Introduction and Anatomical Overview:
Muscle is made up of two types of fibers, intrafusal and extrafusal. Extrafusal fibers are the contractile fibers and intermixed within the extrafusal fibers are intrafusal fibers. Housed within intrafusal fibers is a specific type of mechanoreceptor. Mechanoreceptors, in general, are interspersed through the entire body – hair, skin, ligaments – and are responsible for sensing tissue pressure and distortion and give our body a sense of proprioception by detecting position of our muscles, bones, and joint. There are many types of mechanoreceptors, but one specifically – the muscle spindle – lives within the intrafusal muscle fibers. The muscle spindle transmits sensory data regarding changes in muscle length, and therefore movement, to the central nervous system (CNS) via the primary afferent (sensory) neurons. The intrafusal fibers receive neural stimulation from gamma efferent (motor) neurons. Think of the gamma motor neuron as a type of sensitivity adjuster. The efferent input adjusts the length of the spindle so that it remains at an optimal length to detect changes within the muscle.
The Muscle Spindle
What is Gamma Gain:
Oftentimes, there may be hyperactivity of the gamma motor neurons. This hyperactivity causes continual tightening of the intrafusal fibers, known as gamma gain. Gamma gain leads to hypertonicity in the muscle , causing adaptive shortening of surrounding fascial tissues. These taut bands of fascial tissues lead to hypersensitive trigger points associated with myofascial pain syndrome (MFPS).
The question of what causes gamma gain is unclear. However, acute injury will result in muscular spasm and guarding. Also, pain receptors (nociceptors) increase gamma motor response. Chronic pain, such as that of MFPS is also a result of gamma gain. The aforementioned hyperactivity of the gamma motor neuron, hypersensitivity to stretch and subsequent adaptive tissue shortening becomes hypersensitive to stretch. Stretch of fascial tissue is painful and this results in an increase in gamma motor activity; a vicious cycle of gamma motor hyperactivity, hypersensitivity, hypertonicity, adaptive shortening, and pain. The cycle builds from mild, localized discomfort and will progress to a pain pattern that includes moderate to severe pain with pain referral.
The Gamma Gain Cycle
For effective treatment of this persistent pain pattern, therapy must focus on breaking this cycle. Breaking this cycle by blocking gamma gain is the focus of two manual therapy techniques: myofascial release and strain–counterstrain. The positioning for these techniques decrease gamma motor activity and interrupts the cycle.
Myofascial Release:
Many modalities can be used to treat physically active individuals, and all treatment plans for myofascial pain should include active therapeutic exercise. None of these interventions, however, directly address the problem of fascial restrictions and gamma gain.
Myofascial release techniques, more specifically indirect myofascial release techniques, place muscle and fascia in positions that remove stress from the tissues. The application of light, sustained pressure up to the fascial resistance barrier allows the tissue to relax. The shortened position decreases noxious stimulus from the tissue, which in turn diminishes activity in gamma motor nerves. Direct techniques attempt to stretch bound fascia, by applying load through a restrictive barrier. This will decrease the afferent input from the tissue. Both types of techniques can be used to treat myofascial pain pattern, however, the primary focus of the indirect technique is to address gamma gain, by decreasing efferent stimulus.
Strain–Counterstrain:
Strain–counterstrain, originated by Dr. Lawrence Jones, is another type of manual therapy technique that places emphasis on gamma gain. Strain–counterstrain relieves pain and dysfunction by altering neural activity. Strain–counterstrain is a technique in which a body segment is passively moved into a position of least discomfort. Like indirect myofascial release, moving to a position of comfort decreases noxious stimulus from the tissue and diminishes activity of gamma motor nerves, which are responsible for the dysfunction.
Like myofascial release, strain-counterstrain has both direct and indirect techniques. Direct techniques involve applying force against a restrictive barrier to improve motion, whereas indirect techniques involve moving the body away from a motion-restricting barrier to a position of comfort and relaxation. Hold the passive position for a period of 90-120 sec, then slowly move back to a resting, neutral position. Placing a body segment in a position of comfort reduces gamma motor activity which in turn halts the cycle of hypersensitivity, hypertonicity, and adaptive shortening.
Conclusion:
If you have a client complaining of persistent pain patterns similar to myofascial pain syndrome it may be a result of gamma gain. Using manual therapy techniques such as indirect myofascial release and indirect strain-counterstrain may pay big dividends at relieving pain and improving function. Applying these manual therapy techniques in combination with other modalities and therapeutic exercise can have a positive impact on patient outcomes.
A portion of the content contained in this post was adapted from Denegar et, al., Therapeutic Modalities for Musculoskeletal Injuries, 3rd Edition. Human Kinetics
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