Introduction

Muscle energy technique (MET) is a type of osteopathic manipulative medicine developed by Fred Mitchell, Sr., DO. In 1948, Dr. Mitchell first described the kinematic motion of the pelvis. Building on this concept, and inspired by the neurophysiologist Charles Sherrington, Dr. Mitchell developed a modality to treat muscular action dysfunction by harnessing the patient's muscle action. Sherrington observed that contraction of an antagonistic muscle would help relax the agonistic muscle.[1][2][3] He named the modality muscle energy, which was designed to improve musculoskeletal function by mobilizing joints and stretching tight muscles and fascia, thereby reducing pain and improving circulation and lymphatic flow. 

MET can be applied to all body joints except the cranium.[1][4] MET is a nontraumatic modality, and its application helps treat key lesions that are the root cause of many body dysfunctions. Understanding the intricacies of MET requires knowledge of human biomechanics; this knowledge can facilitate treatment with other modalities. For example, those with an in-depth understanding of human biomechanics can treat lesions typically addressed with a high-velocity, low-amplitude technique (HVLA) using less force and greater precision. Although Dr. Mitchell's initial concept of MET involved muscle activation with post-isometric relaxation, numerous other physiological principles for MET have since been developed. In today's MET, there are a total of 9 different physiological principles: crossed, extensor reflex, isolytic lengthening, isokinetic strengthening, joint mobilization using muscle force, respiratory assistance, oculocephalogyric reflex, reciprocal inhibition, muscle force in 1 region of the body to achieve movement in another, and post-isometric relaxation. Among the 9 approaches, post-isometric relaxation is the most commonly used. This will be discussed further below.

MET is a safe technique that can be used with inpatients to reduce hospital stays.[5][6][7][8] MET with post-isometric relaxation is generally contraindicated in patients with low vitality, certain postsurgical patients, or those in the ICU. They would benefit from MET using reciprocal inhibition, respiratory assist, or the oculocephlogyric reflex. 

Patients with a history of eye surgery are contraindicated for MET with oculocephalogyric reflex. As the treatment requires patient cooperation, patients should be able to understand and communicate easily with the clinician. Complications can be avoided if clinicians correctly diagnose, localize lesions, and apply appropriate force.

Understanding muscle physiology is essential for MET. There are 4 types of muscle contraction: isometric, concentric, eccentric, and isolytic. Isometric contraction occurs when the muscles contract without the origin and insertion of the muscles approaching each other. Concentric contraction occurs when muscles shorten during contraction. Eccentric contraction is when the muscle lengthens with contraction, and isolytic contraction is when an external force lengthens the muscle contraction.[9] The physiology of muscle contractions best explains the mechanism of action in MET.

Issues of Concern

MET has clinical benefits, but there have been some anecdotal reports of injuries associated with this technique. An inappropriate amount of force can cause tendon avulsion in geriatric patients. Rib fractures are also possible in those with osteoporosis. There have been stories of intraocular hemorrhage in a postcataract removal patient who had MET with oculocephalogyric reflex. Patient safety is enhanced by prioritizing precise diagnosis, careful localization, and the use of the minimum necessary force with this technique.

Cellular Level

Understanding muscle anatomy and physiology is essential in MET. A muscle includes many spindles; each spindle comprises 3 to 12 intrafusal muscle fibers surrounded by a large extrafusal fiber. Each spindle has an efferent and an afferent neural component. Motor nerve fibers innervate the extrafusal fibers through the alpha motor neurons, and the gamma motor neurons innervate the intrafusal fibers. The Ia and II fibers innervate the muscle spindles' afferent (sensory) portions. The central portion of a muscle spindle lacks myofibrils and therefore lacks contracting capabilities; however, the ends of these spindles do contract in response to gamma motor neurons. The Ib fibers innervate the Golgi tendon organs (GTO) in the myotendinous junctions.[10]

GTOs are an encapsulated sensory receptor associated with 10 to 15 muscle fibers. They are stimulated to inhibit muscle activity when exposed to contraction or stretching; this stimulation occurs via a negative feedback loop mediated by the alpha neuron. When the tension in a muscle is excessive, the GTO contracts to relax the entire muscle via the Ia fibers.[11] Dr. Mitchell initially hypothesized that the muscle is refractory to contraction after an isometric contraction, where it may be passively stretched without eliciting a reflexive contraction. In MET with post-isometric relaxation, the GTO is activated by increasing muscle fiber tension when the patient is asked to contract against a barrier. Once activated, there is reflexive inhibition and relaxation of the muscle via Ia fibers, and the clinician may further passively stretch the muscle due to the refractory state.

Two reflex systems within a muscle unit contribute to MET: intrinsic and extrinsic. 

Intrinsic Reflex System

The basic functional unit in muscle physiology is called a myotactic unit, which comprises a motor unit and the intrinsic sensory system associated with the muscle fibers. These sensory receptors comprise 2 types of intrafusal fibers: nuclear bag and nuclear chain (bundles together). The nuclear bag fibers extend beyond the capsule to attach to the endomysium, while the nuclear chain attaches to the inner surface of the capsule.[12] Nuclear bag receptors adapt to muscle length, contraction velocity, and acceleration. The nuclear chain fibers slowly adapt to tension. A hypothesis is that alpha motor neurons fire to contract the muscle, thereby reducing tension on the nuclear chain fibers in a somatic dysfunction. During MET with post-isometric relaxation, contracting the extrafusal muscles while the muscle length remains constant engages the nuclear bag fibers. This contracting reduces the nuclear chain, and the nuclear bag fibers quickly adapt to the stretch. The post-isometric stretch is complete without further elongating the bag fibers due to the refractory period from the decrease in gamma efferent discharge to the spindles.

Extrinsic Reflex System

In the extrinsic system, the alpha and gamma efferents of the muscle receive synaptic information from sensory nerves from other organs or muscles. This system includes reciprocal inhibition of antagonist muscles, pain avoidance, conditioned reflex, viscerosomatic reflexes, and muscle spasms.[13]

Development

Before birth, the first evidence of GTOs is observable in aponeurosis, where Ib axons terminate within islets of collagen bundles and myotubules.[14] In the first postnatal week, the innervated core elongates as collagen bundles and Schwann cells proliferate. By day 2, collagen fibrils are deposited between Schwann cells and the terminal nerve ends. These collagen bundles link the muscle fiber tips to the aponeurosis, establishing the relationship of muscle tension to GTO activation. Muscular contraction applies force to the collagen bundles, stimulating the nerve endings within the GTO.[15][16]

Muscle spindle differentiation starts around 11 weeks of gestation when the intrafusal and extrafusal fibers differentiate. The Ia afferent axon communicates to the spindle, prompting the formation of the nuclear bag, a term given to intrafusal fibers with multiple equatorial nuclei.[17][18] Subsequently, the motor nerve supply reaches the spindle.[17][19] The spindle matures between 24 and 31 weeks and increases in length after birth.[19]

Organ Systems Involved

The MET cannot directly affect organ systems, as this technique is used to treat the musculoskeletal system; however, it may alter the functioning of certain organ systems through viscerosomatic reflexes.[20][21] Each organ system in the body has sympathetic and parasympathetic innervations, which depend on where the nervous innervation arises in the spinal cord: sympathetic innervation in the thoracolumbar region and parasympathetic innervation in the sacral and cervical regions. Autonomic formation of the viscerosomatic reflex is beyond the scope of this topic. There is speculation that problems in specific viscera may present with somatic changes due to the innervation at that level, known as spinal facilitation. This facilitation results in increased output from the spinal cord, leading to changes in the alpha motor neuron and sympathetic outflow, which in turn cause increased pain. This facilitation can be treated using specific MET.[20]

Function

MET assumes that a shortened or contracted muscle is responsible for a somatic dysfunction. There are several hypotheses for the behaviors of such muscles: neuroreflexive (most likely), fibrosis, and congestion of muscle tissue (a cause of myofascial trigger points). MET approaches and treats the muscles using physiological principles and is not used to treat subluxations. There is currently no evidence to support the clinical benefit of treating subluxations.[22][23] Another commonly believed restriction mechanism follows the Meniscoid Theory proposed by Emminger in 1967; this theory is more prevalent in Europe than in North America and states that a meniscoid between the facets causes restrictions in joint movement.[24]

Mechanism

As noted in the introduction, METs employ the physiological mechanisms of post-isometric relaxation and reciprocal inhibition, primarily to enhance musculoskeletal function and alleviate pain. MET is "direct" or "indirect" for a given joint based on the indication.[1] 

Post-isometric Relaxation

GTOs are mechanoreceptors in most skeletal muscles. They are sensitive to muscular contractile force, and in contrast to muscle spindles, muscle stretches rarely and inconsistently activate GTOs. These encapsulated bundles of collagen are innervated by fast-conducting type Ib afferent fibers and are present at muscle-tendon or muscle-aponeurosis junctions; they attach to an individual muscle fascicle tendon on 1 end, and the whole muscle-tendon or aponeurosis of the other. This positioning, described as "in-series," means that the receptor is part of the functional unit and contrasts with the muscle spindle, which operates adjacent to the functional unit "in parallel."[15][16] GTOs are activated at high levels of force and, hypothetically, inhibit muscle activity, thereby preventing musculoskeletal injury.[25]

Physiologically, increased tension to the GTO prompts the activation of the type Ib afferent fibers that project to the spine, where they provide positive input on inhibitory interneurons that, in turn, add negative or inhibitory input on the efferent α-motor neurons that receive input from the cortex to the homonymous muscle.[26] In effect, sufficient GTO stimulation can override the brain's efferent output, leading to relaxation. This phenomenon is known as the "inverse stretch" or the "autogenic" reflex.[27][28] Dr. Mitchell Jr. further postulated that there is a refractory state after an isometric contraction, during which passive stretching may be performed without a myostatic reflex opposition.

Patients are usually placed in the barrier and asked to contract against the clinician. They are then asked to relax. This phase is refractory, where a new barrier can be reached, and the process is repeated.

Joint Mobilization Using Muscle Force

This principle is based on the meniscoid theory, as described above. A distortion of articulation and loss of motion leads to reflexive hypertonicity of the muscles crossing the joint. The reflexive hypertonicity further compresses the dysfunctional joint surface, leading to thinning of the synovial fluid layers and adherence of the joint surfaces. Treating the segment requires the maximum force the clinician can tolerate to "reseat" the joint and elicit reflex relaxation of the hypertonic muscle.

Respiratory Assists

The clinician holds a fulcrum using the motion of the ribs or the subtle movement of the spine/pelvis during respiration, allowing the respiratory forces to work. This technique is frequently used to treat somatic dysfunctions of the ribs and sacrum.

Reciprocal Inhibition

Muscle spindles are stretch-sensitive mechanoreceptors found in skeletal muscle. A muscle spindle is a bundle of striated, intrafusal muscle fibers within the fascicles of force-producing, extrafusal muscle fibers. "Fusil" derives from the term "fusiform," meaning spindle-shaped. Any stretch or change in the length of the extrafusal fibers results in a stretch of the intrafusal fibers, which is then detected in the equatorial and polar regions of the muscle spindle. This physiology contrasts with that of GTOs, which are relatively insensitive to passive length changes but respond to increases in muscle force. Two afferents, primary (type Ia) and secondary (type II), measure the stretch sensation. A single Ia fiber is present, along with between 0 and 5 II fibers per spindle.[29]

The Ia fiber is comparable in size and speed of transmission to the previously mentioned Ib fibers and supplies all intrafusal fibers in the spindle at the equatorial region.[15] The exact function of type II fibers is less understood; however, these smaller fibers terminate on the polar ends of the spindle. Muscle spindles are unique among proprioceptors in that efferent fibers innervate them. These myelinated γ-motor neurons derive from the same efferents that supply the extrafusal muscle. Excitation of these γ-motor neurons does not affect overall muscle tension but appears to maintain tension on the muscle spindles to track the length of the extrafusal fibers effectively. Lastly, spindle afferents are tonically active, with a velocity-dependent increase in firing rate in response to passive stretch.[29] 

Physiologically, stretching a muscle fiber produces activation of Ia muscle spindle afferents that project to the spine and activate the efferent α-motor neurons and, subsequently, the γ-motor neurons of the homonymous muscle, leading to contraction of the intra- and extrafusal fibers. Simultaneously, the Ia fibers activate inhibitory interneurons in the spine to inhibit the α-motor neurons of the antagonist's muscle. This circuit is known as the stretch reflex, which is believed to prevent muscle strain and support bipedal walking and posture.[30][31][32][33][34] This principle is used when contracting the antagonist to relax the dysfunctional agonist muscle.

Oculocephalogyric Reflex

The oculocephalogyric reflex approach to MET can gently treat an unstable segment in the upper cervical spine using eye motion. This reflex is not fully understood but is related to the doll's eye and vestibulo-ocular reflex.[35][36] Nerves for the extraocular muscles are sent to the vestibular nuclei via the ophthalmic division of the trigeminal nerve. Information from the vestibular nuclei then travels down the medial and lateral vestibulospinal tracts. The medial tract specifically innervates C1 and may branch into the suboccipital muscles, thereby enabling motion in these muscles.[37] This approach is useful if the patient has severe pain in the upper cervical spine or if upper cervical instability is suspected. The patient is positioned to look toward a stimulus to elicit the reflex. 

Crossed Extensor Reflex

MET utilizes spinal reflex mechanisms, including the crossed extensor reflex, particularly in the extremities, in response to noxious or injurious stimuli. Voluntary contraction of a muscle leads to inhibition of the same muscle on the contralateral side and facilitation of its contralateral antagonist. An illustrative example occurs when stepping on a nail: ipsilateral hip and knee flexors contract to withdraw the limb, while the contralateral extensor muscles activate to stabilize posture.[38] This reflex is mediated by efferent motor pathways that interact with multiple spinal interneurons, 1 of which transmits inhibitory or facilitatory signals to contralateral agonist and antagonist muscle groups.

Isokinetic Strengthening

This approach to MET is to help strengthen the muscle. A concentric contraction is employed, allowing the muscle to shorten at a controlled rate. It is recommended that any shortening of an antagonistic muscle be addressed before performing strengthening exercises. For example, the quadriceps may be weakened due to hypertonic/shortened hamstrings; treatment would begin with treating the shortened hamstring muscles, followed by isokinetic quadriceps strengthening.

Isolytic Lengthening

This approach is used to lengthen a muscle shortened by fibrotic contracture. An isolytic contraction occurs when the clinician applies sufficient force to overcome the patient’s voluntary muscle contraction, resulting in controlled muscle lengthening. The clinician applies a vibratory motion while performing the technique, as there is anecdotal evidence that it can help break up fibrosis and improve circulation.

Coordinated Motor Movement

This MET approach addresses somatic dysfunction by engaging coordinated movement of adjacent body regions. Muscle contractions generated during the motion of these neighboring segments are thought to influence and normalize the dysfunctional area. For example, in treating a bilaterally extended sacrum, the patient is instructed to activate the pelvis and lower extremities to facilitate correction of sacral dysfunction.

Related Testing

Successful application of MET requires an accurate diagnosis of somatic dysfunction. Fryette’s laws of spinal mechanics are commonly used to guide the diagnosis of spinal dysfunction and inform MET treatment. Fryette described 3 laws of spinal mechanics:

1. In a neutral position, the segments side-bend and rotate to opposite sides
2. In a non-neutral spinal position, the segments side-bend and rotate in the same direction
3. A motion in 1 plane reduces the motions in the other 2 planes of the spinal segment [39]

Pathophysiology

The increased muscle tone purportedly treated by MET is comparable to that observed in hypertonicity or spasticity associated with upper motor neuron disease.[1][40] Increased activity of the extrafusal muscle fibers results from either increased activity of the muscle spindle or abnormal sensory processing in the spinal cord. In the former, increased activity of γ-motor neurons leads to abnormally short muscle spindles, resulting in a hyperexcitable state in which movement within the physiologic range elicits reflexive muscular contraction. Similarly, type II fibers are hypothesized to contribute to spasticity by directly activating α-motor neurons.[40]

Clinical Significance

As noted in the introduction, MET primarily improves range of motion and reduces pain.[1][4] These techniques are used by osteopathic and allopathic clinicians, physical therapists, and chiropractors as primary or adjunctive therapy.[41] In musculoskeletal practice, MET is commonly applied to reduce pain associated with muscle hypertonicity in the back, neck, and other major joints, though it may theoretically be used to treat nearly any joint in the body.[42][43] In osteopathic medicine, MET, when used alongside standard-of-care treatment and other osteopathic techniques, has been shown to improve outcomes in conditions such as pneumonia and fibromyalgia. These complementary effects are proposed to result from fascial stretching that enhances lymphatic and hemodynamic function.[44][45] 

A handful of meta-analyses have assessed the effectiveness of MET in treating various conditions, particularly low back pain. While some studies yielded mixed results, there is apparent consensus on the positive effects of MET on low back pain, with insufficient and inconsistent evidence to support its use in other contexts.[1][41][42][46] Additional research has demonstrated decreased lumbar motor excitability, which has been associated with improved function.[47]

The effectiveness of MET depends on accurate diagnosis, precise localization, and appropriate force application. It is essential to distinguish a key lesion from compensatory changes; for example, a somatic dysfunction at L5 often leads to a compensatory alteration at the sacral base.[48] Treating the compensatory change alone do not resolve the patient’s presenting symptoms. Awareness of fascial interactions between lesions is also critical. Even when a segmental dysfunction is identified, restrictions in the segment above can create fascial strain, potentially complicating treatment of the targeted segment. As with all osteopathic care, an accurate diagnosis within the broader clinical context is fundamental.

Excessive force is a common error among clinicians new to MET. Applying excessive force recruits surrounding muscles to stabilize the treated segment, thereby counteracting the intended effects of the technique. A commonly taught guideline is to use 5–10 pounds of force. Experienced clinicians, however, apply just enough force to produce movement in the targeted segment without activating adjacent muscles.

Finally, the precise localization of force is more important than its magnitude. The body should be positioned so that the applied force targets the specific segmental joint. Clinicians should make subtle adjustments to account for anatomic variability between patients. For example, the sacrum has 3 transverse axes and 2 oblique axes. The middle transverse axis allows the sacrum to move relative to the innominate, while the inferior transverse axis allows the innominate to move relative to the sacrum. When treating an anterior innominate, the clinician should flex the hip to engage the inferior transverse axis; over- or underflexion fails to target the correct segment, reducing the likelihood of successful treatment.

MET with post-isometric relaxation is the most commonly used modality and entails the following steps:

1. The target joint or muscle barrier is isolated through joint positioning, generally to a pathologic barrier.
2. Follow with the patient's active muscle contraction in a specific direction, generally away from the restriction, for a specified period, against the clinician-applied counterforce. Conventionally, the force applied by the patient should be the maximum that can be comfortably tolerated by both the patient and the clinician.
3. Have the patient relax the contracted muscle.
4. Use passive movement of the patient's anatomy toward a new pathologic barrier.
5. Repeat steps 1 to 4 as tolerated until physiologic pain is sufficiently relieved or the patient achieves the desired range of motion.[41]

Different protocols have been developed for each step within this framework, including duration and strength of contraction, duration of rest, and the number of repetitions.[1][49][50][51] For example, the Greenman method proposes a 5 to 7second relaxation step and 3 to 5 repetitions overall.[1]

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Disclosure: Joshua Waxenbaum declares no relevant financial relationships with ineligible companies.

Disclosure: Reddog Sina declares no relevant financial relationships with ineligible companies.

Disclosure: Myro Lu declares no relevant financial relationships with ineligible companies.