## 4 Discussion
### Comparison of sEMG Activation across Muscles and Conditions  
Results from the kinematic controlled sEMG analysis revealed that the intensity of the muscular responses differed amongst testing conditions. To classify as low-to-high intensity of these responses, the criteria of related research were used [67, 68]. The _neutral_ condition was considered the reference plank performance for determining the extent of influence of the other two experimental conditions. Both in terms of TI and for each abdominal muscle assessed, this plank variation produced similar levels of moderate activity. Otherwise, the LES activity remained unchanged, showing a low muscle activity--around 5% MVIC--thereby indicating no influence of head position on the lower back muscle activity. The consistency of these findings with previous research indicates a similar standard performance of this traditional bridging exercise [21, 37, 68, 69]. Interestingly, _extension_ caused a similar effect on the core activation as _neutral_, showing parallel amplitudes of moderate abdominal activity as well as low activity in the LES. In terms of TI, the compounded sEMG response only varied by 2.92% MVIC (ES = 0.253) by placing the head at the end of the cranio-cervical extension range. This slight increase in the sEMG response produced a non-significant variation across muscles, as shown in Figure 3.  
However, when participants were asked to place an active head full flexion posture, they experienced a significant intensification in the magnitude of the overall abdominal response, although its extent was different depending on the muscle group analysed. The greatest sEMG response to _flexion_ was detected for the RA and both IO sides. Both muscles almost achieved levels of very high muscle activity, reaching values of 60% MVIC for the RA and around 56% for both sides of the IO. Based on previous research, this may be due to a greater muscular effort to provide the necessary active stiffness by synergistic cocontraction of the deep and superficial core muscles to supply more efficiently the stabilising and \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Total Intensity**} & \multicolumn{3}{c}{**Rating of Perceived Exertion**} \\ \hline \multirow{2}{*}{
\begin{tabular}{c} **Testing** \\ **Conditions** \\ \end{tabular} } & \multirow{2}{*}{**Mean \(\pm\) SD**} & \multicolumn{3}{c}{**95\% CI**} & \multicolumn{3}{c}{**95\% CI**} & \multirow{2}{*}{**ICE\({}_{\text{G,1D}}\)**} \\ \cline{3-3} \cline{5-7}  & & **Low** & & & **Low** & & **High** \\ \hline _Neutral_ & 23.772 \(\pm\) 10.237 & 18.981 & 28.563 & 3.225 \(\pm\) 1.280 & 2.626 & 3.824 & 0.966 \\ _Flexion_ & 39.366 \(\pm\) 15.389 * & 32.163 & 46.567 & 4.750 \(\pm\) 1.717 \(\pm\) 3.947 & 5.554 & 0.962 \\ _Extension_ & 26.692 \(\pm\) 13.634 & 20.310 & 33.073 & 4.525 \(\pm\) 1.686 \(\pm\) 3.736 & 5.314 & 0.931 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of total intensity (mean, SD and 95% CI of %MCIV) and perceived exertion rating (mean, SD, 95% CI, and ICC\({}_{\text{(2,1)}}\) of RPE) between the three experimental conditions. weight-bearing demands for this _flexion_ variation [10; 11; 12; 16; 17; 36]. For the EO muscles, the sEMG response was moderate, differing slightly between both EO sides, and did not change for the LES which continued showing low levels of muscle activity in _flexion_ as well. When comparing across conditions, the sEMG increase from _neutral_ to _flexion_ highlights a large effect for the RA and IO activity and a small effect for the LES and EO muscles, as can be seen in Figure 3. A similar effect occurred when comparing _flexion_ and _extension_--but milder compared to _neutral_--. The integrated sEMG response of TI significantly raised by 15.59% MVIC by placing the head to the end of the flexion range. This effect was similar but smaller when comparing from the extension to the flexion ranges, with a significant variation of 12.67% MVIC.