Upper back or lower?
When you ride (and when you swim), make a conscious effort at relaxing your back and elongating your spine, instead of hunching over.
Also, it's a good idea to stretch both your legs and your core before and after your rides & swims.
Speaking of core, it would also be a good idea to do some abdominal exercises once or twice a week, like crunches for example.
You can ride 5 miles and swim without exerting yourself and getting hurt like this--just breath deeply, stay relaxed, and maintain a smooth, steady effort.
Last edited by calamarichris; 06-21-10 at 02:48 PM.