  
I love forearm plank and all of its variations. It is a powerful pose that builds strength and endurance. It is also a fairly advanced pose for people new to exercise and not always the best place to start. The transverse abdominis muscle of the lower belly is a focus of forearm plank and it should have a certain amount of tone before working on this type of pose. An easy way to know if you should be doing forearm plank is to check in with the support of your head and neck in the horizontal plane. If the transverse lacks proper tone the spine will not support the heavy weight of the head.
When I see a sea of bodies in forearm plank every possible form of misalignment passes before my eyes: hips too high, hips too low, head down, heels splayed open, lower back collapsed, and for this post—upper back rounded. The body in forearm plank should mirror the body standing with good posture, but unlike standing, a lot of muscles are turned on in order to find and maintain the pose.
When standing with good posture our upper chest should be broad with the collar bones wide apart. There are two muscles in the upper chest – pectoralis minor and pectoralis major connect the rib cage to the arm. These muscles tend to be tighter than the muscles of the upper back. They pull the shoulder blades away from each other. And I can feel them as I am typing this and my upper chest shortens habitually.
The Pectoralis Minor Can be Over Used in Forearm Plank
Forearm plank is an easy pose to cheat a little by using pectoralis minor and major to keep the abs from working too much. I often see students in forearm plank with their upper spine pushing or rounding up towards the ceiling. This is the equivalent of pulling your shoulder blades towards each other. The main effect is to shorten your habitually tight pectoral muscles, especially pectoralis minor. These students are feeling their abdominal muscles work but they are using the upper body, the pectorals in particular to bear much of the load.
If the upper chest broadens and you create a small groove or channel between the shoulder blades on the back, the pectoralis minor and major stretch open a little and the abdominal muscles can really activate. Imagine the openness of your chest while standing. This is the same feeling you would like to have in forearm plank.
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