As Triathletes I find that some people rely on the fact there is so much activity required to get in the appropriate training in swim, bike and run that they feel fitness is the by product why worry about adding to that. Well if you look at the movements that are being used in swim, bike and run your legs move forwards and backwards, yet the hip has many positions it can move in, so the problem is the hip needs to be strong in all positions, so you must work it in all positions, the same goes for shoulders. We begin in the swim asking our shoulders to pull us through the water, we may not have the best stroke, so we start to see some disfunction in our movement patterns in the water, if the shoulders are weaker it also leads more strain. We take those tired shoulders and ask them to support us through out the ride, and then to be happy on the run.
I get it that the shoulders are not as important through the bike and run, but ask someone with a shoulder problem if it creates an issue and the answer is yes. That disfunction of the shoulder can become a bigger issue the longer the race is.
Shoulder tweaks, I hear this a lot, "I tweaked my shoulder". Sometimes this is a sign something is not being done correctly and the shoulder is not completely supported. Our internal rotators are a lot stronger than our external rotators, this can become a real imbalance which is made worse by poor posture and bad habits.
My advice is get into the rotators and work them, make sure you are working them thoroughly, external rotation, upwards, external rotation downwards, internal rotation upwards and internal rotation downwards. I use "The New Power Program" by Dr. Michael Colgan. His periodized approach to strength training is exactly what the new triathlete needs to make sure they do the foundation building of their muscles to make sure they are ready for the training of triathlon. His approach to balance in strength and his method of building the strength of our stabilizers and rotators first before we ever move on to the stronger muscles is so common sense and usually so overlooked. Smart training for the smart triathlete.