This last bike ride was just "the straw that broke the camel's back." Cycling in the Aero position causes the pectoralis minor muscle on your chest to shorten to pull your arms forward. Also, your levator scapulae muscle, which originates on your cervical vertebrae and inserts into the top of your shoulder blade, lifts your shoulders up, plus your trapezius is also shortened from the same static position. This is a movement that is made to bring you into the aero position.
Both of the muscles stay contracted the entire time you are in the aero position, and a phenomenon called "muscle memory" then causes the muscles to shorten. When you try to stand up the muscles are too short to allow your arms to go back down to your side, and there is a ripple-effect that then goes to the muscles of your back, so they are also involved.
I've been working with Ironman triathletes, as well as other endurance/Olympic athletes since 1989, and this work caused me to develop self-treatments that work to release tight muscles that can knock you out of the race. My first book was titled "The Pain-Free Triathlete," and that book has recently been greatly expanded to become "The Pain-Free Athlete."
You can also see my self-treatments for your shoulder
(and a lot more
) by going to my Instagram site, @JulstroMethod. I suggest you do the treatment for the top of your shoulder first, and then do the one that works on your chest. Finally, you can do the treatments for your upper back, but in your case I don't think these are key muscles causing your problem, they are just where you are feeling the pain.
It's like pulling your hair and your scalp hurts. You don't need to take aspirin or massage your scalp to stop the pain, you just need to let go of your hair. In the same manner, you feel the pain in your upper back, but it's the end-result of the other muscles pulling on the bones.
Wishing you well,
Julie Donnelly
YOU are your own Best Therapist? Visit www.FlexibleAthlete.com and www.JulstroMethod.com to discover logical solutions to chronic aches and pains, and sports injuries.