Narrator: This is Science Today. Often times in the workplace, physical workload and job stress co-exist. That's why it's been hard to isolate if back injury may be solely stress-related. But Dr. Niklas Krause, of the University of California, Berkeley, was able to disentangle the two effects by studying San Francisco bus drivers.

Krause: We, for the first time with adequate methodology, proved that psychosocial job factors - job stress and physical workload - are both independent predictors of workplace injury - of low back injury - in that group of public transit operators.

Narrator: It's been long thought that stress is not an objective measure but rather a characteristic of one's personality.

Krause: And the firefighters can determine - do they really want to light that fire today or maybe they should wait until a couple days later.

Narrator: For Science Today, I'm Larissa Branin.