Force Couples 

What is a force couple?

A force couple by definition refers to the principle whereby two or more muscles acting on a joint center in opposing directions create movement about the joint in a specific direction. If the two opposing forces are equal in strength, there will be no net movement, thus creating what is called a force closure. One example of a force couple is the relationship of the biceps and triceps around the elbow joint. For instance, when the biceps contract to flex the elbow, the triceps must relax in order to allow this movement. Conversely, the biceps must then relax in order for the triceps to actively extend the elbow.

Agonists, Antagonists

Within a force couple, the opposing muscle groups are intricately linked via the nervous system creating a reflex arc. Within this arc there are muscles known as agonists, which refers to the muscle performing the specific action, whereas the antagonist refers to the muscle or muscle group directly opposing the agonist. The reflex arc is important with respect to this because in order for the agonist to contract, the antagonist must concurrently relax in order to allow for joint motion. In the example previously stated, the biceps would be considered the agonist for elbow flexion, while the triceps would be considered the antagonist.

Synergists and Stabilizers

In addition to agonists and antagonists are synergists, referring to muscles that assist the agonist in performing a movement by stabilizing the joint to improve its efficiency. Synergists can also act in conjunction with the agonist to perform the specific movement, although they are not considered the prime mover. 

An example of a synergistic relationship with respect to the elbow example stated above would include the brachioradialis and the brachialis, which contract to assist the movement of elbow flexion and provide stability to help the biceps move more efficiently. 

Stretch Reflex

The stretch, or myotatic, reflex is a complex neuromuscular connection designed to help protect the joints and soft tissues surrounding those joints. The stretch reflex refers to the active contraction of a muscle in response to its passive stretching. It is intended to maintain a constant length in the muscle to prevent strain or tearing. Within the body of a muscle are small stretch receptors known as muscle spindles, these receptors must adjust their sensitivity relative to the contraction or relaxation of a given muscle and in the presence of pain, deformity, or altered biomechanics, they can become either over or underactive causing deviation in movement patterns. An example of a stretch reflex is the patellar tendon reflex in that rapid stretch brought on by striking of a reflex hammer on the patellar tendon causes a reflexive contraction of the quadriceps muscle to extend the knee.