myofascial pain syndrome ( mps ) can be caused by frequent muscle or fascial stiffness as a 
 result of prolonged tension and muscle fatigue due to repetitive stress of muscles or 
 overuse of particular muscles1 , 2 . mps activates trigger points in 54% of women and 45% of men , and is regarded as 
 the most common cause of pain in the musculoskeletal system3 . in particular , the upper trapezius requires head movement according 
 to the direction of view , but it can be vulnerable to damage when the hands and arms are 
 used repetitively in work that requires precise control4,5,6 . once myofascial trigger points are activated , changes in the 
 structural characteristics and contraction function of the muscle will occur . the most 
 distinctive changes are taut bands , tender nodules , referred pain , local twitch response , 
 muscle weakness , and restricted range of motion3 . to evaluate the contraction function of skeletal muscle , various methods can be used . among 
 them , surface electromyography ( emg ) analyzes functional changes in muscle by measuring 
 quantitative changes in motor unit action potential that is activated by muscle 
 contraction7 , 8 . surface emg is widely used in kinetic analysis to diagnose normal 
 or abnormal function in muscles and nerves according to amplitude and frequency9 . it has several advantages , as it is 
 noninvasive and convenient , and can perform measurement even during dynamic motion10 , 11 . although a number of clinical reports on the pathologic mechanisms or clinical diagnosis 
 and treatment of mps can be found , few studies have been conducted on the changes in 
 contraction function in myofascial trigger point areas or on the electrophysiological 
 characteristics and neuromuscular physiological information . this study aimed to compare the 
 electrophysiological characteristics of normal muscles versus muscles with latent or active 
 myofascial trigger points , and to identify their neuromuscular physiological 
 characteristics , in order to provide a quantitative evaluation of mps and clinical 
 foundational data for its diagnosis . this study was approved by the 
 research agency , and all participants provided written informed consent . subjects were 
 selected among those with no known neurologic disease , no regular exercise habit , and no 
 drug use that could affect the experimental result . subjects in the active myofascial trigger point group were those diagnosed with mps 
 by a physician and whose clinical characteristics were pain in the upper trapezius during 
 rest , taut bands , tender nodules , referred pain , local twitch response , and muscle weakness . subjects in the latent myofascial trigger point group were those not diagnosed with mps but 
 who felt tenderness when the upper trapezius was stimulated . mean age , height , and weight , respectively , were 23.27 
  4.21  years , 172.32  8.44  cm , and 64.14  6.54  kg in the active trigger point group ; 23.74 
  6.47  years , 174.76  6.22  cm , and 66.74  5.74  kg in the latent trigger point group ; and 
 24.17  3.14  years , 171.21  9.64  cm , and 63.44  7.51  kg in the control group . to measure maximum voluntary isometric contraction ( mvic ) , the shoulders and heads of the 
 subjects were firmly fixed and pulled as the dynamometer as maximum power , thereby measuring 
 mvic against the upper trapezius . endurance time was measured from the start of mvic to a 
 50% reduction in contraction force . median frequency ( mdf ) was measured by using emg at the 
 muscle belly of the upper trapezius during mvic . muscle fatigue index was calculated by 
 obtaining the mdf at a section where the mvic force was reduced from 100% to 50% , and remove 
 the 50% mdf from 100% mdf was divided into 100% mdf . the sampling rate for the surface emg 
 signal was set at 1,000  hz , and the frequency band filter was set at 20 to 450  hz , using one 
 channel . storage and analysis of the emg signals was performed using acqknowledge 3.8.1 
 ( biopac systems , goleta , ca , usa ) . the average of three measurements was calculated and used 
 in the analyses ; 10 minutes of rest was given between measurements to prevent muscle 
 fatigue . statistical analyses were performed using spss version 18.0 for windows ( spss , inc . one - way analysis of variance was conducted to examine differences among 
 groups with respect to the measured items , while the tukey test was conducted for post hoc 
 analysis . there were no significant differences in mvic or endurance time among the three groups . 
 however , mdf was significantly different among all three groups ( p < 0.05 ) ( table 1table 1.comparison of mvic , et , mdf , and fi among the three groupsgroup a ( n = 30)group b ( n = 30)group c ( n = 30)post hocmvic ,  kg19.811.5 * 22.19.423.510.2/et , s48.59.749.510.251.28.8/mdf ,  hz102.521.488.523.184.517.4a < cmfi0.150.070.080.040.060.04a < cdata are presented as mean  sd . * significant difference among the three groups ( p 
 < 0.05 ) . group a : active myofascial trigger points ; group b : latent myofascial 
 trigger points ; group c : control . et : endurance time ; mdf : median frequency ; mfi : 
 muscle fatigue index ; mvic : maximal voluntary isometric contraction ) . tukey post hoc results showed that the active myofascial 
 trigger point group was significantly different from the control group . muscle fatigue index 
 also was significantly different among all three groups ( p < 0.05 ) ( table 1 ) . tukey post hoc results showed 
 that the active myofascial trigger point group was significantly different from the control 
 group . data are presented as mean  sd . * significant difference among the three groups ( p 
 < 0.05 ) . group a : active myofascial trigger points ; group b : latent myofascial 
 trigger points ; group c : control . et : endurance time ; mdf : median frequency ; mfi : 
 muscle fatigue index ; mvic : maximal voluntary isometric contraction this study aimed to measure mvic , endurance , mdf , and muscle fatigue index of normal 
 muscles versus muscles with latent or active myofascial trigger points , thereby identifying 
 the neuromuscular physiological characteristics of muscles with active myofascial trigger 
 points and providing clinical foundational data that are applicable for the quantitative 
 evaluation and diagnosis of mps . surface emg , which was used to evaluate the functionality of trigger points in this study , 
 is nonintrusive and convenient ; thus , it is widely used in studies on the functional 
 characteristics of muscle , by analyzing the electrical activity of muscle . the results of 
 mdf according to surface emg show changes in recruitment of fast - twitch muscle fibers and 
 conduction velocity of motor unit action potential12 . in addition , increases in mdf reflect recruitment of type ii 
 fibers , and muscle fatigue index can be analyzed according to the relationship between type 
 ii fibers and mdf13 , 14 . in the present experiment , mdf was significantly higher in the 
 active trigger point muscles than in normal muscles . the above results indicate that as 
 trigger points were activated , recruitment of motor unit action potential of type ii fibers 
 also increased . in addition , muscle fatigue index also increased due to the increased 
 distribution of type ii fibers . however , mvic was lower in active trigger point muscles than 
 in normal muscles . although a high correlation between muscle strength and mdf has been 
 reported15 , 16 , the same correlation was not found in this study . this result was 
 due to increased recruitment of motor unit action potential of type ii fibers in active 
 trigger point muscles , which more easily induced muscle fatigue as a physiological 
 phenomenon , thereby reducing muscle strength . in other words , the greater the fatigue , the 
 weaker the muscle strength17 ; endurance 
 also was decreased due to muscle weakening . therefore , this study s results will be effective in complementing the physical therapy 
 diagnosis of mps  which , until now , has focused on physical examination only  by understanding 
 not only the usefulness of electrophysiological analysis in mps diagnosis , but also the 
 neuromuscular physiological characteristics of active trigger point muscles .
[ purpose ] this study compared the differences in electrophysiological characteristics of 
 normal muscles versus muscles with latent or active myofascial trigger points , and 
 identified the neuromuscular physiological characteristics of muscles with active 
 myofascial trigger points , thereby providing a quantitative evaluation of myofascial pain 
 syndrome and clinical foundational data for its diagnosis . [ subjects ] ninety adults in 
 their 20s participated in this study . 
 subjects were equally divided into three groups : the 
 active myofascial trigger point group , the latent myofascial trigger point group , and the 
 control group . [ methods ] 
 maximum voluntary isometric contraction ( mvic ) , endurance , median 
 frequency ( mdf ) , and muscle fatigue index were measured in all subjects . 
 [ results ] no 
 significant differences in mvic or endurance were revealed among the three groups . 
 however , the active trigger point group had significantly different mdf and muscle fatigue 
 index compared with the control group . [ conclusion ] given that muscles with active 
 myofascial trigger points had an increased mdf and suffered muscle fatigue more easily , 
 increased recruitment of motor unit action potential of type ii fibers was evident . 
 therefore , electrophysiological analysis of these myofascial trigger points can be applied 
 to evaluate the effect of physical therapy and provide a quantitative diagnosis of 
 myofascial pain syndrome .