Muscle physiology: Brief reportThe variation of the strength of neck extensor muscles and semispinalis capitis muscle size with head and neck position
Introduction
The strength of a group of neck muscles varies according to the joint range of motion. By using an isometric muscle strength test Vasavada et al. reported that the strength of neck extensor muscles were the strongest while head and neck were in neutral position. The authors concluded that the force generation capacity of neck extensor muscles was not constantly maintained during the entire cervical range of motion (Vasavada et al., 1998). The function of an individual muscle may also be varied during an entire range of motion accordingly. Ultrasonography has been recommended to study the function of a single small muscle even located in deep layers (Rezasoltani et al., 2002; Lee et al., 2009; Javanshir et al., 2011).
In an ultrasonography study, Wantanabe et al. measured the thickness of the lumbar extensor muscles when subjects were in different positions of the lumbar spine (standing neutral, flexion and extension) (Watanabe et al., 2004). The authors reported that muscles thickness was the largest in the lumber extension position and the smallest at in the lumbar flexion position. McGill et al. combined two methods of muscle strength test and muscle ultrasonography and revealed that the force generation capacity and the dimension of erector spinae muscles decreased when subjects were in a lumbar flexion position (McGill et al., 2000).
The strength related neck muscle size in different ranges of head and neck position have not yet been reported. The study of the function of the neck semispinalis capitis muscle (SECM) is very important since it acts as a prime mover and main head and neck extensor muscle. The aim of this study was to detect the effect of head and neck positions on the strength of neck extensor muscles and size of SECM in healthy subjects.
Section snippets
Methods
Thirty healthy female students (age 18–24 years old) voluntarily participated in this study. They were informed about the aim and experimental aspects of the research prior to the study. The study protocol was approved by the Ethical Committee of Shahid Beheshti University of Medical Sciences.
A real time ultrasonography device (Hitachi EUB 525, Japan) with the frequency of 7.5 MHz linear array probe was applied in this study. The subjects were asked to sit relaxed, on a chair without back
Statistical analysis
To estimate the repeatability of the measurements taken by two examiners, the ICC (Intra-class coefficient correlation) was computed from repeated measurement of ANOVA. A one-way analysis of variance was used to compare any difference in variables of muscles strength and muscle size among positions of neutral, flexion and extension. The statistical analysis was performed using the SPSS statistical software program for Windows.
Results
The index of inter-testers reliability for SECM size measurement was 0.88 < ICC < 0.98 and for muscle strength measurements it was 0.75 < ICC < 0.90 in all three positions. The size of SECM was significantly smaller in flexion than extension (p < 0.001) (Fig. 1). The maximum isometric strength of neck extensor muscles was significantly different between extension and flexion positions and also between extension and neutral positions (p < 0.05) (Table 1). Indicating that, neck extensor muscles
Conclusion
The size and the strength of a muscle may linearly be associated with the joint range of motion (Vasavada et al., 1998; McGill et al., 2000; Watanabe et al., 2004). The authors of two earlier studies stated that lumbar flexion caused a reduction in dimensions of erector spinae muscles (McGill et al., 2000; Watanabe et al., 2004). In our study, the size of SECM was larger in extension than that of flexion position. The difference may be related to the variation of SECM length from extension to
Conflict of interest statement
The Authors certify that there is no actual or potential conflict of interest in relation to this article.
Acknowledgments
The authors of this study would like to thanks to all of the subjects who participated in this study and to Dr Faizei (MD) for his kind cooperation, arrangements and attitude toward this study.
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