"Lift with your legs" -- but how should you push?

Kevin Granata, one of the authors of the work described here, was killed in in the shootings at Virginia Tech on April 16, 2007.

A back injury can destroy a person's life. The pain can be so excruciating that even "passive" activities like sitting up to read a book become intolerable. Whether you work in a steel mill or sit at a desk, a back injury can make it impossible for you to earn a living. Even worse, for many of those who suffer with chronic back injury, is that because it's difficult for others to see what's wrong, there's a tendency to not believe the problem is "real."

Yet there have also been significant efforts to prevent back injuries. For decades, it has been known that lifting heavy objects is an important cause of back injuries -- nearly everyone knows that you should "lift with your legs, not your back." Workplaces have been modified to reduce the amount of lifting workers must do, replacing lifting tasks with pushing or pulling when possible.

But pushing and pulling tasks can also cause back injury -- accounting for 20 percent of all workplace back injuries in the U.S., U.K., and Canada. Fifty percent of all industrial materials handling jobs involve pushing and pulling.

Granata and Bradford Bennet recognized that while much is known about the stresses placed on the back by lifting, little research has been done on the problem of pushing and pulling.

The work that has been done hasn't taken the cocontraction of muscles into account -- the use of opposing muscle groups to stabilize the area of the body doing the work. They developed a mechanical model of pushing that incorporated cocontraction. Then they asked eleven volunteers to push against a special handle that measured the amount of force they applied. Goniometers measured muscle activity on the key areas of their back:

A display showed the volunteers the amount of force they were applying. They pushed until they met each of three target amounts: 15 or 30 percent of their body mass, or the maximum force they could apply. The handle was set at each of three different heights as well, for a total of nine different measurements per volunteer, in random order.

Granata and Bennet found that when cocontraction was not incorporated into the model, the forces on the back were indeed much smaller than those typically encountered in lifting tasks. But when cocontraction was factored in, the force on the spine increased by as much as 400 percent, depending on the height of the handle and the amount of force applied. Cocontraction was greatest when participants bent lower to push on the handle, as they would when pushing the heaviest loads. In these cases, stress on the spine matches stress in lifting tasks and may be what leads to the most injuries.

They also note that as the amount of pushing force increases, the vertical component of the pushing force also increases, because the volunteers need the corresponding downward force on their feet to gain traction. This makes a pushing action more like a lifting action, again potentially increasing the chance of injury.

Granata and Bennet recommend that their model, especially the cocontraction component, be taken into account when injury-prevention specialists determine safe load levels for their workers.

Perhaps if their recommendations are followed, and if sufficient follow-up study is performed, many thousands of workers will some day be spared the life-altering pain of chronic back injury.

Granata, K.P., & Bennet, B.C. (2005). Low-back biomechanics and static stability during isometric pushing. Human Factors, 47(3), 536-549.