Effect of sling exercise therapy on surface electromyography and muscle thickness of superficial cervical muscle groups in female patients with chronic neck pain

Abstract

BACKGROUND:

The persistence of symptoms in patients with chronic neck pain is considered to be associated with variation in the neck muscle structure and associated neuromuscular control. Sling exercise therapy (SET) has been demonstrated to relieve the symptoms of chronic neck pain, whereas it is controversial whether this benefit is correlated to altered neck muscle structure and associated neuromuscular control in the patients.

OBJECTIVE:

To investigate the effect of SET on cervical muscle structure (thickness) and associated neuromuscular control in patients with chronic neck pain.

METHODS:

Twenty-five patients with chronic neck pain were randomly assigned to the SET group ( $n=$ 12) or the control group ( $n=$ 13). The SET group received the SET intervention for 4 weeks, while the control group maintained normal activities of daily living. At baseline and after 4 weeks of intervention, Visual analogue scale and neck disability index were measured in both groups, and changes in the thickness of the superficial cervical muscles were assessed using musculoskeletal ultrasound. Surface electromyography (EMG) was adapted to assess the neuromuscular control of the neck while the participant was performing the cranio-cervical flexion test.

RESULTS:

At 4 weeks, the SET group had a significant reduction of RMS in both UT and SCM of EMG compared to the control group ( $p<$ 0.05). Regarding ultrasound, the SET group had significantly lower muscle thickness compared to the control group in both the rest position and the MVIC position ( $p<$ 0.05). There were no within-group differences in the control group ( $p>$ 0.05), while the SET group showed significant reductions in both RMS and muscle thickness ( $p<$ 0.05).

CONCLUSION:

4-week SET was effective in reducing pain and dysfunction in patients with chronic neck pain, which may be related to improved neck muscle thickness and neuromuscular control of the neck.

Keywords

Exercise prescription musculoskeletal disorders pain ultrasonography

1. Introduction

Neck pain is one of the most common musculoskeletal disorders around the world, and neck pain causes patients to have limited daily function [1]. Whilst the majority of patients with neck pain have a satisfactory prognosis for recovery, one third of them still develop chronic neck pain [2]. Chronic neck pain imposes a heavy medical and financial burden on individuals and society, with symptoms of some patients persisting for life [3].

The causative factors of chronic neck pain are complex and most neck pain is unexplained and idiopathic. While current research demonstrated that the persistence of symptoms in patients with chronic neck pain may be related to alterations in the structure of the cervical muscles and the neuromuscular control of the neck [4]. Results from ultrasound studies have shown that patients with chronic neck pain have increased thickness (enhanced stiffness) of the superficial cervical muscles, accompanied by decreased muscle strength of the deep cervical muscles, and that these changes result in abnormal coordination between the deep and superficial cervical muscles [5, 6]. Accurate treatment is based on precise assessment; structures such as muscle thickness can be explored by ultrasound, but direct assessment of the neuromuscular control of the neck is more challenging. The cranio-cervical flexion test (CCFT) provided an option to assess the neuromuscular control of the neck by combining surface electromyography to measure the activity of the superficial cervical muscles to evaluate the interaction between the deep and superficial muscle groups [7]. The available evidence supports that when the CCFT is performed, patients with chronic neck pain have reduced activation of the deep cervical muscles than healthy individuals, accompanied by hyperexcitability of the superficial cervical muscles [8, 9], a finding that is consistent with the observations carried out by Taş et al. through ultrasound [6]. Evidence from ultrasound and CCFT actually illustrated an association between altered cervical muscle structure and altered neuromuscular control in patients with chronic neck pain.

In rehabilitation clinical practice, long-term benefits of interventions for chronic neck pain are extremely difficult to obtain, probably due to the limitations of the location of the deep cervical muscles and most treatments targeting only the superficial cervical muscles [10]. However, relaxation or strengthening of only the superficial cervical muscles only achieves short-term results and is limited to maintaining effective postural correction and symptom relief [11]. The treatment of chronic neck pain should not only focus on the superficial muscles, but should also incorporate neuromuscular control into the core of the treatment, as it is still possible to restore superficial muscle symptoms without changing the coordination and neuromuscular control of the superficial and deep neck muscles. SET is now widely applied in the treatment of chronic musculoskeletal pain and has been demonstrate effective to relieve pain, increase mobility and enhance neck muscle strength in patients with chronic neck pain [12, 13, 14]. Current research confirms that acute SET can alter the thickness of deep neck muscles and neuromuscular control of the neck, but the long-term adaptation of SET to neuromuscular control in patients with chronic neck pain still needs to be investigated [15]. Accordingly, this experiment will investigate the effects of a 4-week SET intervention on clinical symptoms and the muscle structure (thickness) of the neck muscles and neuromuscular control of the neck in patients with chronic neck pain.

2. Methods

2.1 Study design

This 4-week randomized controlled trial was double-blind, two-group, pre-and-post measurement designed. The intervention was performed by a senior physical therapist advanced in SET training, and the outcome variables were measured by other senior physical therapist and sonographers trained in accurate identification and reading. The intervention with the participants was included in the daily routine of the senior physical therapist, who was not involved in this study. The participants in both groups were unaware of each other, and the intervention was carried out in a separate physiotherapy room in the hospital. Participants were recruited and randomly assigned by a research assistant who had no knowledge of this study using simple computer-generated randomization numbers, and the grouping information of the participants was placed in an opaque sealed bottle to ensure that they had no knowledge of the grouping information of others.

2.2 Participants

Participants recruitment and eligibility screen for this study was conducted between January 2021 and August 2021 by a senior physiotherapist. The inclusion and exclusion criteria were identified by the same physiotherapist based on previous research [15]. Considering the effects of COVID-19, a detailed check of the subject’s nucleic acid report is required when performing the test and treatment, while the environment is disinfected during the treatment, and both the physical therapist and the patient wear masks properly. The study design and experimental procedures of this study were in accordance with the Declaration of Helsinki, and the study plan was obtained the ethical approval from Shenyang Sport University ethics committee [No, 2020(14)], and was clinically registered (registration number: ChiCTR2200061600). All the participants signed informed consent form prior to participation in this study. Since there was no previous relevant research that could be referred for sample size estimation, the minimum sample size for each group was 12 based on a rule of thumb [16], and the number of participants per group in this study was set at 15 considering a potential attrition rate of 20%.

2.3 Inclusion and exclusion criteria

Inclusion criteria: 1) Female patients with chronic neck pain, age range from 40–65 years; 2) The presence of neck and shoulder discomfort, pain, limited movement and other symptoms for more than 6 months; 3) Cervical muscle tension, neck and shoulder pressure points were found by physical examination; 4) Clinicians perform x-ray tests on patients with chronic neck pain, including the following pathologies: changes in physiological curvature, or segmental instability, or intervertebral stenosis, osteophytes, etc. [17].

Exclusion criteria: 1) History of cervical spine surgery, fracture, trauma, infection, tumor, etc.; 2) Other types of cervical spine disease (neurogenic cervical spondylosis, spinal cord cervical spondylosis, vertebral artery cervical spondylosis, sympathetic cervical spondylosis and mixed cervical spondylosis); 3) Combined with severe cardiovascular, pulmonary, hepatic, cerebral, renal, hematological system, severe osteoporosis; 4) Pregnant and lactating women who are not suitable for high-intensity exercise training; 5) People with mental illness or cognitive dysfunction; 6) cervical spine treatment in the past 3 months [17].

2.4 Intervention

As shown in Fig. 1, SET was conducted in the sports rehabilitation laboratory, using the Redcord ${}^{\text{\textregistered}}$ Suspension System (Arendal, Norwegian). The physiotherapist first carried out a weak chain assessment and selected a suitable training program based on the outcome of the assessment. The participant was guided by the physiotherapist through the training program prior to training, and the participant’s treatment tolerance was also assessed to ensure that the entire assessment process was painless. The duration of the SET was 4 weeks, within 3 times per week [13]. The general duration of training program was around 20–30 min, but would be adjusted according to the participant’s training tolerance.

Figure 1.

Visual display of the SET.

2.5 Intervention group

Cervical flexor training: The participant was placed in a supine position, with the occipital bone supported by a solid central parting band, the chest and pelvis supported by a wide elastic band, the upper arms crossed in front of the chest, and a soft pad placed under the knee fossa to avoid compensations. The participant was instructed by the physiotherapist to first perform head and neck flexion, then lift the chest, and finally lift the pelvis so that the trunk is in a neutral posture. In this posture, the participant maintained the neck in a neutral position for 2 min, during which the physiotherapist administered a certain frequency of vibration stimulation on the suspension rope. The duration that the participant held the position was recorded, and as the participant’s control ability enhanced, the physiotherapist increased the difficulty of training through shaking the elastic band suspension rope or through extending the time.

Cervical extensor training: The participant was placed in a prone position, supported by a solid center band for forehead, adjusted the height of the center band to relax the cervical spine, maintained normal cervical lordosis within the range of the center band, and placed a soft cushion under the ankle. In this posture, the participant’s neck maintained in a neutral position for 2 minutes, during which the physiotherapist gave a certain frequency of vibration stimulation on the suspension rope. Record the time that the participant maintained this posture. As the participant’s control ability enhanced, the physiotherapist suspended the rope with an elastic band or prolonged the time to increase the difficulty.

2.6 Control group

The control group did not participate in any exercise intervention and maintained a normal daily life.

2.7 Testing procedures

Visual Analogue Scale (VAS): The validity and reliability of VAS has been demonstrated in the Chinese population [18] and was adapted to measure the pain intensity among participants. A 10-cm VAS ruler has an intensity length of 0 to 10 cm to measure the intensity of pain in the neck area, with extremes defined as “no pain” (0) and “the worst pain imaginable” (10). Participants select a point at VAS that corresponds to the intensity of their pain according to the degree of their competent sensation, and the VAS measurement was repeated three times and the average of the three measurements was taken.

Neck Disability Index (NDI):The Chinese version of the NDI, whose validity and reliability have been demonstrated in the Chinese population, was adapted to assess neck pain and dysfunction. Before filling out the scale, the tester explained to the participant the measurement precautions. During the measurement, the tester explained each option to the participant and took notes accordingly. NDI contains ten self-reported items, including pain (two items), attention (one item), and activities of daily living (seven items). Each item is scored 0-5 according to the degree of cervical spine dysfunction from mild to severe, and the score is proportional to the degree of dysfunction, with 0 indicating no impairment and 5 indicating serious impairment affecting daily life. The interpretation of NDI scores: 0 to 4, no disability; 5 to 14, mild disability; 15 to 24, moderate disability; 25 to 34, severe disability; and greater than 35, complete disability [19].

Musculoskeletal ultrasound testing: Cervical muscle thickness was performed by an experienced sonographer using the GE Logiq e R7 ultrasound with 8–10 linear array transducers (GE Healthcare, Chicago, IL, USA). Ultrasound is a valid and reliable device for measuring the thickness of neck muscle groups (ICC $=$ 0.98–0.99) [20, 21]. The physician primarily tests the patient complaining of pain on one side, or the most painful if both sides are present. The patient was seated in a comfortable position with the head and neck in a neutral position, hands on the legs and feet on the ground, and the sonographer determined the site to be tested, marked it and took pictures to record, and then applies medical couplant to the test site. The thickness of the sternocleidomastoid and superior trapezius muscles was recorded when the participant was in the relaxed position and 10 s maximal voluntary isometric contraction (MVIC) [15]. The ultrasound images were frozen after the test and muscle thickness measurements were performed using the ultrasound drawing software. Muscle thickness was defined as the maximum distance between the upper and lower tendon membranes of the sternocleidomastoid and superior trapezius muscles, and the measurements were averaged three times to reduce measurement error.

CCFT: The CCFT was performed by a clinician with more than 15 years of experience. The participant was guided to lie supine with hips and knees flexed, the patient’s neck in a neutral position, and the head and face in a horizontal line, with a soft cushion placed under the patient’s knees to relax the low back muscles and thus avoid tension compensation in the neck. The physician primarily tests the patient complaining of pain on one side, or the most painful if both sides are present. Before the formal test, the participant studied the test procedure to ensure proficiency in the CCFT, while the participant was allowed to rest to avoid fatigue after mastery. The formal test was carried out by performing the standard maneuver of tucking the chin and raising the head about 10 cm above the bed for 10 s, followed by a relaxed rest for 1 min. Afterwards, the airbag of the pressure biofeedback device (Stabilizer, Chattanooga Group Inc., Austin, TX, USA) was placed under the neck, near the occipital bone, ensuring that the head and face were level at this point, using a spare towel if necessary to bring the head to a horizontal position. The airbag was then inflated to 20 mmHg, allowing the airbag to completely fill the subcervical space. After guiding the participant to perform a jaw-retracting nod, the patient was made to look at the instrument meter so that the needle reached each of the five target values (22 mmHg, 24 mmHg, 26 mmHg, 28 mmHg, and 30 mmHg), and each target value was maintained for 10 s and then relaxed for 30 s [4].

Table 1
Demographic and clinical characteristics of the two groups at baseline

	SET group ( $n=$ 12)		Control group ( $n=$ 13)		$t$	$P$ value
Age (yrs.)	57.86	$\pm$ 3.16	58.31	$\pm$ 4.44	$-$ 0.305	0.763
Course of disease (yrs.)	4.71	$\pm$ 1.29	4.77	$\pm$ 1.03	$-$ 0.131	0.897
Body Mass Index (kg/m ${}^{2}$ )	25.83	$\pm$ 7.61	24.79	$\pm$ 10.94	0.273	0.787

SET: Sling exercise therapy.

Figure 2.

Flow of participants.

2.8 Surface electromyography (EMG) testing.

CCFT was performed using surface EMG analysis and the test environment was controlled at a temperature of 24 ${}^{\circ}$ C and a humidity of 75% to avoid the influence of the environment on the EMG signal of the participant [22]. The test was carried out with a wireless surface EMG system (Noraxon Ultium, USA) operating at 4000 Hz. The testers were professionally trained and experienced in surface EMG testing. The testers first wiped the participant’s skin of the paste area repeatedly with 75% medical alcohol and used scrub to remove hairs if necessary and then selected the position of the electrodes before applying them. The electrodes were placed in the sternocleidomastoid muscle (SCM) from the mastoid process to the position 1/2 below the superior sternal notch; In the upper trapezius (UT) located in the belly of the muscle between the spinous C7 process and the acromion, two electrodes were attached to the belly of the muscle of the above two muscles with a standard distance of 2 cm, and the reference electrode (wireless sensor) was attached to one side. Participants were guided by testers to complete the max voluntary contraction (MVC) and CCFT which referred to Bonilla-Barba et al. [9]. After the test, the EMG data were imported into the MR3.16 software for analysis, and the EMG signals were processed through the following process: cardiac removal, rectification and normalization (Standardized processing using MVC), and finally the root mean square amplitude (RMS) of the above two muscles was calculated for the time domain index during the CCFT.

Table 2
Differences in NDI and VAS between the two groups at baseline

	NDI at		NDI at 4		Mean difference	$t$	$P$ value	VAS at		VAS at		Mean difference	$t$	$P$ value
	baseline		weeks		with 95% CI			baseline		weeks		with 95% CI
SET	27.50	$\pm$ 10.68	19.17	$\pm$ 9.15	8.33 (5.53 to 11.14)	6.535	$<$ 0.001	5.67	$\pm$ 1.37	3.83	$\pm$ 1.27	1.833 (1.38 to 2.29)	8.848	$<$ 0.001
group
( $n=$ 12)
Control	29.00	$\pm$ 9.82	28.62	$\pm$ 10.02	0.38 ( $-$ 0.25 to 1.02)	1.328	0.209	5.62	$\pm$ 1.33	5.46	$\pm$ 1.33	0.15 ( $-$ 0.18 to 0.49)	1.00	0.337
group
( $n=$ 13)
$t$	$-$ 3.66		$-$ 2.453					0.095		$-$ 3.128
$P$ value	0.718		0.022					0.925		0.005

NDI: Neck disability index; VAS: Visual analogue scale; SET: Sling exercise therapy; CI: Confidence interval.

Table 3

Differences in NDI and VAS between the two groups at baseline

	SET group ( $n=$ 12)		Control group ( $n=$ 13)		$t$	$P$ value
SCM
22 mmHg	16.36	$\pm$ 3.56	17.71	$\pm$ 4.01	$-$ 0.883	0.387
24 mmHg	20.84	$\pm$ 3.22	22.49	$\pm$ 5.05	$-$ 0.964	0.345
26 mmHg	26.38	$\pm$ 4.04	30.73	$\pm$ 6.75	$-$ 1.930	0.066
28 mmHg	32.08	$\pm$ 4.20	34.72	$\pm$ 6.69	$-$ 1.166	0.255
30 mmHg	36.29	$\pm$ 5.47	39.84	$\pm$ 8.44	$-$ 1.235	0.229
UT
22 mmHg	38.02	$\pm$ 2.98	37.11	$\pm$ 2.77	0.793	0.436
24 mmHg	42.87	$\pm$ 3.27	40.89	$\pm$ 3.60	1.441	0.163
26 mmHg	47.03	$\pm$ 2.74	44.62	$\pm$ 4.26	1.667	0.105
28 mmHg	49.73	$\pm$ 2.72	47.82	$\pm$ 3.97	1.393	0.177
30 mmHg	53.70	$\pm$ 3.21	52.06	$\pm$ 4.00	1.126	0.272

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Trapezius muscle.

2.9 Statistics

All data were analyzed using SPSS 23.0 (IBM Corporation, Armonk, NY, USA), and the normality of the data was tested using the Shapiro-Wilk test ( $p>$ 0.05). All data were normally distributed. Accordingly, paired $t$ -tests were used to detect within-group differences from baseline to 6 weeks. Independent samples $t$ -test were used to compare the differences between the groups at baseline and at 6 weeks, respectively. Descriptive statistics were expressed as mean $\pm$ standard deviation or median (IQR) with a significance level of $\alpha=$ 0.05 to verify statistical significance.

3. Results

3.1 Participants

For a flow chart of participants, see Fig. 2, 32 subjects were initially recruited for this study and 25 (12 in the intervention group and 13 in the control group) ultimately completed the 4-week intervention and measurement of all outcome variables. Table 1 shows the demographic data of the participants, and there were no significant differences between the two groups at baseline in terms of age, BMI, and course of disease.

3.2 Between-group difference

Tables 2 and 3 show the descriptive data of EMG and ultrasound at baseline for the two groups, respectively, and no significant differences were found ( $p>$ 0.05), which were comparable at baseline.

Table 4
Between-group differences in EMG between the two groups at 4 weeks.

	SET group ( $n=$ 12)		Control group ( $n=$ 13)		$t$	$P$ value
SCM
22 mmHg	8.41	$\pm$ 2.27	15.74	$\pm$ 3.11	$-$ 6.256	$<$ 0.001
26 mmHg	15.78	$\pm$ 3.33	29.50	$\pm$ 6.53	$-$ 6.529	$<$ 0.001
28 mmHg	19.25	$\pm$ 3.57	33.07	$\pm$ 7.70	$-$ 5.829	$<$ 0.001
30 mmHg	22.16	$\pm$ 5.52	37.56	$\pm$ 7.53	$-$ 5.789	$<$ 0.001
UT
22 mmHg	16.59	$\pm$ 4.03	36.21	$\pm$ 2.45	$-$ 14.815	$<$ 0.001
24 mmHg	23.43	$\pm$ 4.67	40.17	$\pm$ 1.73	$-$ 11.702	$<$ 0.001
26 mmHg	27.82	$\pm$ 3.41	43.28	$\pm$ 3.34	$-$ 11.445	$<$ 0.001
28 mmHg	30.88	$\pm$ 3.20	45.22	$\pm$ 4.89	$-$ 8.586	$<$ 0.001
30 mmHg	35.02	$\pm$ 3.50	50.28	$\pm$ 4.27	$-$ 9.727	$<$ 0.001

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Upper trapezius.

Table 5

Between-group differences in EMG between the two groups at 4 weeks.

	SET group ( $n=$ 12)		Control group ( $n=$ 13)		$t$	$P$ value
SCM
Rest position	0.84	$\pm$ 0.12	0.83	$\pm$ 0.15	0.249	0.805
MVIC	1.07	$\pm$ 0.12	1.08	$\pm$ 0.19	$-$ 0.236	0.815
UT
Rest position	1.32	$\pm$ 0.19	1.31	$\pm$ 0.24	0.077	0.939
MVIC	1.58	$\pm$ 0.09	1.60	$\pm$ 0.10	$-$ 0.519	0.608

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Upper trapezius; MVIC: Maximal voluntary isometric contraction.

Table 4 shows the EMG data of both groups at 4 weeks. for SCM, both 22 mmHg, 24 mmHg, 26 mmHg, 28 mmHg and 30 mmHg were significantly lower in the SET group compared to the control group ( $p<$ 0.001). As for UT, both 22 mmHg, 24 mmHg, 26 mmHg, 28 mmHg and 30 mmHg were significantly lower in the SET group compared to the control group ( $p<$ 0.001).

Table 5 shows the ultrasound data of both groups at 4 weeks. the SCM data showed a significant reduction in the SET group compared to the control group in both the resting position and the MVIC position ( $p=$ 0.035, $p=$ 0.002). In contrast, in UT, the SET group was significantly lower than the control group in both the resting and MVIC positions ( $p=$ 0.031, $p<$ 0.001).

Table 6

Between-group differences in ultrasound between the two groups at 4 weeks

	SET group ( $n=$ 12)		Control group ( $n=$ 13)		$t$	$P$ value
SCM
Rest position	0.72	$\pm$ 0.15	0.83	$\pm$ 0.10	$-$ 2.246	0.035
MVIC	0.91	$\pm$ 0.13	1.08	$\pm$ 0.12	$-$ 3.482	0.002
UT
Rest position	1.13	$\pm$ 0.17	1.30	$\pm$ 0.21	$-$ 2.292	0.031
MVIC	1.30	$\pm$ 0.11	1.59	$\pm$ 0.10	$-$ 6.650	$<$ 0.001

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Upper trapezius; MVIC: Maximal voluntary isometric contraction.

3.3 Within-group difference

Table 6 depicts the pre- and post-group comparisons of the EMG data for both groups. In the control group, there was no significant difference between the pre- and post-intervention ( $p>$ 0.005) for any of the indicators at either SCM or UT. The UT group showed significant reductions in 22 mmHg, 24 mmHg, 26 mmHg, 28 mmHg and 30 mmHg from 18.68 to 21.43 mmHg.

Table 7 depicts the ultrasound data for both groups, there was no significant difference between the control group in either SCM or UT. the SET group showed a significant reduction in resting level of 0.012 ( $p=$ 0.029) in SCM and a significant reduction in MVIC of 0.016 ( $p<$ 0.001). In the UT, the SET group showed a significant decrease of 0.19 ( $p<$ 0.001) in the resting position and 0.28 ( $p<$ 0.001) in the MVIC position.

Table 7
Within-group differences in EMG from baseline to 4-weeks intervention

	SET group ( $n=$ 12)			Control group ( $n=$ 13)
	Difference from	$t$	$P$ value	Difference from	$t$	$P$ value
	baseline to 4 weeks			baseline to 4 weeks
SCM
22 mmHg	7.95 (6.67, 9.23)	13.695	$<$ 0.001	1.97 ( $-$ 0.26, 4.20)	1.924	0.078
24 mmHg	7.46 (6.15, 8.78)	12.486	$<$ 0.001	0.43 ( $-$ 1.23, 2.08)	0.561	0.585
26 mmHg	10.60 (9.24, 11.97)	17.096	$<$ 0.001	1.23 ( $-$ 0.28, 2.74)	1.779	0.101
28 mmHg	12.84 (10.81, 14.86)	13.931	$<$ 0.001	1.65 ( $-$ 0.18, 3.48)	1.961	0.074
30 mmHg	14.13 (12.34, 15.92)	17.406	$<$ 0.001	2.27 ( $-$ 0.33, 4.88)	1.904	0.081
UT
22 mmHg	21.43 (19.71, 23.14)	27.526	$<$ 0.001	0.90 ( $-$ 0.48, 2.27)	1.42	0.181
24 mmHg	19.44 (17.93, 20.95)	28.388	$<$ 0.001	0.72 ( $-$ 0.95, 2.39)	0.936	0.368
26 mmHg	19.22 (17.99, 20.43)	34.597	$<$ 0.001	1.34 ( $-$ 1.19, 3.87)	1.155	0.271
28 mmHg	18.85 (17.75, 19.95)	37.790	$<$ 0.001	1.45 ( $-$ 0.54, 3.44)	1.586	0.139
30 mmHg	18.68 (17.37, 20.00)	31.249	$<$ 0.001	1.78 ( $-$ 0.23, 3.78)	1.933	0.077

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Upper trapezius; MVIC: Maximal voluntary isometric contraction.

Table 8

Within-group differences in ultrasound from baseline to 4-weeks intervention

	SET group ( $n=$ 12)			Control group ( $n=$ 13)
	Difference from	$t$	$P$ value	Difference from	$t$	$P$ value
	baseline to 4 weeks			baseline to 4 weeks
SCM
Rest position	0.12 (0.01, 0.23)	2.501	0.029	0.00 ( $-$ 0.07, 0.06)	$-$ 0.123	0.904
MVIC	0.16 (0.09, 0.23)	5.154	$<$ 0.001	0.00 ( $-$ 0.07, 0.07)	$-$ 0.024	0.981
UT
Rest position	0.19 (0.12, 0.26)	6.270	$<$ 0.001	0.01 ( $-$ 0.05, 0.06)	0.329	0.748
MVIC	0.28 (0.22, 0.33)	10.489	$<$ 0.001	0.01 ( $-$ 0.03, 0.04)	0.342	0.738

SET: Sling exercise therapy; SCM: Sternocleidomastoid muscle; UT: Upper trapezius; MVIC: Maximal voluntary isometric contraction.

4. Discussion

This study is the first to investigate the effects of SET on neck muscle structure and associated neuromuscular control in patients with chronic neck pain. Previous studies [12, 13, 14] have demonstrated the therapeutic effects of SET in chronic neck pain, including pain alleviation, increased joint mobility, and enhanced neck muscle strength in this population. Whilst potential clinical benefits of SET have been identified, the mechanism by which it mediates improvement in neck pain symptoms is ambiguous. Indeed, changes in neck muscle structure and neuromuscular control are responsible for the persistence of symptoms in patients with neck pain, whereas it is difficult to test the neuromuscular control of the neck directly. Accordingly, this study elected CCFT to detect the activation of superficial cervical muscles to indirectly reflect the neuromuscular control of the neck to assess the effect of SET. Meanwhile, in order to assess the structural changes of the neck muscles before and after the intervention, this study used musculoskeletal ultrasound to detect the thickness of the main cervical muscles. Due to the poor accuracy of the structural description of the deep cervical muscles, superficial cervical muscles were used as the focus of the study. This study actually assessed the possible improvement mechanism of SET by focusing on the structure of the superficial cervical muscles and indirect neuromuscular test.

This study focused on ultrasound images of the superficial neck flexion and extension muscles represented by the superior trapezius and sternocleidomastoid muscles, which are the most active muscles in chronic neck pain, and patients with chronic neck pain usually complain of pain in this area. Taş [6] et al. demonstrated by ultrasound that the superficial neck muscle groups are more rigid (thickness) in patients with chronic neck pain compared to asymptomatic individuals. CCFT also illustrated those higher levels of superficial flexor activity are a sign of reduced activation of deep cervical flexors and that tension in superficial cervical extensors leads to weakness in deep cervical extensors [9, 23]. Ultrasound and CCFT findings suggest altered neuromuscular control in patients with chronic neck pain, which may be closely related to muscle tension between the superficial and deep muscle groups. Janda et al. provided a possible explanation for these phenomena by suggesting that in the presence of pain, the superficial muscles of the neck and back were susceptible to protection, while the deep muscles were susceptible to weakening [24]. Muscle protection may be caused by pain, and when pain persists, a vicious cycle may lead to more muscle protection [25]. We hypothesize that the above phenomenon is related to the functional differences between the superficial and deep cervical muscles. Unlike the superficial cervical muscles that are involved in the regulation of cervical spine motion, the deep cervical muscles mainly involved in maintaining the stability of the cervical spine during prolonged posture and movement [26]. Abnormalities in the function of the deep cervical muscles resulting from prolonged incorrect posture or other causes can therefore lead to compensatory involvement of the superficial cervical muscles in the maintenance of cervical stability, which in the long term can result in tension (hyperactivation) of the superficial cervical muscles and weakness (inhibition) of the deep muscles, ultimately leading to altered neuromuscular control.

This study found that the improvement in pain and clinical symptoms in the SET group may be related to structure of the cervical muscles and the variation of neuromuscular control of the neck. This study found that the recruitment level of SCM and UT decreased with increasing pressure during the CCFT in the SET group after the intervention, whereas this change did not occur in the control group. An explanation for this change has also been provided by many studies [9, 27, 28], which demonstrated that during CCFT, compared to the pain-free group, the neck pain group recruited more SCM and UT with increasing pressure, whereas the pain-free group mobilized more of the deep neck flexors. The ultrasound study also found that the thickness of the superficial cervical muscles also decreased significantly with SET, and our post-intervention results suggest that a potential improvement in neck muscle structure and neuromuscular control occurred in chronic neck pain after the SET. In fact, patients with chronic neck pain who have both muscles seem to have a habit of maintaining a fixed posture for long periods of time, and patients with chronic neck pain experience fatigue of the superficial cervical muscles due to overcompensation, which manifests itself as a decrease in neck muscle endurance in patients with neck pain, and the specific physiological mechanism may be related to a shift in muscle type from slow-twitch type-1 fibres to fast-twitch type-IIB fibres in the above mentioned muscles [29, 30, 31]. SET may have activated the deep layers while avoiding superficial compensations due to fatigue, with eventual symptomatic improvement.

Therefore, in clinical practice, relaxation or strengthening of superficial muscles can only achieve short-term results and is limited to symptomatic relief [11]. Stability training for the deep cervical flexors, on the other hand, can reduce EMG activity in the SCM, anterior trapezius and cephalic grip muscles by increasing the endurance of the deep cervical flexors [32], which is also consistent with the results of our present study. Therefore, based on the above results we hypothesize that SET improving symptoms in patients with chronic neck pain may be based on the following hypothesis: Suspension decreases the compensatory and thus the tension and fatigue of the superficial cervical muscles by activating the deep cervical muscles, which ultimately improves the neuromuscular control of the neck and achieves symptomatic improvement.

4.1 Limitations and further recommendations

The first limitation of this study is that the participants were all female, which can reduce the validity of the results. Therefore, the finding obtained cannot be generalized to neck pain patients of all genders. The second limitation is that the age of the participants was concentrated in middle-aged and elderly people, and the obtained results cannot be generalized to neck pain patients of all ages. Third, the sample size was not pre-estimated, but it can be used as a reference for sample size estimation in subsequent large-scaled studies. Finally, this research only conducted the measurement of ultrasonography and surface EMG for one side, thus the difference between both sides was not investigated. Follow-up studies will need to include patients of different genders and ages with chronic neck pain in long-term interventions to further investigate the long-term effects of SET.

5. Conclusion

The effectiveness of 4-Week SET in reducing pain and dysfunction in patients with chronic neck pain may be related to improved cervical muscle structure and neuromuscular control of the neck.

Ethical considerations

The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by Shenyang Sport University Ethics Committee (No. 2020 [14]).

Funding

This research was funded by a grant from the Basic Scientific Research Projects of Colleges and Universities in Liaoning Province (Grant no. LQN2017ST03 to Yan Gao).

Informed consent

All participants signed an informed consent form prior to enrollment.

Author contributions

ZWY and MZ: Conceptualization, Methodology, Funding acquisition.

ZWY and ZY: Writing-Original draft preparation and Reviewing.

FLZ and YG: Designed the figures.

ZW and JLW: Visualization and Investigation.

All the authors have read and approved the final version of the manuscript and agree with the order of presentation of the authors.

Footnotes

Acknowledgments

None to report.

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Effect of sling exercise therapy on surface electromyography and muscle thickness of superficial cervical muscle groups in female patients with chronic neck pain

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Study design

2.2 Participants

2.3 Inclusion and exclusion criteria

2.4 Intervention

2.6 Control group

2.7 Testing procedures

Table 1 Demographic and clinical characteristics of the two groups at baseline

Table 2 Differences in NDI and VAS between the two groups at baseline

3. Results

3.1 Participants

3.2 Between-group difference

Table 4 Between-group differences in EMG between the two groups at 4 weeks.

Table 7 Within-group differences in EMG from baseline to 4-weeks intervention

4.1 Limitations and further recommendations

5. Conclusion

Ethical considerations

Funding

Informed consent

Author contributions

Footnotes

Acknowledgments

References

Table 1
Demographic and clinical characteristics of the two groups at baseline

Table 2
Differences in NDI and VAS between the two groups at baseline

Table 4
Between-group differences in EMG between the two groups at 4 weeks.

Table 7
Within-group differences in EMG from baseline to 4-weeks intervention