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(Last updated 11/27/00)
The Ohio State University Institute for Ergonomics is dedicated to ensuring the design of safe and effective work environments and consumer products.

Previous stories involving Professor Marras' work:
"Lifting Patients Poses High Risk For Back Injuries," 1/20/98.
"Back Support Belts Don't Prevent Lifting Injuries, Study Shows," 6/20/97.
Previous stories involving Professor Heaney's work:
"Chronic Job Insecurity Making Manufacturing Workers Ill," 3/27/96.
"Workers Not Using 'Mental Health Days,' Possibly With Ill Effects," 4/25/96.
"Stress Encourages Smokers To Consider Quitting, Study Shows," 1/21/97.


COLUMBUS, Ohio - On-the-job stress doesn't just strain the nerves -- it can strain the back, too.

That's the implication of a study at Ohio State University that provides the first-ever link between stress and back pain. The findings should be significant to businesses that depend on physical labor from their workers.

"We found that psychological stress seems to amplify the physical demands of lifting for certain personality types."

The Impact

So, how important is this research? A look at the scope of such injuries gives some new insight.

  • Back injury is the most prevalent and costly work-related musculoskeletal disorder in the US.
  • In the US in 1998 (most recent stats), more than 440,000 people missed work for one or more days because of a back injury.
  • This accounts for one-fourth of all nonfatal injuries/illnesses that caused people to miss work that year.
  • People with back injuries missed an average of 6 work days that year. However, almost 20 percent of people with back injuries missed 31 work days or more.

Researcher Bill Marras says that the medical costs for treating a lower back injury can add up to as much as $50,000. That of course does not include personal income lost, or the cost to the business that lost the worker for that time.

Source: US Bureau of Labor Statistics

The study, which appears in the December 1, 2000 issue of the journal Spine, found that people with certain personality types may increase their risk of back injury if they experience workplace stress.

Ohio State co-authors William Marras, professor of industrial, welding, and systems engineering, and Catherine Heaney, professor of public health, and their colleagues tested how 25 college students reacted to critical and unsupportive supervision while lifting boxes.

Students who were distressed by the criticism used their muscles in ways that might lead to injury over time, the researchers said.

The results take a first step toward explaining why people with certain personality types -- namely, introverted people and
those who dislike performing repetitive tasks -- are more likely to report back pain on the job, Marras said.

"Sometimes, work isn't physically demanding, but psychologically demanding," Heaney said. "We found that psychological stress seems to amplify the physical demands of lifting for certain personality types."

Previous studies have shown a link between psychological work stress and back pain, but have not provided an explanation as to why.

"I always thought there must be some kind of pathway between the two, something that damages the spine," Marras said. "One theory is that stressed people move differently when performing the same tasks."

To test that theory, the researchers first gave each student a common psychological test called the Myers-Briggs Type Indicator. The test rates certain aspects of a personality, such as introversion or extroversion.

Alone in the laboratory with a supervisor, each student then lifted a 25-pound box several times. To complete the lift successfully, the student had to lift the box at a particular velocity.

The students wore the Lumbar Motion Monitor (LMM), a device Marras invented to record information about back motion. Because the LMM was hooked up to a computer monitor, the students could gauge their own performance and confirm whether they attained the appropriate velocity when lifting the box. The LMM recordings, in addition to measured electrical activity of the students' trunk muscles, enabled the researchers to create a mathematical model of forces on the spine.

For the first half of the experiment, the supervisor played the student's favorite music and offered words of encouragement, such as "Good job!" or "Way to go!"

For the second half of the experiment, the supervisor left the room, and then returned pretending to be in a foul mood. He switched off the music and told the student that the laboratory director wasn't pleased with how the experiment was going.

The researchers rigged the second half of the experiment, so even when the student lifted the box correctly, the monitor indicated that he or she had failed.

The supervisor then began criticizing the student's efforts, saying things like, "You can do better than that," or "What happened that time?"

All but two of the students experienced a rise in blood pressure during the second half of the experiment, indicating that they may have felt stressed.

But of the 23 students who felt stress, only two groups responded by using their muscles differently -- those who were rated as either "introverts" or "intuitors" by the Myers-Briggs Type Indicator.

"The criticism just rolled right off the extroverts, but introverts changed the way they used their muscles, so that lifting became much more mechanically stressful," said Marras.

When stressed, introverts began employing muscles in their abdomen or sides -- muscles that weren't necessary for lifting. As a result, different forces on the spine increased as introverts lifted the box. The same held true for intuitors.

For introverts, spinal compression increased by almost 14 percent, while sideways forces on the spine increased by about 27 percent. For intuitors, spinal compression increased by almost 11 percent, and sideways forces by about 25 percent.

Marras and Heaney said that over time such extra spinal forces could lead to back injury.

Why would introverts and intuitors react so strongly? According to psychologists, introverts commonly internalize feelings of frustration, while intuitors tend to dislike repetitive tasks. The authors speculated that introverts may have been particularly upset by failing at the task, while intuitors may have disliked having to repeat the task when they failed.

Heaney was surprised by the results. "I didn't think we would find much of a change in lifting from such a short experiment," Heaney said. "I thought, 'what could happen in 20 minutes?' But the students responded to the stress very quickly."

She added that real workers would experience a more dramatic response than the students, who had volunteered for the experiment and weren't required to perform the task day in and day out.

As to why stressed people may lift differently, Marras said the only analogous situation occurs for people who already have a back injury. They contract many extra muscles to support the spine as they lift, so if they need to stop lifting suddenly because of pain, they can. This is known as "guarding" behavior.

"I hesitate to call this behavior true guarding behavior, but it's the only thing I know of that's similar," Marras said.

While these results suggest that certain personality types may be better suited to handling the stress of particular jobs, Heaney feels making workplaces less nerve-racking in general is the best way to prevent stress-related back injuries.

Marras agreed. "It makes a whole lot more sense to design the workplace to be less stressful, because otherwise you're mixing and matching the needs of different employee personality types, and that would be really difficult to do. What may be a tolerable situation for one employee may be stressful for another," he said.

Marras added that while this research has revealed a potential pathway between psychological stress and back pain, such situations contribute to relatively few of the overall incidences of back injury.

"The vast majority of back injuries can be explained by the weight of the object, or the circumstances under which the person lifts. We're looking at a small subset of back injuries you can't readily explain," Marras said.

Marras and Heaney conducted this work with graduate students Kermit Davis and Anthony Maronitis, and former graduate student Gary Allread.

In the future, the researchers want to extend this work to actual workplace situations. Marras and Heaney are also co-supervising a graduate student who will continue the project for his dissertation.


Contact: William Marras, (614) 292-6670; Marras.1@osu.edu
Catherine Heaney, (614) 293-5837; Heaney.1@osu.edu

Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu