It makes sense that repeated loads on the spine put the back at risk for injury. Many studies show the risk of back stress or strain increases when the same movement is repeated over and over.
But how many repetitions are needed before this happens? And how much rest is needed to restore overused tissues? Doctors and engineers tested the limits in cats.
The animals were put in three different grous. Each group had the lumbar spine flexed over and over for 10 minutes followed by 10 minutes of rest. This cycle was repeated in group one three times. Group two had six cycles of movement and rest. Group three had nine cycles of movement and rest.
The researchers found that the number of repetitions on the lumbar spine does make a difference. With repeated motions the muscles contract and increase the stiffness of the joints. This is one way the body tries to prevent or limit damage to the soft tissues. The more a motion is done over and over, the stronger and longer the muscle contractions become.
At the same time the ligaments, joint capsules, and fascia lose their elasticity. The ability of these tissues to stretch and loosen up is called creep. Repeated motions reduce the creep of soft tissues.
Even after rest, the tissues are unable to recover fully. The worker exposed to repetitive motions starts the next work day with less creep than the day before. Tissues that aren’t allowed to heal with limited creep are at risk for chronic inflammation and disability.
According to these authors seven hours wasn’t enough to restore creep in cats. More studies are needed to find out just how many repetitions it takes to cause a cumulative trauma disorder of the human spine. How many hours of rest is needed to restore creep each day is also being studied.