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

Background: The increasing prevalence of neck pain emphasizes the need for techniques that can analyze neck muscles more objectively than physical examination, yet less invasively and more conveniently than electromyography, computed tomography (CT), or magnetic resonance imaging (MRI). Here, we explored high-frequency ultrasound (HFUS) and shear wave elastography (SWE) for measuring neck muscle thickness and stiffness. Methods: Eight muscles in the right posterior and anterior neck of 100 healthy Chinese individuals were analyzed while the neck was in different positions. HFUS was used to measure muscle thickness, whereas SWE was used to measure stiffness. Measurements were repeated in 20 individuals in order to calculate intra- and inter-observer reliability. Results: HFUS of the eight muscles showed good reliability, with inter- and intra-observer correlation coefficients for muscle thickness ranging from 0.895 to 0.988 (P<0.001); as did SWE, with inter- and intra-observer correlation coefficients for muscle stiffness ranging from 0.751 to 0.917 (P<0.001). During forward flexion, posterior muscles became thinner (P<0.05). Except for the trapezius, the other posterior muscles were thinner in extension than they were in the neutral position (P<0.05). As an example, for the trapezius, in neutral, forward flexion, and extension positions, the thickness was 0.16±0.05, 0.15±0.04, and 0.16±0.05 cm, respectively, whereas that of splenius capitis was 0.53±0.10, 0.43±0.08, and 0.50±0.11 cm, respectively. Among the anterior muscles, the sternocleidomastoid and longus capitis became thinner in neutral position (P<0.05), whereas the longus colli was the opposite (P<0.001). Posterior muscles were stiffest in forward flexion, softest in extension (P<0.001); anterior muscles were stiffer in deflection (P<0.001). For example, the average stiffness of semispinalis capitis was 2.99±0.57, 4.61±0.78, and 2.35±0.44 m/s in neutral, forward flexion and extension positions, respectively, and sternocleidomastoid in neutral and deflection position was 2.67±0.45 and 4.13±0.72 m/s, respectively. The thickness of all muscles significantly correlated with gender (P<0.05). Trapezius thickness correlated only with gender (r=0.660, P<0.05); other muscles negatively correlated with age (e.g., semispinalis capitis: r=-0.327, P<0.05) and positively with body mass index (BMI; except trapezius and semispinalis cervicis). For stiffness, gender effects differed significantly: trapezius and splenius capitis stiffness showed an opposite pattern to that of multifidus, longus capitis, and longus colli (P<0.05). Age inversely correlated with splenius capitis stiffness (r=-0.200, P<0.05), but positively with semispinalis cervicis (r=0.237, P<0.05). Longus capitis/colli stiffness linked to desk-bound time (P<0.05), whereas regular exercise habits affected only longus colli stiffness (P<0.05). Conclusions: HFUS and SWE can reliably measure the thickness and stiffness of neck muscles in healthy individuals. Demographic and lifestyle factors, particularly gender, age, BMI, and time spent sitting at a desk or exercising, should be taken into account during neck muscle assessment.