Well, now I've done it. After making a solid commitment to get to my golf and with plenty of time (and a pass from the wife), I've overdone it and have strained a significant muscle in my rib area. It's called the serratus anterior and from what I can glean on the web (plenty of discussion on the Sandtrap) it's a fairly common, but painful, golf injury.
To all of those golfers who laid off for a while and are at it again or are just older, do yourself a favour and strengthen your core muscles AND this muscle in particular.
It is just above your obliques and does a fair amount of the pull on your downswing. The pain started in my lat/shoulder area and I laid off for a day or two (thought that was enough). After I started up again, it moved to my side then just below my pectoral muscles. It is now very painful -- feels like a broken/bruised rib.
Initially I thought it might be the intercostal muscles, also a common golf injury. This is what B.Sneds and P.Casey had.
Anyway, just thought I would remind all of you that golf fitness is just important as the actual swing. Something I'm now learning the hard way.