Assessment of neck muscle thickness and stiffness in Chinese healthy individuals by ultrasound

Introduction

As modernization of society and the workplace continues, the prevalence of chronic neck pain is increasing (1). Neck pain is one of the five major types of chronic pain. Compared with low back pain, there are few studies on neck pain. Although most acute episodes resolve spontaneously, more than a third of patients still have mild symptoms or relapse more than a year later (2). Timely detection of these changes may be helpful for early intervention in order to prevent neck pain, reduce neck pain, and improve function (3).

The function of neck muscles is traditionally assessed based on medical history and physical examination, which can be subjective. Potentially more objective methods include electromyography, which is invasive and susceptible to interference; isokinetic muscle strength testing, which requires expensive equipment and is inappropriate for patients with limited mobility (4,5); as well as computed tomography (CT) and magnetic resonance imaging (MRI), which also require expensive equipment, are unsuitable for patients with claustrophobia or metal implants, and do not allow analysis of the neck muscles while they are actively functioning (6,7). An alternative imaging may be high-frequency ultrasound (HFUS), which can image fine muscle structure better than CT and can follow muscle movement in real time (8). The technique has already proven useful for detecting changes in muscle structure and thickness in neuromuscular disorders (9), and such changes indicate alterations in muscle mass and activity (10,11). In shear wave elastography (SWE), an ultrasound probe sends shear waves of acoustic pulses through the target tissue, and their speed of propagation provides information about the tissue’s stiffness (12). SWE has already been used to analyze healthy and diseased tissues in the liver (13), thyroid (14), breast (15), prostate (16), and musculoskeletal system (17,18). The feasibility and reliability of HFUS and SWE in quantifying neck muscle thickness and stiffness in normal individuals have been confirmed (19-22). However, those studies analyzed relatively few muscles in small samples, and did not systematically analyze the effect of neck position on the measurements or examine SWE.

Here, we explored whether HFUS and SWE, would allow reliable measurement of, respectively, the thickness and stiffness of eight anterior and posterior neck muscles while the neck was in different positions. At the same time, we explored whether the thickness and stiffness of neck muscles in the healthy Chinese population correlate with demographic or lifestyle factors, which may inform the ongoing debate about what contributes to chronic neck pain. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-435/rc).

Methods

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Ethics Committee of West China Hospital, Sichuan University {approval number: 2021[2509]}, and informed consent was provided by all participants.

Study participants

Chinese healthy volunteers were recruited from November 2021 through May 2022 at West China Hospital, Sichuan University. The inclusion criteria were age older than 18 years and no neck pain or discomfort during the previous 6 months. The exclusion criteria were clinically diagnosed cervical spondylosis or history of cervical spine injury; history of use of glucocorticosteroids, muscle relaxants, or other drugs that might affect structure or function of muscles; scoliosis, ankylosing spondylitis or other disease affecting one’s ability to sit or stand; rheumatological, immune, metabolic, or other systemic disease that might affect muscles; scarring of the skin at the site of measurement; and pregnancy.

Interviews were conducted to obtain information from participants about gender, age, body mass index (BMI), left- or right-handedness, whether they spent more or fewer than 40 hours per week sitting at a desk (desk-bound time), and whether they engaged in some form of exercise (e.g., working out, swimming, running, playing football) at least three times per week (regular exercise habits).

HFUS

The Aixplorer ultrasound system with SuperLinear SL10-2 MHz probe (SuperSonic Imagine, Aix-en-Provence, France) was used in “superficial musculoskeletal” mode, with the same brightness mode. Depth was fixed at 3–4 cm, and the focus was adjusted based on the muscle being targeted. Based on the literature (22,23) and our pilot study (unpublished data), we selected five posterior cervical extensor muscles (trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis, multifidus) and three anterior cervical flexor muscles (sternocleidomastoid, longus capitis, longus colli) on the right side of the neck for imaging.

During imaging of posterior cervical muscles, thickness was measured at the site marked in Figure 1. The participant was asked to sit with his or her back to the examiner and to look straight ahead, while keeping the neck and shoulder in a neutral position, as confirmed by a reading of 0° on a joint mobility meter. The C7 spinous process was located by palpation, the probe was placed transversely at that position, then it was moved upward to C4 and positioned 2 cm next to the spinous process. Thickness (in cm) was measured in transverse cross-section while the neck was in each of three positions: neutral, forward flexion (+40° on the joint mobility meter), and extension (−10° on the mobility meter).

Figure 1 Measurement of the thickness of posterior cervical muscles while the neck was in different positions. (A-C) Photographs illustrating: (A) neutral position; (B) forward flexion; and (C) extension during measurement. (D-F) Thickness measurement with high-frequency ultrasound while the neck was in the position shown in the corresponding image in the upper row. The cervical muscles, from top to bottom, are as follows: trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis and multifidus. This image is published with the patient’s consent.

During imaging of anterior cervical muscles, thickness was measured at the site marked in Figure 2. The participant was supine, and the ultrasound probe was placed laterally 2 cm below the highest part of the thyroid cartilage, then moved to the level of C5–C6 and shifted 2 cm to the side. Thickness (in cm) was measured in transverse cross-section when the neck was in each of two positions: neutral and deflection (±30° on the joint mobility meter).

Figure 2 Measurement of the thickness of anterior cervical muscles while the neck was in different positions. (A,B) Photographs illustrating: (A) neutral position, (B) deflection position during measurement, and (C,D) thickness measurement with HFUS in the position shown in the corresponding image in the upper row (from shallow to deep are sternocleidomastoid, longus capitis, and longus colli). This image is published with the patient’s consent. HFUS, high-frequency ultrasound.

SWE

In tissues that are homogeneous and isotropic, such as liver, tissue elasticity can be expressed in terms of Young’s modulus, and calculated with the formula E = 3ρC², where E is tissue elasticity, ρ is tissue density, and C is shear wave velocity (SWV). Muscle tissue, however, has significant anisotropy and its density is highly variable, which makes it difficult to apply the formula. Thus, it is more accurate to measure the stiffness of muscle tissue based on SWV (24,25). In our study, a region of interest (ROI) was drawn using Q-box (1–3 mm) to enclose a region of uniform elasticity signal. ROIs had diameters of 1 mm in the case of trapezius and longus capitis; 2 mm in the case of splenius capitis, semispinalis capitis, semispinalis cervicis, multifidus and longus colli; and 3 mm in the case of sternocleidomastoid. Scale was adjusted to 0–180 kPa during acquisition of optimal and standardized images. For each muscle, after determining the thickness as described above, the probe was turned in the direction of the long axis (18) (Figures 3,4), and mean shear velocity (in m/s) was measured three times and averaged. Higher average velocity was interpreted as greater stiffness (26,27).

Figure 3 Measurement of the stiffness of posterior cervical muscles while the neck was in different positions. (A-C) Photographs illustrating: (A) neutral position, (B) forward flexion, and (C) extension during measurement. (D-F) SWV measurements of trapezius in the position shown in the corresponding image in the top row. (G-I) SWV measurements of semispinalis capitis in the position shown in the corresponding image in the top row. This image is published with the patient’s consent. SD, standard deviation; SWV, shear wave velocity.

Figure 4 Measurement of the stiffness of anterior cervical muscles while the neck was in different positions. (A,B) Photographs illustrating: (A) neutral position, (B) deflection position during measurement, and (C,D) SWV measurements of sternocleidomastoid in the position shown in the corresponding image in the top row. (E,F) SWV measurements of longus capitis in the position shown in the corresponding image in the top row. This image is published with the patient’s consent. SD, standard deviation; SWV, shear wave velocity.

Reliability study

We selected 20 of the participants for repeat measurements in order to measure the intra- and inter-observer reliability of muscle thickness and stiffness. In the case of inter-observer reliability, measurements of the same right neck muscle in a neutral position during the same measurement session were compared between two experienced ultrasonographers (sonographers A and B). The measurements by each ultrasonographer were the average of three separate determinations. In the case of intra-observer reliability, measurements of the same neck muscle in the same position by the same ultrasonographer (A) were compared between one measurement session and another session one week later. Intra- or inter-observer reliability was considered poor if the correlation coefficient <0.40, moderate if 0.40–0.75, or good if >0.75.

Statistical analysis

Data were analyzed statistically in SPSS 22.0 (IBM Corp., Armonk, NY, USA). Continuous data were expressed as mean ± standard deviation (SD) if normally distributed or as median (quartile) if not. Pairwise differences were assessed for significance using the t- or t’-test if data were normally distributed, or using the Wilcoxon or Mann-Whitney U test if not. Differences among three or more groups were assessed for significance using Friedman’s or Kruskal-Wallis test. Categorical data were expressed as n (%), and differences were assessed for significance using the chi-squared test.

Potential correlations between variables were assessed using Pearson analysis if the variables showed normal distribution or using Spearman analysis if not. Correlations were categorized based on the absolute value of the correlation coefficient as follows: |r| ≥0.8, very strong; 0.6≤ |r| <0.8, strong; 0.4≤ |r| <0.6, moderate; 0.2≤ |r| <0.4, weak; |r| <0.2, very weak or absent. Variables that significantly predicted the thickness or stiffness of neck muscles were identified using stepwise multiple linear regression. Multiple two-by-two comparisons were corrected using the Bonferroni method.

All statistical tests were two-tailed, and P<0.05 indicated statistical significance.

Results

The demographic characteristics of the 100 participants (43 male and 57 female) are summarized in Table 1. The participants had a mean age of 38±12 years (range, 19–69 years), and a mean BMI of 22.12±2.75 kg/m² (range, 16.41–25.83 kg/m²). All participants were right-handed, and there was no significant difference in age between men and women (P=0.699), but men had significantly higher height, weight, and BMI than women (P<0.05).

We analyzed eight muscles in the right posterior and anterior neck of all participants with HFUS and SWE, and the resulting measurements of thickness and stiffness showed good intra- and inter-observer reliability for all muscles. In the measurement of neck muscle thickness, the intra-observer correlation coefficients were all greater than 0.973 (P<0.001), the highest was 0.988 (P<0.001), and the inter-observer correlation coefficients were all greater than 0.895 (P<0.001). The intra-observer correlation coefficients of neck muscle stiffness measurement were all greater than 0.752 (P<0.001), the highest was sternocleidomastoid muscle stiffness measurement (0.917, P<0.001), and the inter-observer correlation coefficients were all greater than 0.751 (P<0.001) (Tables 2,3).

Next, we compared thickness or stiffness of the cervical muscles in different neck positions. In the posterior cervical muscles, the average thickness of trapezius in neutral, forward flexion, and extension positions was 0.16±0.05, 0.15±0.04, and 0.16±0.05 cm, respectively; that of splenius capitis was 0.53±0.10, 0.43±0.08, and 0.50±0.11 cm, respectively; that of semispinalis capitis was 0.59±0.12, 0.51±0.10, and 0.53±0.12 cm, respectively; that of semispinalis cervicis was 0.70±0.11, 0.60±0.09, and 0.66±0.11 cm, respectively; and that of multifidus was 0.88±0.13, 0.77±0.12, and 0.86±0.13 cm, respectively. Posterior muscles were all thinner in forward flexion than in neutral and extension positions (P<0.05). With the exception of the trapezius, the other posterior muscles were thinner in extension than in neutral position (P<0.05) (Figure 5). Among anterior muscles, the average thickness of sternocleidomastoid in neutral position and deflection position was 0.66±0.10 and 0.70±0.12 cm, whereas that of longus capitis was 0.29±0.05 and 0.31±0.07 cm, and that of longus colli was 0.77±0.10 and 0.66±0.10 cm, respectively. The sternocleidomastoid and longus capitis were thinner in the neutral position than in deflection (P<0.05), whereas the longus colli was the opposite (P<0.001) (Figure 6). In terms of neck muscle stiffness, the stiffness of posterior muscles was greatest in forward flexion, lower in the neutral position, and lowest in extension (P<0.001), and the average stiffness of trapezius was 2.46±0.38, 3.77±0.61, and 2.00±0.13 m/s in neutral, forward flexion, and extension positions, whereas that of splenius capitis was 2.37±0.42, 3.80±0.67, and 1.96±0.29 m/s, that semispinalis capitis was 2.99±0.57, 4.61±0.78, and 2.35±0.44 m/s, that of semispinalis cervicis was 3.55±0.68, 5.42±0.63, and 2.73±0.51 m/s, and that of multifidus was 3.86±0.82, 5.53±0.69, and 3.02±0.66 m/s, respectively (Figure 7). The stiffness of anterior muscles was greater in deflection than it was in the neutral position (P<0.001), and the average stiffness of sternocleidomastoid in neutral and deflection position was 2.67±0.45 and 4.13±0.72 m/s, respectively, whereas that of longus capitis was 3.05±0.45 and 4.05±0.72 m/s, respectively. Meanwhile in longus colli, they were 3.40±0.63 and 4.49±0.76 m/s, respectively (Figure 8).

Figure 5 Comparison of the thickness of the posterior neck muscles in different positions. Numbers on short lines in the figure are P values.

Figure 6 Comparison of the thickness of the anterior neck muscles in different positions. Numbers on short lines in the figure are P values.

Figure 7 Comparison of SWV during SWE of the posterior neck muscles in different positions. Numbers on short lines in the figure are P values. SWE, shear wave elastography; SWV, shear wave velocity.

Figure 8 Comparison of SWV during SWE of the anterior neck muscles in different positions. Numbers on short lines in the figure are P values. SWE, shear wave elastography; SWV, shear wave velocity.

At least one of the three variables of gender, age, and BMI showed significant associations with thickness of all cervical muscles that we examine. The thickness of all the anterior and posterior cervical muscles was correlated with gender (P<0.05), but not with desk-bound time or exercise habits (P>0.05). Furthermore, the thickness of trapezius was only correlated with gender, with a correlation coefficient of 0.660 (P<0.05), whereas semispinalis capitis, semispinalis cervicis, sternocleidomastoid and longus, colli were negatively correlated with age, with correlation coefficients of −0.327, −0.198, −0.255, and −0.275, respectively (P<0.05). Except for trapezius and semispinalis cervicis, other muscles were also positively correlated with BMI (P<0.05). For muscle stiffness, gender had an opposite effect on trapezius and splenius capitis than it did on multifidus, longus capitis, and longus colli (P<0.05). The stiffness of splenius capitis was negatively correlated with age (r=−0.200, P<0.05), whereas the stiffness of semispinalis cervicis was positively correlated with age (r=0.237, P<0.05). The stiffness of the longus capitis and longus colli was also related to desk-bound time (P<0.05), whereas regular exercise habits only affected the stiffness of the longus capitis (P<0.05) (Figure 9).

Figure 9 Matrix of pairwise correlations between thickness or shear wave velocity in the indicated neck muscles and the indicated demographic or lifestyle variables. Pearson or Spearman r correlation coefficients are shown in the cells, which are colored according to how positive (red) or negative (blue) they are; gray indicates that the corresponding parameters have no correlation with thickness and stiffness. BMI, body mass index.

Discussion

Here, we demonstrate HFUS and SWE for measuring thickness and stiffness of posterior and anterior neck muscles is reliable. We also demonstrate that these characteristics of the muscles depend on demographic factors and, in some cases, on whether or not the person has a primarily sedentary lifestyle or engages in regular exercise.

We observed good intra- and inter-observer reliability for our measurements of thickness, regardless of the muscle or neck position, with correlation coefficients ranging from 0.895 to 0.988. These coefficients are comparable to those reported in other studies using ultrasonography when neck muscles were at rest or during isometric contraction (23,28). Similarly, we observed good reliability for our measurements of stiffness, regardless of the muscle or neck position, with correlation coefficients ranging from 0.751 to 0.917. These coefficients are comparable to, or even better than, those reported in studies that measured SWV in medial gastrocnemius and tibialis anterior muscles (18), in anterior and middle scalene muscles (29), or in neck extensor muscles (30). Nevertheless, previous work has suggested that certain factors may affect the reliability of SWV measurements, such as the depth of the target area below the skin, whether bone is present below the target area, and whether blood vessels are nearby (31). Future work should explore the most reliable ways to measure the stiffness of neck muscles.

We found that posterior cervical muscles were thicker when the neck was in a neutral position or in extension than they were when the neck was in forward flexion. This is consistent with a study showing greater thickness of the semispinalis capitis when in extension than in the other two positions (32). We found that, among anterior neck muscles, sternocleidomastoid and longus capitis were thinner in the neutral position than in deflection, whereas the opposite was true of longus colli. In contrast, a previous study found that the thickness of both the longus colli and sternocleidomastoid gradually increased with cranio-cervical flexion (33). Thus, cervical muscles may contract differently when the neck is in different positions, highlighting the importance of measuring neck muscle thickness in different neck positions.

In our study, neck position affected not only the thickness of cervical muscles but also their stiffness. The stiffness of all posterior cervical extensors was greater when the neck was in forward flexion than it was when it was in a neutral position or in extension, which is consistent with a previous study (34). The stiffness of all anterior cervical extensors was greater in the deflected than neutral position. These observations highlight the importance of standardizing neck position when assessing muscle stiffness in order to maximize reliability and clinical usefulness.

Our results go even further by highlighting the importance of comparing muscle parameters within the same gender: the men in our sample showed significantly thicker cervical muscles than the women did. A previous study found similar results for sternocleidomastoid and all cervical extensor muscles except trapezius or longus colli (35). Gender showed a more complex relationship with muscle stiffness in our sample: among all the muscles, the only gender differences were that the trapezius was stiffer in women, whereas the longus colli was stiffer in men. In contrast, one previous study found no gender difference in stiffness of the trapezius (35), whereas another (36) found the sternocleidomastoid to be 29% stiffer in men than women, regardless of the direction of force. Although the details remain to be clarified, neck muscle parameters appear to vary with gender, which should be controlled for in neck muscle analyses.

Whether comparisons of neck muscle parameters should also control for age is unclear. Age correlated weakly with the thickness of only two muscles in our study, namely, semispinalis capitis and longus colli, whereas it correlated weakly with the stiffness of only splenius capitis and semispinalis cervicis. Previous studies have failed to detect a correlation between age and stiffness of the anterior and middle scalene muscles (29), or have detected a decrease in stiffness of the sternocleidomastoid with increasing age (37). Future work should clarify whether age significantly influences thickness or stiffness of neck muscles and, if so, which muscles in which types of individuals.

Similarly, whether comparisons of neck muscle parameters should control for BMI is unclear. We found a weak positive correlation between the thickness of each neck muscle and BMI, but only specific muscles, such as splenius capitis, multifidus, longus capitis, and longus colli showed significant differences among individuals stratified by BMI. Stiffness of only trapezius and splenius capitis showed a weak negative correlation with BMI, whereas a previous study reported a robust association between BMI and stiffness of the trapezius (38). As in the case of age, future work should clarify for what muscles in what populations clinicians should control for BMI when assessing neck muscle parameters.

The main objectives of our study were to demonstrate whether HFUS and SWE can reliably measure the thickness and stiffness of neck muscles, as well as to explore what demographic and lifestyle factors might influence those muscle parameters. We took advantage of the collected data to begin to explore whether two lifestyle factors that have been proposed to contribute to chronic neck pain might also influence thickness and stiffness. We found that muscle thickness did not differ significantly between individuals in our study who reported sitting for shorter or longer than 40 hours per week, whereas stiffness differed significantly between the two groups only in the case of longus capitis and longus colli. Similarly, muscle thickness did not differ significantly between individuals in our study who reported exercising at least three times per week or not, whereas stiffness differed significantly between the two groups only in the case of longus capitis. Our findings differ with previous studies that have linked longer time sitting in the workplace with thinning of neck muscles and greater risk of chronic neck pain (39). Our findings also contrast with previous work showing that at least certain forms of regular exercise can improve the specificity of neck muscle activity (40) and can thicken the sternocleidomastoid and longus colli in individuals with chronic neck pain (41). Some of the discrepancies between our work and previous studies may reflect that we analyzed individuals who did not report chronic neck discomfort and who showed no obvious morphological or mechanical abnormalities in the neck muscles. Another reason for our failure to detect an influence of exercise may be that we did not focus on neck-specific exercises, in contrast to previous work (41). In any case, the link between neck muscle stiffness and chronic neck pain has yet to be demonstrated conclusively (42).

Our findings should be interpreted with caution given that our entire sample was right-handed, we assessed cervical extensor parameters only at the C4 level and cervical flexor parameters only at the C5–C6 level, and our sample was too small to establish reliable references ranges for thickness or stiffness. Future studies should verify our analysis of healthy individuals and extend it to other populations.

Conclusions

Our work demonstrates that HFUS and SWE can reliably measure the thickness and stiffness of neck muscles. Assessment of these parameters should take into account the individual’s gender. It may also need to take into account age and BMI, which should be explored in more depth in future work. The non-invasive, multimodal technique that we describe here should be applied to larger samples whose risk of chronic neck pain varies because of their occupation, exercise habits, and other lifestyle factors. A sufficiently large dataset could bring us much closer to understanding whether and how demographics and lifestyle influence the risk of chronic pain.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-435/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-435/dss

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-435/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Ethics Committee of West China Hospital, Sichuan University {approval number: 2021[2509]}, and informed consent was obtained from all participants.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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