TITLE: Electrophysiological characteristics according to activity level of
myofascial trigger points

  ABSTRACT:
[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.

BODY.INTRODUCTION:
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). Most individuals experience MPS at least once in their
lifetime. 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.

BODY.SUBJECTS AND METHODS:
Ninety adults in their 20s participated in this study. 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 were equally divided into three
groups. 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. Subjects in the control group
had no voluntary pain or tenderness. 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.
Chicago, IL, USA). 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. The statistical significance level of α was set at 0.05.

BODY.RESULTS:
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, kg19.8±11.5*22.1±9.423.5±10.2/ET, s48.5±9.749.5±10.251.2±8.8/MDF, Hz102.5±21.488.5±23.184.5±17.4†a<cMFI0.15±0.070.08±0.040.06±0.04†a<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.

BODY.DISCUSSION:
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.

Question: Compared to Control group what was the result of Active myofascial trigger point group, or Latent myofascial trigger point group on Maximum voluntary isometric contraction or endurance?
Evidence: No
significant differences in MVIC or endurance were revealed among the three groups.
Answer: no significant difference