why does it hurt to swim too fast at low SPL?

[Danny]

This question arose in Werner's thread, but the subject there quickly moved into other technical stuff so I'll pose it here separately. According to the coaches, as you up your stroke rate and try to hold your SPL constant, it can start hurting your shoulders. I would like to understand this better.

As your stroke rate increases, your rotation tends to decrease, and this can make it more taxing on the shoulders to get a good catch. Is this the reason?

I view all shoulder stress as occurring in the front quadrant. The more you rotate, the less stress you will have when you start your catch. When I try to sprint (largely a pathetic undertaking..) I focus on rotating faster, but I try to keep the amplitude of my rotation from decreasing as I up the tempo. As a result, I don't think I experience this shoulder stress. Instead, I just notice that my timing is starting to fall apart when I go too fast, and I am getting out of breath. When the timing falls apart, I start trying to do too much of my stroke with my arms, as opposed to with body rotation. This can also hurt shoulders, but I'm not sure.

Can the coaches, or anyone else, clarify?