From Fitness & Beauty, 2014 August issue, P73, written by Bob Chen
 
Plank exercise is an isometric exercise that has particularly high demands on the stability of joints, especially on the shoulders, lumbar vertebra, cervical vertebra and so on.  When you first start doing this exercise, it is normal to experience some muscle aches.  However, if you experience joint pain that continues to the next day, then you should go see a doctor or a rehabilitation specialist to resolve the problem. 
 
Discomfort in the neck is generally associated with raising the heads while looking ahead when athletes are in the prone position.  This causes neck muscle spasms, which stimulate the cervical spine, causing discomfort in this area.  The correct way should be extending the head naturally along the direction of the spine, eyes looking downward, the cervical vertebra maintaining a natural physiological curve, without bending overly backward. 
 
Discomfort in shoulders is more commonly seen in female athletes.  Many women experience shoulder discomfort, resulting in a slightly twisted posture, after doing plank for a short time, and eventually they have to give up the exercise.  The first reason for this to happen is that, throughout human evolution, the upper limbs have not been able to bear weight like the walking-tool-lower extremities have, and their strength and stability are relatively weak.  The second reason is that some women lack fitness.  Their bodies are weak, and their shoulder stability is poor, resulting in their upper limbs being unable to bear their bodyweight.  Therefore, when they are doing plank exercises, the shoulder joints, compressed for long periods, will naturally experience strain and pain.  The correct shoulder posture is to keep shoulders flat as the name suggests – the plank is a flat board, and there should be no collapse between the two shoulder blades.  In addition, the whole body (from head to toes) should be in a straight line like a bench, and the upper arms and the upper body form 90 degrees.  Obviously, this requires the shoulders to have a certain degree of strength. 
 
As for discomfort in the lower back, this is the main area on which this exercise works, and it is also a disaster area for athletes.  From the point of view of anatomical structure, in order to withstand the weight of the upper body, the lower lumbar vertebrae of the human body are thicker and stronger than higher lumbar vertebrae.  Furthermore, there is a physiological curve, a forward bend, in lumbar vertebrae that is an adaptation for walking upright.  However, this upright advantage would cause problems for lumbar vertebrae when doing the plank exercise, which tends to increase the risk of downward movement of the lumbar vertebrae.  Because the lumbar vertebrae are connected to the pelvis, if one is not careful during exercise, it can cause the pelvis to be pushed forward, or the lower back to collapse.  If accompanied by poor posture, such as raising the head, the lower back would collapse even more, which would increase the burden and physiological curve of the lumbar vertebrae.  One must remember that when an animal is on all fours, its lumbar vertebrae are flat or arched. 
 
In short, when one begins doing plank exercise, it is best to consult with a coach or an experienced person in order to get it right and to build good habits from the start.  Please remember, performing the motions correctly according to standards should always come first.  If the motion is deformed due to fatigue, one should stop the exercise immediately.  What we want to challenge is the time required to perform the correct movements.