Force Couple: Although we usually think of a joint's movement as being the action of one muscle, muscles are actually recruited as groups. Each muscle, because of its different attachment points (origins and insertions) and angle of pull, tends to pull the bone in a different direction. These divergent pulls, collectively, culminate in a certain joint action. A force couple can be defined as a pair of muscle forces that act together on a joint to produce rotation, and these forces may actually exert pulls in opposite directions. These muscles may be synergistic pairs, consisting of prime movers and groups of stabilizers, for instance, or agonist/antagonist pairs. Force couple, then, is actually another way of referring to muscle synergists or a synergistic action. For more on synergists see muscle roles.
One example of an important force couple in the human body is the deltoid-rotator cuff force couple. The rotator cuff is more than one muscle but each of these works with the deltoid to create its own important force couple. The major function of this force couple is to prevent migration of the humeral head out of the glenoid socket during shoulder abduction, preventing impingement of the rotator cuff against the acromion. As the deltoid moves the humerus in an upward direction, the pull of the deltoid can exert a shearing force that can pull the humeral head out of its socket in a superior direction, if left unchecked. The rotator cuff muscles each exert a force that checks this tendency, known as the "superior component."
Weakness or dysfunction in any of the rotator cuff muscles can compromise this normal shoulder function, causing a shoulder impingement or impingement of the rotator cuff. Typically, during shoulder movements that raise the arm, this causes the humeral head to impinge the supraspinatus tendon where it runs underneath the acromion process, causing the tendon to become inflamed (see image below).
The supraspinatus exerts an approximating component or, in other words, acts as a compressor, which pulls the humeral head into the glenoid socket. The infraspinatus, teres minor, and subscapularis exert an inferior component, making them depressors, which exert a downward pull counteracting the upward pull of the deltoid.
|internal and external obliques||trunk rotation|
|deltoid and rotator cuff||shoulder abduction|
|upper and lower trapezius fibers||scapula upward rotation|
This page created 26 Feb 2012 20:50
Last updated 12 Mar 2016 22:33