Biomedical engineer Jacek Cholewicki is beginning to answer that question and to explain this surprisingly common form of injury. Cholewicki has determined that the spine of the lower back is more vulnerable to damage when the body is at rest than it is during periods of physical exertion.

This finding comes from his research into the relationship between the motor control system of the lower back and the incidence of chronic low back pain, a common ailment that is poorly understood.

Low back pain is the No. 2 reason---behind the common cold---that people see a doctor. Most cases resolve themselves in about six weeks. But for many sufferers, the pain keeps coming back.

"The spine itself is a very unstable structure," Cholewicki says. "It's a bunch of joints piled on top of each other. Without muscle support, the spine is very flexible and will buckle under very low loads." Studies have suggested a link between sudden and unexpected movements and low back injuries. But the underlying mechanisms have so far eluded researchers.

Cholewicki, Ph.D., an assistant professor of orthopedics and rehabilitation at Yale University, is investigating the problem in a new way by combining data from anatomical three-dimensional models with data from a series of other models that describe muscle tension and stiffness, external forces and their impact on the lumbar vertebrae, and the flexing of the tiny muscles and passive tissues swaddling the spine itself.

The results are shedding new light on chronic low back pain and may lead to new ways of preventing and treating the disorder.

In recent experiments published in Clinical Biomechanics, Cholewicki measured the nerve, ligament and muscle action of three young men holding hand weights while standing, lifting, bending, sweeping, twisting, pushing and pulling.

Each volunteer performed the seven activities twice while being videotaped with two synchronized cameras set at right angles to each other. From these images, digitized at 15 frames per second, Cholewicki calculated three-dimensional coordinates for the actual center points of each joint in the lower spinal column. As the video rolled, the test subjects were also recorded by a 14-channel electromyograph sensitive to the electrical activity of muscles. These recordings came from electrodes taped at 3-centimeter intervals over nine muscle groups of the upper torso.

Data recorded by the video cameras and the electromyograph were combined and analyzed. The results showed, among other things, that spinal stability increased with the increase in stress. "The lowest level of stability was present when there was no demand on high muscular forces, such as upright standing just prior to the beginning of a lifting trial," Cholewicki notes. At that time, spinal stability relied predominantly on the passive stiffness of the intervertebral joints.

Cholewicki speculated that since spinal damage is unlikely during periods of rest, the body conserves energy by reassigning the job of safety from large muscle groups to passive tissues. A quick jump from a state of rest to one of exertion requires that the motor control system immediately arouses muscle support to cushion the spine. If there is a motor control problem, injury can result.

"This highlights the importance of motor control in switching between the large muscles and the small, intrinsic spine muscles when handling small loads," Cholewicki says.

In his current work, Cholewicki is taking a closer look at the differences between the patterns of activity in torso muscles of chronic back patients and those of healthy individuals. He believes there may be a fundamental difference between the two groups that underlies the disorder.

For this study, Cholewicki is collecting measurements from 30 volunteers, 15 back patients and 15 healthy individuals. So far, he has completed tests on 11 from each of the two groups and has begun to analyze the data. In the study, a seated test subject is fastened into a restraint that holds the waist secure but allows freedom of movement from the waist up. The individual is pulled or pushed in one direction with gradually increasing force, then suddenly released. The test subject's reaction is recorded by video cameras and by an electromyograph.

An electromagnetic field source and sensor are used to record trunk motion. The position and orientation of the sensor can be measured in three-dimensional space by detecting the strength of the magnetic field. This gives a clear picture of trunk position independent of the video image.

The two study groups are reacting very differently. In pulling tests, healthy individuals contract all their muscles at the point of release, while back patients do not. In pushing tests, normal individuals react to the release in 40 or 50 milliseconds, while back patients respond after 200 milliseconds or more.

"We don't know whether the differences are causes or effects. At this point we are just observing the differences," Cholewicki says.

In the next phase of his study, Cholewicki will investigate whether the slower reaction times of back patients are a result of injury or a predisposing factor. If the latter is true, it may be possible to devise therapies to improve a patient's reaction time and thus decrease the likelihood of chronic injury.