Reciprocal inhibition describes the process of muscles on one side of a joint relaxing to accommodate contraction on the other side of that joint. In some allied health disciplines this is known as reflexive antagonism. Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony for smooth movement. When a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle. This inhibition is accomplished by the actions of an inhibitory interneuron in the spinal cord.
The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.
If opposing muscles were to contract at the same time, a muscle tear can occur. This may occur during physical activities, such as running, during which muscles that oppose each other are engaged and disengaged sequentially to produce coordinated movement. Reciprocal inhibition facilitates ease of movement and is a safeguard against injury. However, if a "misfiring" of motor neurons occurs, causing simultaneous contraction of opposing muscles, a tear can occur. For example, if the quadriceps femoris and hamstring contract simultaneously at a high intensity, the stronger muscle (traditionally the quadriceps) overpowers the weaker muscle group (hamstrings). This can result in a common muscular injury known as a pulled hamstring, more accurately called a muscle strain.
"When the central nervous system sends a message to the agonist muscle (muscle causing movement) to contract, the tension in the antagonist muscle (muscle opposing movement) is inhibited by impulses from motor neurons, and thus must simultaneously relax. This neural phenomenon is called reciprocal inhibition." Taken from Massage Therapy Principles & Practices by Susan Salvo 1999, pg 161
The phenomenon is now known to be fleeting, incomplete, and weak. By example, when the triceps brachii is stimulated, the biceps is reflexively inhibited. The incompleteness of the effect is related to postural and functional tone. Also, some reflexes in vivo are polysynaptic, with entire muscle groups responding to noxious stimuli (Nociceptive Withdrawal Reflex).
Application in Physical Therapy
Reflexive antagonism is the basic original notion behind indirect muscle energy techniques. While this notion is now understood to be incomplete, the clinical mechanism of Reflexive Antagonism continues to be useful in Physical Therapy. Reciprocal inhibition is a synonym.
Muscle energy techniques that use reflexive antagonism, such as rapid de-afferentation techniques, are medical guideline techniques and protocols that make use of reflexive pathways and reciprocal inhibition as a means of switching off inflammation, pain, and protective spasm for entire synergistic muscle groups or singular muscles and soft tissue structures.
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