Studies on individual muscle exercise have attempted to evaluate muscle exercises on the basis of the new concept of myofascial continuity. Myofascial continuity can be defined as a connection between two structures that are aligned in the same direction within a myofascial webbing (Myers, 2009). Wilke and Banzer (2017) found that the range of motion of the neck increases in the sagittal plane following the stretching of the muscle triceps surae (calf's ms.) and hamstring. Weisman et al. (2014) revealed muscle contraction correlations by using electromyography (EMG) of muscles along the back side of the body [superficial back line, Myers (2001)]. In addition, Huijing (2007) reported cases of epimuscular myofascial force transmission. This fascial webbing is associated with the widespread motor system of fibrous connective tissues, and transfers an impact to distal structures (Wike, Krause, Vogt and Banzer, 2016).

The concept of myofascial meridians proposed by Myers (2001), who studies the Rolfing technique, divides the body into continuous myofascial meridians. He illustrated the fascial webbing in myofascial meridians with an anatomical image for easier understanding (Figure 1).

In a recent study that used myofascial meridians, myofascial meridian stretching led to more-significant improvements in the performance of Taekwondo side kick than in individual muscle stretching (Jeong, 2017). Moreover, myofascial meridian relaxation techniques improved the gait and balance ability of stroke patients (Han, 2015), and myofascial meridian massage led to increased range of motion of the neck (Cheon, 2012). However, studies on myofascial meridians have used myofascial meridian stretching or massage only, and research on cor- relations relating to myofascial continuity is lacking.

In this study, muscle contraction correlations between the muscles along 8 myofascial meridians that vertically connect the body were analyzed by contracting the terminal muscles of the myofascial meridians in the supine and upright positions. By doing so, we aimed to assess the possibility for functional compensation among muscles, and provide research data for clinical applications in the field of kinesiology and rehabilitative medicine.

Figure 1. The fascia meridian that connects the body vertically. From the left superficial back line (SBL), superficial front line (SFL), lateral line (LL), spiral line [back] (SL[B]), spiral line [front] (SL[F]), functional line [back] (FL[Back]), functional line [front] (FL[Front]), deep front line (DFL) (Myers, 2009).

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1. Research participants and period

Seven male students who were studying physical education at Konkuk University participated in this study between September 15, 2017, and September 22, 2017. Students who had a muscle disorder or pain in the neck and ankle, received treatment for musculoskeletal disease in the last 6 months, undergone surgery of the neck or ankles, a previous or current history of neurological problems, or a cardiopulmonary disease were excluded. All the participants received an explanation about the purpose and methods of this study, and provided informed consent. The general characteristics of the participants are summarized in (Table 1).

2. Electrode placement

Muscles along the myofascial meridians to be assessed were selected on the basis of the anatomy of myofascial meridians (Myers, 2009). Electrodes were attached to the muscles along the spiral line (SL) that connects the body in a diagonal line and the functional line (FL) on both the right and left sides of the body. Surface electrode pads were attached at the most developed parts of the muscles by referring to the electromyogram analysis (Table 2, Figure 2) (Kim et al., 2013).

Figure 2. Measurements of muscle activity in prone and standing postures (left: FL[B], right: SBL)

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3. Measurement procedure

All the participants received an explanation of the purpose and precautions for the experiment, and provided written consent. They were then instructed on how to perform exercise movements precisely.

1) MVC

For the normalization of EMG data, maximum voluntary isometric contraction was performed for all muscles along each myofascial meridian prior to the measurement (Kendall, 2001).

2) Measurement of muscle activity along the myofascial meridian

To assess the muscle contraction correlations of each muscle, active voluntary contraction was performed for 1 second three times after matching muscle functioning between the two sides. For the superficial front line (SFL), lateral line (LL), superficial front line (SL[F]), functional front line (FL[F]), and deep front line (DFL), contraction was performed in the supine and upright positions. For the superficial back line (SBL), superficial back line (SL[B]), and functional back line (FL[B]), contraction was performed in the prone and upright positions. During the measurement, the legs were spread apart at shoulder width, and the lower limbs, trunk, and neck were linearly aligned. During the measurement in the prone position, the feet were kept outside the bed. During the measurement in the supine position, the feet were spread apart at shoulder width, and measurements were obtained while the participant looked straight ahead. For the muscles of the lower legs, ankle exercise was performed. For the neck muscles, neck exercise was performed to measure muscle contraction. The latissimus dorsi and pectoralis major, which are muscles of the upper body, were measured while the examiner put resistance at the angle of maximum contraction. Contraction of the muscles of the hip joints, namely the adductor longus and adductor magnus, were measured while the examiner applied resistance in the hip joints in an abducted position. Contraction of the muscles of the knee joints, namely the vastus lateralis, was measured while the examiner applied resistance at the angle of maximum contraction. The participant was asked to make a half squat in the supine position. (Table 3) shows the measurement methods used for each myofascial meridian.

4. Electromyography

Telemyo 2400T G2 (USA) was used to obtain EMG images of muscles along myofascial meridians. Surface electrodes (Single Electrodes, Noraxon, USA) were used. The sampling rate was set at 1,000 Hz; and the bandwidth, at 20~350 Hz. The root mean square method was used for EMG signals from each muscle.

For the normalization of EMG data, measurement data were expressed in terms of maximum voluntary isometric contraction (MVCmax) and were compared.

%MVC = Measurement data/MVCmax

5. Statistical analysis

SPSS 22.0 was used to analyze muscle contraction correlations among the muscles along the myofascial meridians using EMG data. The level of statistical significance was set at p < .05. Descriptive statistics were used for the general characteristics of the participants. EMG data of terminal muscle contraction in the supine and upright positions were combined, and Pearson correlation coefficients were calculated to analyze muscle contraction correlations.

The muscle contraction correlation results for each myofascial meridian are as follows:

1. Superficial back line

The gastrocnemius, lumbar erector, thoracic erector, and semispinalis were significantly correlated. However, the biceps femoris was not significantly correlated with any of the muscles along the superficial back line (Table 4).

2. Superficial front line

The tibialis anterior, pectoralis major, and SCM were significantly correlated with all muscles along the superficial front line. The rectus femoris was significantly correlated with the tibialis anterior, pectoralis major, and SCM. The rectus abdomen lower and upper were significantly correlated with all muscles except the rectus femoris (Table 5).

3. Lateral line

The peroneus longus was significantly correlated with all the muscles except the external abdominal oblique. The TFL was significantly correlated with all the muscles except the external abdominal oblique. The muscle gluteus medius was significantly correlated with all the muscles except the external abdominal oblique. The external abdominal oblique was not significantly correlated with any muscles along the lateral line. The SCM was significantly correlated with all the muscles except the external abdominal oblique (Table 6).

4. Spiral line [back]

The peroneus longus, lumbar erector, thoracic erector, and semispinalis were significantly correlated with all the muscles along the spiral back line except the biceps femoris. The biceps femoris was not significantly correlated with any muscles along the spiral back line (Table 7).

5. Spiral line [front]

The tibialis anterior significantly correlated with the TFL, rhomboids, and splenius capitis. The TFL significantly correlated with the tibialis anterior only. The internal abdominal oblique did not significantly correlate with any muscles. The external abdominal oblique significantly correlated with the serratus anterior only. The serratus anterior significantly correlated with the external abdominal oblique, and the rhomboids significantly correlated with the tibialis anterior and splenius capitis. The splenius capitis significantly correlated with the tibialis anterior and rhomboids (Table 8).

6. Functional line [back]

The latissimus dorsi significantly correlated with the thoracic erector and vastus lateralis. The thoracic erector significantly correlated with the vastus lateralis. The lumbar erector and gluteus maximus significantly correlated with each another. The vastus lateralis significantly correlated with the latissimus dorsi and thoracic erector (Table 9).

7. Functional line [front]

The pectoralis major significantly correlated with all the muscles along the functional front line. The rectus abdomen upper significantly correlated with the pectoralis major and rectus abdomen lower. The rectus abdomen lower significantly correlated with all the muscles except the adductor longus. The adductor longus significantly correlated with the pectoralis major only (Table 10).

8. Deep front line

The tibialis posterior significantly correlated with the adductor magnus and scalenes. The adductor magnus significantly correlated with the tibialis posterior and iliopsoas. The iliopsoas significantly correlated with the adductor magnus and scalenes. The scalenes significantly correlated with the tibialis posterior and iliopsoas (Table 11).

Myofascial continuity refers to the connections between ligaments, tendons, and muscles, or the connections between myofascial structures, and is based on the study of anatomy. Myers (2009) explained that tension and movements are transferred through myofascial continuity and referred to direct myofascial connections as "fascial connections" and the transfer of tension from tendons or a force while the body is in a specific position or performing a certain function as "mechanical connection".

In this study, positive correlations were observed for the myofascial connections on the superficial back line, superficial front line, and spiral back line, and negative correlations were observed for the mechanical connections on the same myofascial meridians. On the superficial and spiral back lines, the biceps femoris and lumbar erector connect to one another via the sacrotuberosity Ligaments. On the superficial front line, the rectus femoris mechanically connects to the rectus abdomen without any anatomical connections upon the extension of the trunk. On the lateral line, the gluteus medius, external oblique, and SCM functionally connect to one another upon the lateroflexion of the trunk. Positive and negative correlations are observed among the muscles along these myofascial meridians before and after these connection points, respectively.

Myofascial continuity was observed before the spiral front line and functional line during the trunk rotation exercise. The two myofascial meridians that seemed functionally connected to one another were no significantly correlated, as all exercises were performed without tilting the trunk.

Although no correlation was found between all the muscles along the myofascial meridians, some muscles showed strong positive correlations. They were the lumbar erector-thoracic erector-semispinalis on the superficial back line, tibialis anterior-rectus femoris on the superficial front line, and peroneus longus-TFL on the lateral line. This result suggests strong myofascial connections between the corresponding muscles that cause them to contract together. On the other hand, some muscles strongly negatively correlated, and they were the tibialis anterior-SCM on the superficial front line, the peroneus longus-SCM on the lateral line, and the tibialis anterior-splenius capitis on the spiral front line. These muscles appear to contract when muscles on one end of a myofascial meridian contract to increase contraction efficiency.

In this study, the muscles along the myofascial meridians were found to be partially connected in the supine and upright positions. Our results suggest the possibility for changes in muscle contraction depending on the posture and trunk position.

In this study involving 7 male university students, muscle contraction correlations among terminal muscles along 8 myofascial meridians that vertically connect the body were analyzed in the supine and upright positions. As a result, positive correlations were observed for the myofascial connections, and negative correlations were observed for the mechanical connections between the muscles along the myofascial meridians. Based on these results, muscles that show significant contract correlations with one another may be expected to be used as an effective clinical marker in muscle strengthening or relaxation therapy, and rehabilitative training. In this study, the intrinsic functional position of the myofascial meridians was not considered; therefore, results regarding the overall correlation of the myofascial meridians may not be precise. These limitations must be accounted for in future studies.