Core stability is defined as the ability of the motor control system to coordinate the osteo-articular and muscular structures to maintain or restore a position or trajectory of the trunk when it is subjected to internal and/or external forces. Good core stability of the trunk-pelvis section improves the stability of the spine and ensures an efficient transfer of the forces generated by the upper limbs to the lower limbs, and vice versa. Studies show that this work is just as essential in athletes looking for performance as in sedentary subjects who would like to avoid problems with the lower back.
The aim of core training is to develop patterns aimed at improving the stability of spinal structures. The prone plank is a classic bodyweight exercise, used to strengthen the abdominal muscles (transverse, internal oblique, external oblique and right abdomen) and spinal muscles (especially, erector spinae), which we have already discussed in a previous article. In combination with the thoraco-lumbar fascia, the co-contraction of these core muscles increases the intra-abdominal pressure to provide functional stability to the lumbar spine.
Performing correctly this exercise involves maintaining the pelvis and spine in a neutral position, resting on the hands or forearms and feet, while controlling the natural curvature of the spine. However, the anterior or posterior pelvic tilt influences the muscle activity of the rectus abdominis. In addition, the position of the shoulders, and more precisely the scapular abduction/adduction can influence the inclination of the trunk. This core exercise is one of the most practiced, so it is essential to understand what the right position is to adopt.
To answer this problem, a team of Spanish researchers tested the exercise of the prone plank under 4 different conditions involving 2 different positions of the scapulae (abduction or adduction) and 2 different positions of the pelvis (anterior or posterior tilt). For this protocol, 15 people participated (5 women and 10 men). The experiment consisted in performing 4 variants of the prone plank in 3 sets of 10 seconds, in a randomized order. The variants were:
For each position tested, the researchers measured the muscle activity by electromyography (EMG) of the rectus abdominis, external oblique, internal oblique and erector spinae. In addition, just after each set, the rate of perceived exertion (RPE) was collected from each participant. Finally, to ensure good positions during the experiment, a 2D video kinematic analysis in the sagittal plane was carried out using sensors positioned on the participants.
The main results of this study showed that, during the prone plank exercise, the posterior pelvic tilt and the scapular adduction were associated with a muscle activity significantly superior to the other variants for the abdominal muscles and erector spinae. These results demonstrate the influence of the pelvis and scapulae positions on muscle recruitment during a core exercise. These results are confirmed by the RPE data which show a significantly higher level of perceived exertion for the AdRet variant. This is why, in a difficult situation, during this exercise, people tend to let their pelvis goes down while going in anteversion to limit the work of the abdominal muscles and to place their scapulae in adduction.
The influence of the scapular adduction/abduction on the increased muscle activity of the abdominal muscles could come from the fact that in adduction, the tension provided by the latissimus dorsi is limited. The latter being connected to the thoraco-lumbar fascia, the scapular adduction would reduce the tension provided by the fascia causing greater recruitment of the abdominal muscles.
Figure 1. Muscle activation of the abdominal muscles and erector spinae during the prone plank exercise according to the position of the pelvis and scapulae: scapular abduction and anterior pelvic tilt (AbAnt), scapular abduction and posterior pelvic tilt (AbRet), scapular adduction and anterior pelvic tilt (AdAnt) and scapular adduction and posterior pelvic tilt (AdRet). MVC: Isometric Maximum Voluntary Contraction.
This study focused on a popular and widely practiced core training exercise and in the ways of performing it. The results show that not only the position of the pelvis is important but that of the scapulae is just as important. Even if this study is only a one time test, and it will be necessary to wait for other studies to fully understand the mechanisms of the scapular position influence on the muscle activity, it is the first to take into account the shoulder position during this exercise. It then seems necessary for a coach to indicate to his client/athlete not only to practice a retroversion of the pelvis but also to perform a retraction of the shoulders in order to further stress the abs muscle group. It will also be necessary to wait for other studies to obtain more information on the influence of the position adopted during this exercise on the risk of injury.
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