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Reciprocal inhibition is aneuromuscular process in whichmuscles on one side of a joint relax to allow the contraction of muscles on the opposite side, enabling smooth and coordinated movement.[1] This concept, introduced byCharles Sherrington, a pioneeringneuroscientist, is also referred to asreflexive antagonism in some allied health fields. Sherrington, one of the founding figures in neurophysiology, observed that when the central nervous system signals an agonist muscle to contract, inhibitory signals are sent to the antagonist muscle, encouraging it torelax and reduce resistance. This mechanism, known as reciprocal inhibition, is essential for efficient movement and helps prevent muscle strain by balancing forces around a joint.[2]
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Joints are controlled by two opposing sets of muscles calledextensors andflexors, that work in synchrony for smooth movement. When a muscle spindle is stretched, thestretch reflex is activated, and the opposing muscle group must be inhibited to prevent it from working against the contraction of the homonymous muscle. This inhibition is accomplished by the actions of an inhibitor interneuron in the spinal cord.[3]
The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates thealpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which then innervates thealpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposingalpha 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 opposing muscles engage and disengage 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 thequadriceps femoris andhamstring 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 apulled hamstring, more accurately called amuscle strain.
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The phenomenon is fleeting, incomplete, and weak. For example, when thetriceps brachii is stimulated, thebiceps is reflexively inhibited. The incompleteness of the effect is related topostural and functional tone. Also, some reflexesin vivo arepolysynaptic, with entire muscle groups responding tonoxious stimuli.
As the body ages, the control of voluntary inhibition decreases in conjunction with the torque of the synapse as joints stiffen and their motor output is reduced. However, this reduction in ability tends to be insignificant.[4]
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Reciprocal inhibition is the basic original notion behind indirectmuscle energy techniques. While this notion is now understood to be incomplete, the clinical mechanism of reflexive antagonism continues to be useful in physical therapy.
Muscle energy techniques that use reflexive antagonism, such as rapid deafferentation techniques, aremedical 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 andsoft tissue structures.
