myofascial pain syndrome ( mps ) is described as the sensory , motor , and autonomic symptoms caused by myofascial trigger points ( trps ) .
trps are defined as exquisitely tender spots in discrete taut bands of hardened muscle that produce local and referred pain , among other symptoms .
an individual contraction knot appears as a segment of a muscle fiber with extremely contracted sarcomeres and an increased diameter .
the integrated trp hypothesis postulates that in myofascial pain motor endplates release excessive acetylcholine , which is evidenced histopathologically by the presence of sarcomere shortening .
it prevents full lengthening of the muscle , weakens the muscle , and mediates a local twitch response of muscle fibers when adequately stimulated . when compressed within the patient s pain tolerance , it produces referred motor and often autonomic phenomena , generally in its pain reference zone .
a latent trp is  clinically quiescent with respect to spontaneous pain ; it is painful only when palpated .
a latent trp may have all the other clinical characteristics of an active trp and always has a taut band that increases muscle tension and restricts range of motion  .
trps are very common , although literature about their prevalence is sparse [ 37 ] .
knowing the potential causes of trps is important to prevent their development and recurrence , but also to inactivate and eliminate existing trps .
there is general agreement that any kind of muscle overuse or direct trauma to the muscle can lead to the development of trps .
muscle overload is thought to be the result of sustained or repetitive low - level muscle contractions , eccentric muscle contractions , and maximal or submaximal concentric muscle contractions .
although muscle damage is not required for the development of trp , there may be a disruption of the cell membrane , damage to the sarcoplasmic reticulum with a subsequent release of high amounts of calcium - ions , and disruption of cytoskeletal proteins , such as desmin , titin , and dystrophin .
ragged red ( rr ) fibers and increased numbers of cytochrome - c - oxidase ( cox ) negative fibers are common in patients with myalgia , which are suggestive of an impaired oxidative metabolism .
mechanical muscle overuse can be defined as the result of muscle contractions that exceed muscle capacity . since the capillary blood pressure ranges from approximately 35  mm hg at the beginning ( arterial side ) to 15  mm hg at the end of the capillary beds ( venous side ) , the capillary blood flow is temporarily obstructed during muscle contractions .
the blood flow recovers immediately with relaxation , which is consistent with its normal physiological mechanism . in dynamic rhythmic contractions ,
intramuscular blood flow is enhanced by this contraction - relaxation rhythm , also known as the muscular pump . during sustained muscle contractions , however , muscle metabolism is highly dependent upon oxygen and glucose , which are in short supply .
even contractions performed at only 10  % and 25  % of capacity or maximum voluntary contraction ( mvc ) may produce intramuscular pressures high enough to significantly impair the intramuscular blood circulation .
the association of the percentage of mvc and intramuscular pressure ( imp ) is highly dependent upon the architecture of the muscle .
it has been postulated that the percentage of mvc reported as the upper limit for localized muscle fatigue varies between muscles , because of the variation of imp during a contraction .
for example , 10  % of mvc of the supraspinatus muscle produces approximately 50  mm hg of imp , while 25  % of mvc of the trapezius muscle is good for only 22  mm hg of pressure .
the emg signs of localized muscle fatigue did not recover until the muscle contraction pressure was below 20  mm hg in the biceps muscle . according to otten ,
the increased pressure gradients during low - level exertions may contribute to the development of pain   and eventually to the formation of trps ( personal communication 2005 ) . since oxygen and glucose are required for the synthesis of adenosine triphosphate ( atp ) , which provides the energy needed for muscle contractions , sustained contractions may cause a local energy crisis due to the lack of oxygen . to guarantee an adequate supply of atp , the muscle can switch within a few seconds to anaerobic glycolysis . during the initial phase of glycolysis ( sugar splitting ) 1 glucose molecule is broken down into 2 pyruvic molecules releasing enough energy to form 2 atp molecules . under aerobic circumstances
, oxygen reacts with pyruvic acid producing a high amount of atp ( 16 molecules per pyruvic acid molecule ) , carbon dioxide and water . under anaerobic circumstances ,
however , most of the pyruvic acid produced during glycolysis is converted into lactic acid , thereby increasing the intramuscular acidity ( ph ) .
most of the lactic acid diffuses out of the muscle into the bloodstream ; post - exercise lactic acid is washed out within 30 minutes after exercise .
unfortunately , when the capillary circulation is restricted , as in sustained low - level contractions , this process comes to a standstill .
researchers at the us national institutes of health found that in the direct environment of active trps , the ph may be well below 5 , which is more than sufficient to excite muscle nociceptors , including acid - sensing ion channels ( eg , asic 1 and 3 ) , and the transient receptor potential vanilloid receptor trpv1 [ 13 , 14 ] . small increases of the h concentration , as seen with inflammation , heavy muscle work , and ischemia , are sufficient to excite muscle group iv endings , contributing to mechanical hyperalgesia and central sensitization . furthermore , a low ph downregulates acetylcholinesterase ( ache ) , increases the efficacy of acetylcholine ( ach ) , and maintains the sarcomere ( super- ) contraction .
it also triggers the release of several nociceptive substances , such as calcitonin gene - related peptide ( cgrp ) , which can enhance the release of ach from the motor endplate and simultaneously decrease the effectiveness of ache in the synaptic cleft .
cgrp also upregulates the ach - receptors ( achr ) at the muscle and thereby creates more docking stations for ach .
miniature endplate activity depends on the state of the achr and on the local concentration of ach , which is the result of ach - release , reuptake , and breakdown by ache .
after atp is attached to the myosin molecule , the link between myosin and actin weakens , and the myosin head detaches from actin . in other words , the cross - bridge between myosin and actin 
breaks. simultaneously , the ca ion detaches from the troponin molecule , which blocks tropomyosin . under normal physiological circumstances ,
large amounts of free calcium - ions will reenter the sarcoplasmic reticulum by the ca pump ( calcium atpase ) , which places a high demand on atp during relaxation . in case of severe energy depletion
, the sarcomeres may stay contracted , until enough atp is available to resolve the intracellular ca accumulation .
ca accumulation due to sustained motor unit activity has been suggested to play a causative role in the development of muscle disorders and trps . in a preliminary study using doppler ultrasound , sikdar et al
have shown that blood flow waveforms show significant differences between active trps , latent trps and normal sites .
the flow waveforms near active sites showed increased systolic velocities and flow reversal with negative diastolic velocities [ 17 ] .
they identified 2 contributing factors , namely an increase in the volume of the vascular compartment , and an increased outflow resistance .
increased outflow resistance could be due to muscle contractures at the trp that compress the capillary or venous bed .
sustained low - level contractions are common in the workplace where many occupations rely on prolonged postures , as seen in musicians , supermarket cashiers , computer operators , hairdressers , and dentists , among others .
henneman , working with anesthetized cats , showed that in response to increasing physiological excitation , motor neurons are recruited in order of increasing size .
this fundamentally important finding was verified in humans by milner - brown et al   and is now generally accepted .
smaller motor units have a smaller alpha - motor neuron cell body , smaller axons and fewer muscle fibers to activate compared with larger ones . according to the size principle , small motor units innervating type i red colored , slow
oxidative fibers are recruited first , followed by red to pink colored , fast oxidative fibers , and finally by white colored , fast glycolytic fibers .
furthermore , small type i muscle fibers are activated during prolonged tasks , although a few studies have described some degree of motor unit substitution .
hgg suggested that the continuous activity of these motor units in sustained contractions causes overuse muscle fiber damage , especially to the type i fibers during low - level activities , which he summarized in his cinderella hypothesis .
it is conceivable that in sustained low - level contractions and in dynamic repetitive contractions , ischemia , hypoxia and insufficient atp synthesis in type i motor unit fibers are responsible for increasing acidity , ca accumulation , and subsequently sarcomere contracture .
furthermore , starting with the sarcomere ( super- ) contraction , the intramuscular perfusion slows down and ischemia and hypoxia will occur .
this may lead to the release of several sensitizing substances causing peripheral sensitization [ 15 , 22 ] .
henneman , working with anesthetized cats , showed that in response to increasing physiological excitation , motor neurons are recruited in order of increasing size .
this fundamentally important finding was verified in humans by milner - brown et al   and is now generally accepted .
smaller motor units have a smaller alpha - motor neuron cell body , smaller axons and fewer muscle fibers to activate compared with larger ones . according to the size principle , small motor units innervating type i red colored , slow
oxidative fibers are recruited first , followed by red to pink colored , fast oxidative fibers , and finally by white colored , fast glycolytic fibers .
furthermore , small type i muscle fibers are activated during prolonged tasks , although a few studies have described some degree of motor unit substitution .
hgg suggested that the continuous activity of these motor units in sustained contractions causes overuse muscle fiber damage , especially to the type i fibers during low - level activities , which he summarized in his cinderella hypothesis .
it is conceivable that in sustained low - level contractions and in dynamic repetitive contractions , ischemia , hypoxia and insufficient atp synthesis in type i motor unit fibers are responsible for increasing acidity , ca accumulation , and subsequently sarcomere contracture .
furthermore , starting with the sarcomere ( super- ) contraction , the intramuscular perfusion slows down and ischemia and hypoxia will occur .
this may lead to the release of several sensitizing substances causing peripheral sensitization [ 15 , 22 ] .
during ( sub- ) maximal concentric contractions high amounts of energy ( atp ) are required .
after about 46 seconds , the muscle shifts to direct phosphorylation of adp by creatine phosphate ( cp ) .
phosphorylation produces atp by coupling 1 phosphate group to an adp molecule catalyzed by the enzyme creatine kinase .
stored atp and cp provide enough energy for maximum muscle power for approximately 1416 seconds .
hereafter , a short period of rest is needed to replenish the exhausted reserves of intracellular atp and cp .
when ongoing atp demands are within the capacity of the aerobic pathway , muscular activity can continue for hours in well - conditioned individuals .
however , when the demands of exercise begin to exceed the ability of the muscle cells to carry out the necessary reactions quickly enough , anaerobe glycolysis will contribute more and more of the total generated atp .
finally , the muscle will run out of atp and sustained sarcomere contractions may occur , starting the development of trps .
classic examples of this are walking , whereby the knee extensors are active while being lengthened just after heel strike while the knee flexes ; placing an object gently downwards , whereby the arm flexors are active while being lengthened to control the rate of movement of the object . in other words ,
eccentric contractions are commonly used to control the rate of movement . in another scenario ,
the load on the muscle may increase to the point where the external force on the muscle is greater than the force that the muscle can generate . in that case
there is no solid evidence that eccentric loading would lead to the development of trps , but as gerwin et al summarized , there is much overlap between the mechanisms of eccentric contraction and the development of trps . in one study ,
itoh et al found that healthy volunteers presented with tender ropy taut bands , which were painful on compression , immediately after 3 sets of eccentric exercises of the middle finger extensor muscle .
after 7  days the muscles were recovered . while this study suggests that eccentric loading does contribute to the development of trps , the study also employed isometric loading in addition to eccentric loading , which precludes definitive conclusions .
there is evidence from biopsy studies that during eccentric contractions disruption of the cytoskeletal structures occurs , especially of the protein desmin that interconnects the adjacent myofibrils [ 26 , 27 ] and of titin , the largest protein in the human body .
titin connects myosin filaments to the z - bands and is also linked to actin filaments [ 28 , 29 ] .
microscopic research revealed increased sections of abnormal fibers in eccentrically exercised muscles [ 30 , 31 ] , which appeared to be approximately 4 times the normal size .
this was only observed with eccentric exercise , and not with concentric exercise or passive stretching .
furthermore , all enlarged fibers were exclusively of the fast glycolytic type or fast contracting type ii fibers that are highly fatigable . it is hypothesized that early in the exercise period fast glycolytic fibers fatigue as they are unable to regenerate atp and as a result they enter into a rigor or high stiffness state .
subsequent stretching of the stiff fibers mechanically disrupts the fibers resulting in cytoskeletal and myofibrillar damage .
there is also evidence that in eccentric exercised muscles the intracellular concentration of calcium is increased , probably because of disruption of the sarcoplasmic reticulum . as we have seen before high concentrations of
furthermore , increased ca has the potential for activating several mechanisms that leads to further damage of the cell membrane and cytoskeletal disruption .
in contrast , hocking has postulated that eccentric loading does not provide a good model for the trp pathogenesis .
he suggested that sustained partial depolarization or plateau depolarization of an -motor neuron dendrites leads to lasting alterations in the function of the entire -motor neuron   due to an upregulation of l- or n- type voltage dependent calcium channels and 1-adrenergic receptors and a downregulation of calcium - activated potassium channels , which would lead to an increase in the motor terminal cytosolic calcium ion concentration ( personal communication , 2012 ) .
in other words , according to hocking , the increased ach release would be the cause and not the result of the energy crisis .
alpha-1-adrenergic receptors are linked to l - type voltage dependent calcium channels , which would suggest that sympathetic activity would increase the cytosolic calcium ion concentration and the excessive release of ach . rather than looking at overuse mechanisms
, hocking maintains that persistent nociceptive input causes the formation of trps through central sensitization of the c - fiber nociceptive withdrawal reflex and plateau depolarization of withdrawal agonist alpha - motor neurons and compensatory reticulospinal motor facilitation of antigravity muscles and plateau depolarization of withdrawal antagonist alpha - motor neurons .
eccentric contractions occur during all activities of daily life , but have been researched especially in sports .
overhead throwing , spiking in tennis and volleyball , javelin - throwing , downhill running , jumping and running ( landing phase ) involve eccentric activities . during the deceleration phase of throwing , the posterior shoulder muscles such as the infraspinatus , teres minor and major , posterior deltoid , and latissimus dorsi muscles ,
contract eccentrically not only to decelerate horizontal adduction and internal rotation of the arm , but also to resist shoulder distraction and anterior subluxation forces .
a shoulder compressive force slightly greater than bodyweight is generated to resist shoulder distraction , while a posterior shear force of 40  % 50  % bodyweight is generated to resist shoulder anterior subluxation .
consequently , high activity is generated by the posterior shoulder musculature , in particular the rotator cuff muscles .
the percentage of maximum voluntary isometric contraction ( mvic ) is calculated in studies using needle electromyography .
for example , the teres minor exhibits its maximum activity ( 84  % of mvic ) , the infraspinatus 37  % , supraspinatus 51  % , posterior deltoid 69  % , latissimus 88  % , subscapularis 115  % , and triceps 89  % during the deceleration phase .
scapular muscles also exhibit high activity to control scapular elevation , protraction , and rotation during this phase . the lower trapezius muscle , which exerts force on the scapula in the direction of depression , retraction and upward rotation , generated its highest activity during the eccentric phase [ 35 ] .
high emg activity from glenohumeral and scapular musculature during eccentric contraction illustrates the vulnerability of the posterior musculature for developing trps in the overhead - throwing athlete .
posterior shoulder pain is a common complaint of throwing athletes and is mostly explained by posterior impingement [ 36 , 37 ] .
interestingly , stretching exercises have been shown to assist in reducing the athlete s pain and to normalize shoulder motion , particularly shoulder internal rotation and horizontal adduction .
it is common for the overhead thrower to exhibit loss of internal rotation of 20   or more , referred to as  glenohumeral internal rotation deficit ( gird ) .
 this internal rotation deficit has been suggested to be a cause of specific shoulder injuries .
wilk expressed that loss of internal rotation is most often due to osseous adaptations of the humerus and posterior muscle tightness , and not posterior capsular tightness . stretching exercises , including the sleeper s stretch and supine horizontal adduction with internal rotation ,
are advised to improve the flexibility of the posterior musculature , which may become tight because of the muscle contraction during the deceleration phase of throwing .
trps can be found in many sportsmen , including elite volleyball players , swimmers , and runners [ 3941 ] .
in spite of a lack of well - designed studies , the best available evidence supports that trps develop after muscle overuse .
several potential mechanisms may play a role , such as eccentric overload , submaximal sustained , and ( sub)-maximal concentric contractions .
a key factor is the local ischemia , which leads to a lowered ph and a subsequent release of several inflammatory mediators in muscle tissue .
whether overuse mechanisms are the crucial initiating factor or persistent nociceptive input remains a point of debate and further study .
 myofascial pain syndrome ( mps ) is described as the sensory , motor , and autonomic symptoms caused by myofascial trigger points ( trps ) . knowing the potential causes of trps is important to prevent their development and recurrence , but also to inactivate and eliminate existing trps . 
 there is general agreement that muscle overuse or direct trauma to the muscle can lead to the development of trps . 
 muscle overload is hypothesized to be the result of sustained or repetitive low - level muscle contractions , eccentric muscle contractions , and maximal or submaximal concentric muscle contractions . 
 trps may develop during occupational , recreational , or sports activities when muscle use exceeds muscle capacity and normal recovery is disturbed . 