Though I'm not a sprinter, on long runs around an hour (especially on flat, straight roads) I often find that my low back is also tight. Assuming you are doing your sprints on a track or other flat, even surface, the cause may be the same.
Similar to how your back or legs may ache after sitting in the same position in a car/bus/plane for a long time, while running on a flat surface your back is being held in a fairly constant position -- with the added stress that many of the muscles may be contracted.
To alleviate this, I find it helpful to pause every 15 to 20 minutes (or when waiting to cross the road) and do two simple exercises:
- Squat down all the way to the ground so that my knees and hips go through a full range of motion
- Standing up straight, bend forward at the waist as far as I can, adding a slight knee bend as necessary for my low back to go through a full range of motion
I'll do each of these, slowly, once or twice. I find that this releases some tension and keeps me from feeling so sore at the end of the run.