It's always fun to see forum discussions about whether seat or waist harnesses are better, especially when the opinions are presented with crusader-like intensity. I have to admit that I am faithless in this particular issue...
I had used a seat harness for more than 20 years until Andy Brandt told me to switch to a waist harness for freestyle (and, I believe, to improve my stance). So I did, without any problems. When I had tried a waist harness a few years earlier, it had given me back problems, but not this time. A bit later, when I got more into speedsurfing, I tried the seat harness again, but I did not like it, not one little bit, anymore.
Fast forward to the East Coast Windsurfing Festival: after the first day of races in high wind, which gave me plenty of opportunity to practice my catapults, and a night in a random hotel, I woke up with considerable back pain. I blamed the bed - some beds just screw up my back. Over the next few days, my back got better, but after a light wind session, some of the pain came back. So when we went windsurfing in Fogland yesterday, I decided to bring my seat harness again.
The wind yesterday was marginal. I'm a lazy sailor who likes to hook in as soon as I start moving, and thing about planing later. That did not work well with the seat harness; however, I did get to practice jumping up on the board to get out of the harness lines again. Things got better when the wind picked up so I could plane. I was quite amazed how different the feel of the seat harness was; it sure felt good on my back. It also felt better on my hips, where a few small muscles sometimes start complaining after sailing for a couple of hours. At the end of the day, the pain in my back was gone completely!
Besides the back pain, another motivation to try the seat harness again were the many blog and magazine articles that state that seat harnesses are better for speed and slalom. Indeed, I did have the feeling that the seat made it easier to keep the nose of the board quieter, probably because it's easier to keep constant harness line pressure. The GPS tracks did not show any difference in speed relative to the waist harness tracks, but the wind was very much up and down, so that means little.
So - why is the waist harness giving me a bit of back problems now, when it it worked perfectly fine for more than a year? I think the problem is that I stopped cross training - windsurfing is pretty much the only sport I am doing now. When I originally switched to the waist harness, I still did karate on a regular basis, and to the gym (boring!) every now and then. Both involved quite a bit of core training. So, time to start some core & cross training again! But at least for now, I think I'll switch back and forth between waist and seat harness, depending on what the conditions are, and what I'm doing in a session.