## 4 Discussion
### Practical Applications and Implications for Future Practice  
The way in which the plank exercise is executed is essential since it can involve different loads on the spine and different levels of core activation [34; 35; 36; 37; 39]. Consequently, it is important to investigate all the variables influencing the proper performance of this exercise because, otherwise, this misunderstanding could make the same exercise painful [6; 33]. In this sense, the present findings provide a new understanding of the behaviour of the trunk musculature in this essential core exercise. Therefore, when using the prone plank as part of core stabilisation training, clinicians, physiotherapists, and fitness professionals should consider, this demonstrated increase in sEMG during _flexion_ and no change in sEMG during _extension_, when compared to _neutral_.  
Although the different head positions in the plank and the corresponding spinal loads for each of these positions need to be evaluated in future studies, the influence of these postures on core muscle activity has been described in this study. Knowledge of these results could help to graduate the intensity of muscle recruitment during the plank based on the influence that the changes produced in the cranio-cervical region have on the sEMG activity of the core musculature. In addition, these findings may be useful in providing practical guidance on the initial challenge, corrective technique, progression, and choice of exercise variation.  
Our findings also indicate that future studies evaluating the core sEMG activity during bridging exercises should monitor not only the pelvic and shoulder girdle areas, following previous research [37; 39], but also the cranio-cervical region. Moreover, when performing field tests using the plank protocol to assess the postural control of the lumbar spine, the posture of the head should be controlled to guarantee the validity of the evaluations. Additionally, future research should assess the extent to which the head and other pelvic, scapular and/or limb positions interact and influence each other during the prone plank or other bridging exercises such as side and back bridges, to gain a deeper understanding of trunk muscle behaviour during core stabilisation training.  
The present findings have some methodological limitations. First, the relatively small sample size of 20 participants could have affected the results, principally to explain why some statistical effects on abdominal muscles were not more clearly observed, especially related to the bilateral differences in EO muscle activity. Therefore, more research with a further sample size should be proposed to clarify the activity on the EO muscles. Second, this investigation included only healthy participants, experienced in core stabilisation training, without back or neck pain or disorders. Although analogous findings could be expected in clinical populations, it could be possible that the existence of injuries or back or neck pain could result in different sEMG responses [79]. Consequently, these findings cannot directly extrapolate to symptomatic populations. Further cross-sectional studies with symptomatic participants should be addressed to prove our results in these clinical samples.