A recent study funded by the National Institute for Occupational Safety and Health shows the relationship words that start with i between muscle fatigue from repetitive lifting and back injury. It was conducted by an Ohio State industrial systems engineer and an MD who is studying biomechanics. The study which recently appeared in Clinical Biomechanics is the first to examine muscle oxygenation throughout the workday.
Ten people participated in the study; six had at least one year’s experience in a job that requires lifting, such as in stocking shelves. Four were considered novice lifters. Each person wore a Lumbar Motion Detector, a device that was designed to measure the spine’s movement. They wore oximeters, a gauge that measures the oxygen level of the muscles – just like the pulse oximeter, a device that clips onto a finger, measures oxygen level of the blood.
Study participants lifted a box from a waist high stand and set it on a chest-high conveyor belt. The box traveled on a conveyor belt to the next participant, who would lift the box and set on another conveyor belt. The boxes weighed 2 pounds, 11 pounds, and 26 pounds. Participants worked 8 hours with a half-hour lunch break and two 15 minute breaks.
Researchers said that the 26 pound box weighed less than half as much as loads that some workers are routinely required to lift in industry. The researchers studied the oxygen levels in the muscles. After 2 hours of lifting, the oxygen level gradually increased until it reached 11% above the resting level. During hours 2 to 4, it rose to 13%. During the lunch break, the oxygen level returned to the resting level but immediately rose to 11 % with lifting during hours 4 to 6.
During the last 2 hours of the work day, the oxygenation level rose to 16%, the highest level of the day. This oxygen level indicates how hard the muscles were working and whether they were becoming fatigued. Citing his study, Professor Marras of Ohio State, says, ” 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.” As muscles fatigue and begin to hurt, workers tense up and try to lift with other muscles that are not as painful.
Tense muscles constrict blood vessels, preventing the flow of oxygen and nutrients to the tissues and the removal of waste produced by muscle activity. While using different muscles may bring relief at first, it increases stress on the joints and the spine, increasing the chance of injury. The muscles of inexperienced lifters tensed up more quickly as they needed more oxygen. Injuries were most likely to happen during the last 2 hours of a shift when muscles were the most fatigued.