They mainly target the serratus anterior. I also notice this muscle especially prominent on Frank. It is a great stretch for the chest and back but I have to be honest, I dont do them very often except for a stretch.
As said by Creator, the mainly work your seratus anterior.
I do not do them because I work my seratus in a different way.
I'm really not sure about this exercise being able to expand the ribcage, but here's a short anecdote about my training last summer.
I used to do DB pullovers religiously with every back workout. I could feel the stretch really well in my lats. But it also felt like my ribcage was being pulled at, like an intense stretch. Which makes sense considering the serratus muscle is being worked so much in this exercise. Anyway, I was doing this exercise one back day, on my last set, one of my last reps, and I thought I pulled my shoulder out of its socket. It hurt like crazy for the next couple of weeks. So I went to the chiropractor, and she checked me out to see what the problem was. I didn't tell her much, I just told her my shoulder was really bothering me from lifting. After the examination she told me she had to put one of my ribs back in place. The shoulder injury was secondary infact, that only happened because my ribs were pulled on so hard from the exercise that it got out of place, and it shifted all the force to my shoulder.
So still, the idea that an exercise can change your skeletal configuration does seem far fetched, but my injury seems like the exercise was trying to do just that.
On another note, I don't do these anymore. hahah
I used to do these and did like them. I would use them at the end of my chest workout. I felt it really hit my upper chest when i sqeezed at the top of the movement and also the serratus. I did feel a stretch in my rib cage, but a stretch was all it was, nothing more like it expanding or anything.
I have heard it isn't the bones that lengthen but the cartilage between the ribs can grow? Is that true?
^^ i doubt it....the myth of expanding rib cages is more thanx to the position u develop when standing after doing some, chest goes high and it "feels"like the ribcage is extended, plus i think long term use of the exercise can change ur posture to where u walk more upright thus feeling like having a expanded rib cage