Too Far, Too Much, and Too Often: Knowing Safe Limits for the Low Back
Past studies have shown that some low back postures and heavy lifting at work can put the spine at risk. But how can you know when you're bending or twisting too far, or if that box you're about to lift weighs too much? Scientists haven't carved that answer in stone--yet.

So how far is too far, and how much is too much? Researchers in the Netherlands have started to put some numbers to these questions. They recorded workers on videotape to see how far and how often they had to bend or twist. They also tallied how much weight the workers lifted over the course of the workday. They followed up with the workers yearly for three years. Here's what they found.

The more people have to bend, twist, or lift at work, the higher the risk. Bending forward more than 60 degrees and twisting more than 30 degrees appeared to be the angles that mattered the most. The chance of back pain was higher for bending, especially when people did it more than 5% of the day. Lifting less than 55 pounds didn't seem to be a problem. But when workers lifted at least 50 pounds more than 15 times a day, the incidence of back injuries gradually went up. These values are summarized below:

In their concluding remarks, the authors say the risk for low back pain is "moderate" for people who flex, twist, and lift at work, especially when they move too far and lift too much--too often.
References
Wilhelmina E. Hoogendoorn, MSc, et al. Flexion and Rotation of the Trunk and Lifting at Work Are Risk Factors for Low Back Pain: Results of a Prospective Cohort Study. In Spine. December 15, 2000. Vol. 25. No. 23. Pp. 3087-3092.