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Components of muscle function

This is an excerpt from Dynamic Human Anatomy 2nd Edition With Web Study Guide by William Whiting.

The control of even the simplest joint movement typically requires the cooperative action of several muscles working together as a single unit. This cooperative action is called muscle synergy. Synergistic muscles work together, but other muscles with opposite functions may work against a particular movement. The overall, or net, effect of all muscles acting at a joint determines the ultimate mechanical effect, or movement.

Several concepts of muscle function are important for understanding how muscles cooperate and compete to control movement: agonists, neutralization, stabilization, antagonists, and coactivation.

• Agonists. Muscles that actively produce or control a single joint movement or maintain a single joint position are called agonists. In most movements, several muscles act together as agonists, with some playing a greater role than others.
• Neutralization. Muscles often perform more than one movement function at a given joint. At the ankle complex, for example, a muscle might act as both a plantar flexor and invertor (e.g., tibialis posterior). To produce pure plantar flexion, another muscle whose action produces plantar flexion and eversion (e.g., peroneus longus) would also need to be involved. The eversion action of the second muscle would cancel out, or neutralize, the inversion action of the first muscle. This process of canceling out an unwanted secondary movement is called neutralization.
• Stabilization. During concentric action, a muscle attempts to shorten by pulling its two bony attachment sites together. In most cases, the bone with the least resistance to movement (inertia) will move. When the inertia of both bones is similar in magnitude, both ends tend to move. If movement of only one end is desired, the other end must be prevented from moving, or stabilized. This stabilization is provided by other muscles or an external force. As an example, consider hip flexion created by the anterior thigh musculature. In attempting to move the femur in flexion, the hip flexors also tilt the pelvis anteriorly. If pelvic tilt is unwanted, then the abdominal musculature must act isometrically to stabilize the pelvis and prevent its movement.
• Antagonists. Muscles acting against a movement or position are called antagonists. To perform a movement most effectively, when the agonists actively shorten in concentric action, the corresponding antagonists passively lengthen. When agonists actively lengthen in eccentric action, the associated antagonists passively shorten. In many movements, then, the agonists and stabilizers are active while the antagonists are passive.
• Coactivation . Simultaneous action of both agonists and antagonists is called coactivation (also co-contraction). Coactivation might occur, for example, when an unskilled performer is unsure of the necessary muscle recruitment strategy. Skilled performers, however, do not exhibit an absence of coactivation. At least four possible explanations exist for coactivation in skilled performers: (1) less overall effort may be required in agonist - antagonist pairings for movements that involve changes of direction when the muscles maintain some level of activity, as opposed to working in an on - off manner; (2) coactivation increases joint stiffness and consequently joint stability, which may be desired for movements involving heavy loads; (3) coactivation of a single-joint muscle (e.g., gluteus maximus) and a two-joint muscle (e.g., rectus femoris) can increase the torque at a joint (e.g., knee) acted on by the two-joint muscle; and (4) given the neural complexities of the forearm and hand, fine movements of the fingers require complex coactivation strategies (Enoka, 2002).

Note that the term coactivation is limited to the concurrent action of agonists and antagonists and should not be used to describe the simultaneous action of multiple agonists. For instance, concurrent activity of the biceps brachii and triceps brachii during an elbow curl exercise would be considered coactivation. On the other hand, if the triceps was passive (i.e., inactive), simultaneous action of the three elbow flexors (biceps brachii, brachialis, brachioradialis) would not be considered coactivation.

With these concepts in mind, we now consider a simple yet fundamental question: How do we determine which muscles are active in producing or controlling a given movement?

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