Honestly, I neever knew the importance of this muscle until recently during one of the yoga class.
As it turns out, we were doing visvamitrasana pose anusara styled by holding the lifted leg with the hand (see picture here : Shiva Rea calls it the super pose…). It’s like doing the sundial pose while doing vasisthasana aka the side plank. Apparently even though I can do the normal sundial pose, I cannot do it in this pose. Some how I managed to pull the pectoral muscle running up my left chest.
During the 1st 2 days, I thought I had pulled some muscles near the arm area that is connected to the chest as everytime I move my arms overhead, my chest muscle hurts. Then at times when I move my left arm, there was no pain. It seems as if it was ok and I have recovered. However when I go back into class, the pain came back. This was just too weird and I kept trying to figure out why it was so inconsistent and it comes and goes so fast. As I told my friend, I always have weird funny problem ( like last time when I could not bend my knee in full pigeon )
Just the day before, I realised that the pain was not caused by the left arm. instead it was due to the muscle that is connecting from the left chest towards the head. So once I lift my head up as in like upward dog or cobra pose, I can feel the pain along that muscle.
At one time it was so painful that I cannot even do the upward dog or the sun salutation. Luckily the pain is almost gone now. That is one lesson learnt – every muscle in your body is important and crucial + never let your ego get ahead of yourself.
And I am thankful of all the concern and help by all my yoga friends when they knew I had this problem. 
OM~
