In reply to Dave the Rave:
You're spot on as it turns out. I had an appointment with a fantastic doctor this afternoon who took the time to properly examine me and he said it was the serratus anterior muscle. Everything he described made sense. I'd never heard of the muscle before but I did suspect on a couple of occasions that it was something other than the intercostals (or in addition).
It's a long story (but quite an interesting one). I picked up an ankle injury way back in 2003 which, although pretty painful and swollen at the time, didn't cause me any problems until 2007 when the bottom of my foot (around the ball of the foot) swelled up. I later (not until 2013) discovered this was because of a lack of dorsiflexion in the ankle (brought on by that initial injury in 2003), which was exacerbated by wearing winter climbing boots. Because of the rigid sole, every time I took a step (so on the approach to routes or the walk out) my toes would claw at the ground. Over the course of a winter, this would cause swelling that was so painful I used to wince if the sole of my foot even touched the sides of the boot when I was tying to put the boot on. Ironically, when climbing it was absolutely fine.
The swelling would go down when I hadn't been wearing the boots for several months but over the following few years the swelling would occur sooner and sooner until the very first day of wearing them would result in severe swelling. I sought a lot of help during this time but the problem was always diagnosed as a biomechanical issue, and even though the swelling was only occurring in one foot, corrective aids like orthotics were given for both feet. The actual core problem - the lack of dorsiflexion - wasn't identified as the problem until 2013 when I met with an excellent podiatrist. It might seem like a no brainer in hindsight but he was the only person who checked my dorsiflexion in both legs and, when he noticed that one was much worse than the other, asked if I'd ever had an injury in that ankle. I had to think initially, it hadn't ever occurred to me that the swelling in my foot could be linked to an injury I sustained 10 years before when I missed the kerb when walking home from work one day and went over on it.
The podiatrist gave me a bunch of different exercises to work on and things started to progress ever so slightly by the end of 2013. The main exercise I was doing was to stand facing a wall, a few feet back, and just bend my knee in toward the wall while keeping my foot flat. By February 2014, I had only gained a few mm of movement but at least it was movement. And then during one session, the area I was stretching just gave way and my knee lurched forward. I had suddenly gained about 2 inches of extra movement.
The net result, however, was that over the next few days I was flooded in pain. One night, I experienced a tingling sensation down my whole left side, from my head to my toes, when lying in bed. I didn't know what was happening but it scared me. It had gone by morning but was replaced with a deep throbbing ache around my right shoulder blade, shoulder, and right arm. Over the course of the next few weeks I discovered my shoulder blade would protrude straight out of my back whenever I did a plank or lay flat on a bench with my arms raised. This winging caused a bit of alarm with a fairly new physio I saw, but her more experienced colleague had seen it before.
More exercises, and a referral to a professor, who told me that the cause of it was because my immune system had experienced so much pain that it led to an electrical misfiring of my nervous system, with the nerves controlling my shoulder blade ceasing to work as normal. When he carried out nerve tests on the area around the start of 2015 there were positive signs of response but he did warn me that it could take up to three years from the time of the initial winging to fully recover. I still do four sessions a day of ankle physio to try and increase my dorsiflexion and I'm pleased to say that I now have almost full flexibility restored. But there could still be a long road ahead, I just don't know.
When I was doing the pull up session that's caused this latest injury, it felt like a slippage of the muscles from the shoulder blade area round to my lower rib cage. The injury just felt like it wrapped around my right chest wall. And there have been a couple of instances since then when I could feel an ache coming from the shoulder blade that made me stop and think it could be related to that previous weakness.
Sorry for such a long post, it's been quite therapeutic to write all of this down. It really emphasises just how linked our muscles are, and that a seemingly innocuous injury can have huge consequences later on.
If anyone has any experience of an injury to the serratus anterior, I'd be grateful if you could let me know.