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 .