User:Ken Saladin/Prime mover draft

A prime mover, or agonist, is any muscle that produces the greatest amount of force in a particular joint movement (Saladin, 2007: 325–326; Williams, 1995: 786). In human anatomy, for example, the prime mover of elbow flexion is the brachialis muscle on the anterior side of the humerus. Its unmoving origin is the anterior side of the lower half of the humerus. Its moving insertion is the ulna of the forearm. Contraction of the brachialis draws the ulna toward the humerus and thus flexes the elbow. Although other muscles aid in flexing the elbow, the brachialis provides the most force.

Muscles Aiding or Opposing a Prime Mover
Most joints in the skeletal system are acted on by two or more muscles. Muscles other than the prime mover may act as s, antagonists, or fixators. A synergist is a muscle that aids the prime mover in producing its action. In elbow flexion, for example, the brachialis is aided by the biceps brachii muscle, which exerts an upward pull on the radius and other forearm structures. Although better known, especially because of its interest to body builders, the biceps brachii does not produce as much force as the brachialis. . An antagonist is a muscle that opposes the prime mover. In elbow flexion, the antagonist is the triceps brachii muscle on the posterior side of the humerus. When one flexes the elbow, the triceps maintains a degree of opposing muscle tension so the elbow flexion is not excessively abrupt or jerky. Unrestrained movements tend to be uncoordinated and inefficient. In addition, joint action of an prime mover and antagonist is often needed to hold a joint in a particular position, such as holding the elbows partially flexed while reading a book. An antagonist also typically produces joint actions opposite from those of the prime mover. For example, the triceps brachii acts to extend (straighten) the elbow when it contracts more powerfully than the flexors.

A fixator is a muscle that holds a bone stationary, preventing unwanted movement. Elbow flexion provides another example. The biceps brachii originates on the scapula (shoulder blade) and crosses both the shoulder and elbow joints. In principle, its contraction could not only flex the elbow but also pull the scapula anteriorly. Depending on the circumstances, this might be an undesirable movement because some of the power of the biceps would be wasted on scapular movement instead of helping to flex the elbow. However, the scapular movement is prevented by a pair of rhomboid muscles (the rhomboideus minor and rhomboideus major), which extend from the medial border of the scapula to the spine (vertebral column). Their contraction draws the scapula toward the spine. In elbow flexion, they act as fixators by preventing anterior motion of the scapula while the biceps moves the forearm.

Terms Change As a Muscle's Role Changes
It is important to note that a muscle cannot simply be classified as an prime mover, synergist, antagonist, or fixator under all circumstances. These terms describe not the muscle per se, but the role that the muscle plays in a particular movement (Williams, 1995: 786). The same muscle may serve as an prime mover in one movement and antagonist in another. For example, in elbow flexion, the brachialis is the prime mover, the biceps is a synergist, and the triceps is the antagonist; but in elbow extension, the triceps becomes the prime mover and the brachialis and biceps are now antagonists.

Nonredundancy of Prime Movers and Synergists
It is also important to note that even though a prime mover and synergist may appear to have an identical function, such as elbow flexion, they are not redundant. The biceps, for example, is not only an elbow flexor but also a powerful supinator that rotates the forearm in actions such as turning a doorknob or using a screwdriver. Thus, its function is not merely repetitious of the brachialis.

Another reason for the nonredundancy of a prime mover and synergist is that the two may be different physiological classes of muscle. For example, two calf muscles, the gastrocnemius and soleus, share a common calcaneal (Achilles) tendon to the heel. The gastrocnemius is the prime mover of plantar flexion, a motion that points the toes downward as in pressing the gas pedal of a car. The soleus is a synergist. Although they share a common tendon and therefore produce the same joint action at the ankle, they are not redundant. The gastrocnemius is composed mainly of fast glycolytic (fast-twitch) muscle fibers adapted for quick movements such as jumping, but it fatigues easily. The soleus is composed mainly of slow oxidative (slow-twitch) muscle fibers; it produces slightly less force at the heel, but is more fatigue-resistant and better adapted for the slow and sustained contraction employed in maintaining posture (Neumann, 2002: 514) and for endurance exercises such as jogging and snow skiing (Saladin, 2007: 430).