muscles are voluntary; the brain sends a signal to the muscles via nerves, and the muscles react accordingly.

Non-skeletal muscles are basically the same for all horses. Smooth muscles are located in such places as the digestive tract, respiratory and circulatory systems, as well as the urogenital system. These muscles are many and varied, and they respond to stimuli within the respective systems