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

[Danny]

I guess this is different for each person, and Talvi is doing a good job of trying to describe how to start exerting force, but for whatever it's worth, here is the gauge I use. Sometimes when I am swimming, I start to notice that my arms are getting tired and I feel like I am working too hard. When this happens, I start waiting longer to apply the pressure, and I focus more on spearing forward as opposed to pushing back. I feel like I've got the timing correct when my arms aren't tired any more and I sense that most of the power is coming from body rotation, not from working the arms. It's a wonderful feeling because I haven't lost any speed, but I no longer find it tiring!