## 3 Results  
Eighty-two women diagnosed with neck pain with an average age of 35.56 (9.91) years were included in the study. Pain intensity measured with a VAS, neck disability measured with NDI and the impact of headache measured with HIT-6 presented low levels in most of the patients. None of the women presented co-morbidities. The mean and standard deviations of the demographic data, endurance tests and morphological characteristics of the DN flexors and extensors of all the included women are shown in Table 1.  
Figure 2: Cross-sectional area of the deep neck extensor muscle marked by dotted lines.  
The correlation analysis showed weak to moderate negative correlations between the endurance tests and neck disability and headache impact. The chin tuck test showed a moderate negative correlation with neck disability (r = \(-\)0.38), and a weak negative correlation with headache impact (r = \(-\)0.26). The extensor muscles test showed a weak negative correlation with neck disability (r = \(-\)0.27) and with headache impact (r = \(-\)0.26). These correlations were all statistically significant, but no significant correlation was found for any of the endurance tests and pain intensity. Concerning the CSA of the flexor and extensor muscles, only flexor muscles' CSA showed a weak negative correlation with neck disability (r = \(-\)0.22). No associations between the flexor and extensor muscles' CSA were found with pain intensity or headache impact. The correlation values and the significance of all the tests are shown in Table 2.  
In a simple linear regression analysis, the statistic R squared indicates the percentage of the variance in the dependent variable that the independent variables explain collectively. NDI and HIT-6 were considered the dependent variables, and endurance test and DN muscles' CSA as independent variables. For headache impact and pain intensity, the linear regression analysis showed an R squared below 20% and was not statistically significant (HIT-6: R squared = 0.165; F = 1.70; \(p\) = 0.113; VAS: R squared = 0.129; F = 1.27; \(p\) = 0.271). For neck disability, the simple linear regression analysis showed an R squared of 26.7% and was statistically significant (NDI: R squared = 0.267; F = 3.13; \(p\) = 0.004). The chin tuck neck flexion test, extensor muscles endurance test, and DN extensor CSA were significantly associated with the NDI score. Conversely, the DN flexor was not significantly associated with the NDI score. The full results of the linear regression analysis for NDI are shown in Table 3.  
\begin{table}
\begin{tabular}{l c c} \hline
**Variable** & **Mean** & **SD** \\ \hline Age (years) & 35.56 & 9.91 \\ Height (cm) & 165.32 & 6.10 \\ Weight (kg) & 62.75 & 10.64 \\ BMI & 22.93 & 3.59 \\ HIT-6 & 51.23 & 8.53 \\ NDI & 8.26 & 2.82 \\ Pain intensity (mm) & 2.65 & 1.66 \\ Chin tuck neck flexion test (s) & 19.79 & 11.39 \\ Extensor muscles test (s) & 34.10 & 18.91 \\ DN Flexor CSA (cm\({}^{2}\)) & 0.83 & 0.31 \\ DN Extensor CSA (cm\({}^{2}\)) & 1.25 & 0.36 \\ \hline \end{tabular} Abbreviations: BMI: body mass index; HIT-6: headache impact questionnaire; NDI: neck disability index; DN: deep neck muscle; CSA: cross-sectional area.  
\end{table}
Table 1: Demographic and clinical characteristics of the sample.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Pain Intensity** & _p-Value_ & **NDI** & _p-Value_ & **HIT-6** & _p-Value_ \\ \hline Chin tuck neck flexion test & \(-\)0.15 & 0.18 & \(-\)0.38 & \(<\)0.001 & \(-\)0.26 & 0.02 \\ \hline Extensor muscle test & \(-\)0.18 & 0.10 & \(-\)0.27 & 0.01 & \(-\)0.26 & 0.02 \\ \hline DN Flexor CSA & \(-\)0.19 & 0.14 & \(-\)0.22 & 0.05 & \(-\)0.12 & 0.28 \\ \hline DN Extensor CSA & 0.05 & 0.63 & 0.07 & 0.54 & 0.10 & 0.36 \\ \hline \end{tabular} Abbreviations: NDI: neck disability index; HIT-6: headache impact; DN: deep neck muscle; CSA: cross-sectional area.  
\end{table}
Table 2: Correlation analysis between symptoms and muscle function and thickness.