more about muscles

The contraction of muscles forms the basis of all movement - in horse and humans alike. Our skeletons provide leverage and the boney framework, but it is the muscles that are designed to create movement.

When a muscle is stimulated by a nerve, a series of chemical reactions take place that cause the muscle fibers to shorten. When a muscle contracts it generates tension while pulling on its attachment points (typically on bones). If the tension generated is great enough to overcome the resistance of opposing muscles and gravity, then the muscle shortens and movement occurs.

Generally speaking, most skeletal muscles are arranged in opposing pairs at joints: flexors-extensors, abductors-adductors, etc. Within opposing pairs, one muscle, acting as the agonist or prime mover, contracts to cause an action while the other muscle, the antagonist, stretches and yields. The respective roles of the muscles, however, can switch for different movements.

In the forelimb of the horse, for example, the triceps are the prime mover, or agonist in extending the elbow. While the limb is in stance phase the biceps brachii yields. When the foot leaves the ground the biceps brachii acts to flex the elbow and bring the leg forward in movement.

In well-balanced activity, each half of the agonist-antagonist pair contributes to the mechanical efficiency of the movement. For optimal functioning these muscles should be balanced in tone and lengthening capabilities. Chronic shortening of a flexor, for example, requires the antagonist (the extensor) to maintain constant counter-balancing tension or contraction to a similar degree. This requires the expenditure of more energy to maintain balance in movement, the quality of which will be poorer as a result of excess tension.

Muscles act in concert with each other and no single muscle creates a bodily movement on its own. Muscles that aid in the movement of the prime mover are called synergists. If a prime mover is not functioning properly, the synergists may compensate to ensure the movement occurs.

Stabilizers are smaller muscles that lie deeper in the body and typically perform precise small-scale adjustments, rather creating external movement. They function mainly to stabilize bones and joints so that other skeletal muscles can execute a movement more effectively. The better the stabilizers are able to do their job the more fluid and smooth the movement, and the safer the joints.

Studies have found that the deep stabilizing muscles, particularly those of the spine, may become “turned off” in response to injury and inflammation and that they may not always turn back on again after the primary injury has healed. In some cases specific exercises may be required to get them back to proper functioning and ensure a truly complete recovery.

Muscles are enclosed by a sleeve of connective tissue that serves to separate one muscle from another, and allows for the easy, gliding movement of one muscle over another. Within the muscle, connective tissue surrounds each muscle fiber and bundles of the fibers, called fascicles (this is what gives a cut of meat its characteristic “grain”).

These layers of muscular connective tissue are continuous with one another and with the connective tissue that attaches the muscle to bone. In some cases the connective tissue will extend beyond the muscle fibers to form a tendon – like the flexor tendons of the forelimb, which originate from muscles further up the leg. A broad, flattened tendon is called an aponeurosis.

The intimate association between muscle fibers and the surrounding connective tissue plays an important role in the maintenance of the integrity and proper functioning of the entire muscular system. Adhesions in the connective tissue can develop as a consequence of any condition that causes inflammation – the body’s response to injury.

During inflammation, blood vessels become “leaky” and secrete healing agents into the affected tissues. A side effect is that some of these agents act like glue, binding the tissue together. As it heals, connective tissue tends to become denser and shorter. Lack of movement can also cause a thickening of connective tissue within a muscle, even in as little as two days. These changes can affect the elastic behavior of muscles and in this way may be implicated in the development of abnormal patterns of movement.

Because the body functions as a unit, any undue tension, restriction, or injury to one part will compromise the effective functioning of the whole. When all the muscles and associated connective tissue are functioning properly, efficient, balanced and fluid movement is possible. When a horse is able to use their body well, it allows them to perform longer with less fatigue and strain, helping them to stay sound longer.