The inner thighs tend to be weak. The inner thigh muscles (the adductors) are so important in the yoga practice because they are weak in relation to the outer thighs and therefore are often underused. I consider my weak inner thighs and excessive hyperextension to be the driving force behind the multiple knee surgeries I went through a number of years ago.
I made a lot of changes to my body and posture building many different muscles but I credit the strength that I built in the inner thigh muscles as the main reason that I am pain free 99% of the time.
There are five adductors of the inner thigh and their main function is to pull the legs towards each other. They range from very short (pectineus) to very long (gracilis) with three in the middle (brevis, longus, and magnus). They all attach on the rim of the pubic bone at the front of the pelvis.
One of them though has a slightly different, and from my perspective way more important function than the rest. Adductor magnus has a second head that attaches to the ishcial tuberosity, or sit bone, of the pelvis. This second attachment is important to our backbends because it can help with an internal rotation that sets the sit bones back and apart and helps to create space in the SI joints.
This second attachment of adductor magnus also helps to set the psoas in its proper position for the magic discussed in the prior post to happen. When the psoas lives in the back plane of the body at its top and bottom, the back of the spine will be free to extend up as it is designed.
When watching someone do wheel pose, I focus on the wrinkles, or lack of wrinkles, in clothing where the leg meets the pelvis. If we use the inner thighs to draw the legs towards each other and then add the slight internal rotation from adductor magnus, wrinkles of clothing miraculously appear in the groins and this sets the stage from true spinal extension rather that spinal compression.
All parts of the body are important when building a successful backbend. But if the inner thighs are not carrying their weight the integrity of the whole pose will be compromised.