Fruit & Vegetable Magazine

Equipment Spraying
More breaks may be needed to avoid back injury, study says

April 25, 2008  By Fruit & Vegetable

Workers who lift for a living need
to take longer or more frequent breaks than they now do to avoid back
injury, according to a new study.

Workers who lift for a living need
to take longer or more frequent breaks than they now do to avoid back
injury, according to a new study.

The study also suggests that people who are new on the job need to take
breaks even more often than experienced workers, and that the risk of
injury is higher at the end of a work shift.


People who participated in the study where required to lift boxes onto
conveyor belts for eight hours, while researchers measured the amount
of oxygen that was reaching the muscles in their lower back.

The oxygen level indicated how hard the muscles were working, and
whether they were becoming fatigued, explains Dr. William Marras,
professor of industrial welding and systems engineering at Ohio State
University. His research has shown that muscle fatigue is linked to
back injury.

The study, which appeared in the journal Clinical Biomechanics, is the
first to examine what happens to muscle oxygenation over a full workday.

Despite the fact the study participants were performing the same job at
the same pace all day, their back muscles needed more oxygen as the day
went on.

Taking a half-hour lunch break helped their muscles recover from the
morning’s exertion, but once they started working again, their oxygen
needs rose steeply and kept climbing throughout the afternoon.

“That was alarming to us, because it means that their muscles were
becoming fatigued much faster during the afternoon, and we know that
fatigue increases the risk of back injury,” Marras says.

Two 15-minute breaks – one mid-morning and the other mid-afternoon –
helped muscles recover a little, but not as much as the half-hour lunch.

“This tells us two things,” says Marras. “First, rest is good – a
half-hour break does a good job of helping muscles recover. But it also
tells us that people are especially at risk for back injury at the end
of the day, and the only way to counteract that effect is with more
breaks as the day goes on.”

Ten people participated in the study, six of who had at least one
year’s experience in a job that requires lifting, such as stocking
store shelves. The other four were considered novice lifters.

One person would lift a box from a waist-high stand and set it on a
chest-high conveyor belt in Marras’ lab, which simulates a typical
shipping center. The box then traveled down the belt to the other
person, who would lift it and set it on another conveyor belt. They
lifted boxes of three different weights – two pounds, 11 pounds, and 26
pounds – and they worked for the entire eight hours, except for the
half-hour lunch break and the two 15-minute breaks.

Each person wore a Lumbar Motion Monitor, a device that Marras designed
to measure the movement of the spine. They also wore oximeters on their
lower back – devices that measured the oxygen level of their muscles
through the skin. Just like the pulse oximeters that doctors clip to a
patient’s finger, these sensors use an LED light to detect the flush of
colour to the skin when blood carries oxygen to the tissues underneath.

Study coauthor Gang Yang, a medical doctor who is now earning a
doctoral degree in biomechanics at Ohio State, said that the
researchers’ top priority was making sure the subjects didn't grow
fatigued enough to become injured during the study. The heaviest box
they had to lift, 26 pounds, weighed less than half as much as the
loads that some workers are routinely required to lift in industry.

In Clinical Biomechanics, the researchers detailed the oxygen levels in
the muscles of the typical study participant. During the first two
hours of lifting, the oxygenation level gradually increased until it
reached 11 per cent above resting level. During hours two to four, it
rose to 13 per cent. It returned to resting level during lunch, but
immediately rose 11 per cent as the people started lifting again during
hours four to six. During the last two hours of the day, oxygenation
rose to its highest level – 16 per cent above resting level.

“Because the oxygen demand at the end of the day was so much higher,
that’s when we’d expect people to get hurt on the job,” Marras says.
“And the data I see coming out of industry bear that out – people tend
to hurt their back toward the end of a shift.”

Meanwhile, data from the Lumbar Motion Monitor showed that the
participants used their muscles differently as they became fatigued – a
finding that meshes with Marras’ previous work. He’s found that when
people’s back muscles begin to hurt, they tense up, and try to lift
with other muscles that don’t hurt as much.
“Now because of this study, we have a clinical reason for why that’s happening.

It’s because the muscles are becoming fatigued, because they have such a high demand for oxygen” Marras said.

Tensing muscles prevents proper blood flow, so the muscles are even
further deprived of oxygen. And using different muscles to lift may
lessen pain at first, but it increases the stress on the joints and the
spine, and increases risk of serious injury in the long run.

“When that happens, it’s like the muscles fight each other,” Marras
said. “You have back muscles that fight the abdominal muscles, and when
they both contract, it's like a seesaw effect, except you’re pulling
down on both ends, and your spine is in the middle.”

The researchers found that participants who had never lifted for a
living let their muscles tense up during the study. Their muscles also
needed more oxygen than the experienced lifters, who generally relaxed
their muscles and used the proper muscles for lifting.

“The bottom line is that it’s much more costly from a physiological
standpoint for novices to do the same work as experienced people,”
Marras said.

Taking half-hour breaks instead of the standard 15 minutes might help
reduce back injury, Marras says, although he acknowledged that such
long breaks might not be practical in industry. He pointed to other
studies, however, which showed that shorter breaks, taken more
frequently, have a similar positive effect.

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