Originally Posted by Carbonfiberboy

Here's my primer on how to check a fit: How can I fitting my bike


How to get the back fit? The best way is to ride lots. Ride until it hurts, rest it for a day, then do that again. After not all that many repetitions, the pain won't start for many more miles. That can be eventually increased to 100s of kilometers.

Of course off-bike exercises help too, and will shorten the time to fitness, but nothing makes your back as fit as cycling, lots of it.