hello, here is your information. at the apex of the rib cage there are around 4 muscles on each side. front abdominal wall contains total of 4 muscles on each side. all four muscles are attached to the rib cage at different levels at upper end and to the hip bone at lower end and this way they cover the whole of the front. now, as per you might have noticed, a normal human contains apex in the rib cage on both sides.... i.e. the downward projections of ribs these both projections meet each other in the midline somewhat higher. three of the muscles are attached to this projection or apex what you call it and the muscle which forms six packs is rectus abdominis muscle which is situated in the middle of the belly on both sides of the centre midline. now this rectus abdominis help you flex the belly and raise the torso from lying down position as per we do in abdominal crunches or such other excercises. and rest 3 muscles on each sides, they help in respective turning of the torso to the respective sides. so if you do excessive abdominal work out, obviously your rectus muscle is put to extra tension and force. and this extra force is exerted on the attached end of a working muscle. and due the same reason, because of the heavy exercise, the muscle attachments become tender and sore. some fibres might get torn at local level if a muscle is overly exerted. but this happens particularly in severe cases, mind it. all you need to do is to give it some rest, allowing the muscle to rest and regenerate. to deal with the pain you can have some pain killers and anti inflammatory drugs, which will help. hope your query is dealt with. thanks regards.