I have chronic back issues. Here's what I found if you are looking for a full scale bass:
1. Shop at
Sweetwater. They list the weight of all instruments over $400.
2. Ibanez makes many full scale basses that can go down to 7 pounds. All models. You just need to keep an eye out for a light one. Additionally The Ibanez Mezzo (med scale) can go as light as 6 pounds.
3. Spector NS Pulse basses can be very light (mine is 7.2 pounds).
4. In the higher range look at EBMM Stingray Specials (mine is 7.5 pounds).
5. Fender mades a Lyte P-Bass that can get into the 7 pound range. I believe the Aerodynes can get into the 7 pound range as well.
6. Most short scale basses can be very light. I have a 7 pound Gretsch Electromatic that balances and plays well.
The only issue I have found with some of the lightweight basses is that they can sometimes be neck divey. Try before you buy, or shop at a place with a good return policy (or be OK with swapping the tuners for some Hipshot ultralites). Additionally, string selection will be critical on short scale basses. Expect to burn some money on finding the right strings (and gauges).
Stay away from Luthite bodies.....nasty stuff that shrinks over time and will cause you a plethora of issues.