The pectoral muscles are not supposed to be major players in plank and forearm plank.  There are both favorite poses of mine. They are both excellent tools for building the core though I don’t always think they are the right place to start. When it comes to the core, a deep low belly muscle called the transverse abdominis needs to have a certain amount of tone in isolation before moving into core exercises that require the use of more muscles.
There are a lot of muscles involved in the plank variations. The abdominals of the low trunk all want to be firing as well as the muscles of the upper trunk—the rhomboids and the serratus anterior in particular. The rhomboids  and serratus anterior want to tone the shoulder blades evenly onto the back so the upper chest can broaden. Unfortunately the pectoral muscles tend to get involved and get in the way.
Muscular balance across the upper back and chest is an elusive thing. Most people are tight in the pectoral muscles at the front of the chest and loose in the rhomboids that connect the shoulder blade to the spine. As a result of this common imbalance, many people rely on the pectoral muscles way to much in the plank poses.
Looking across a sea of upper backs in either plank or forearm plank, I hope to see a shallow groove between the shoulder blades that situate the spine slightly lower than the blades. This gully in the upper back indicates an expansion of the upper chest and tone in the rhomboids.
Instead, many students round the upper back slightly, elevating the spine and making the chest more concave. This shortens or engages the pectoral muscles and takes a great deal of work out of the abdominal muscles which are meant to be providing support for the spine.
If you try either of the plank poses, first feel your tendency in the pose and check out what you do with your upper back and then try softening the upper spine down between the shoulder blades to feel the abdominal muscles really kick in.
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