Probably more than you think. Picture the back muscles as rubber bands. When you bend forward, you stretch them out. It takes time for the muscles to recover their strength afterward. Without adequate recovery, they're not able to properly support the vertebrae in the spine. This makes the spine loose and unstable, setting the stage for back injury.

A recent study looked at back muscle recovery after just 20 minutes of uninterrupted bending. Researchers found that even seven hours of rest weren't enough to return the back muscles to 100 percent. After seven hours, the muscles had only reached 79 percent of their normal elasticity. (Most of this recovery happened within the first ten minutes of rest.) Researchers suspect that 24 hours may be needed for muscles to recover fully after keeping the spine bent forward for long periods.

You may not have 24 hours to rest between bending movements. Still, it's important to take precautions. Talk with a doctor or physical therapist about specific things you can do to protect your low back.