Karate people spend a great deal of time punching. Punching requires stable shoulders. When the chest is overdeveloped in relation to the back, the shoulder may actually loose stability. This is especially true if the joint is not properly stretched. Many groups do a cursory shoulder stretch before an intense workout, this can cause even more problems.

My shoulders are grindy garbage from years of judo, aikido, and karate. In the last few years, thanks to some very helpful coaches and personal trainers, I have learned to spend as much time taking care of my shoulders as I do abusing them. They still flair up from time to time and I end up looking like a bad Irish clogger when I am on the dojo floor (no arm movements and no rhythm either).

Here are a couple of sites that you may find beneficial; Shoulder Exercises/Shoulder Injury Prevention, and Rotator Cuff Exercisers.

The site Shoulder Exercises/Shoulder Injury Prevention contains a discussion of shoulder stretching as it relates to training and conditioning about halfway down the page.

If you spend time in a local gym, find one of the certified personal trainers and arrange an appointment. Most gyms have them. There are differences in quality between the various organizations that certify personal trainers, however I have found that most of them are pretty reliable as far as communicating the basics of rc injury prevention. They can be expensive, but a one off in which you tell them about your training and ask them for suggestions, is likely to be well worth the expense.

I do not think that being traditional means we have to ignore good sports science.