Recent studies show various soft tissues with elasticity have a property called creep. This tells how much tightness or looseness there is in ligaments, discs, joint and capsules. There is also a certain amount of creep in joint capsules.
As tissues shorten from use, they loose elasticity and the creep increases. With repeated work loads or movements, creep doesn't fully recover or return to normal after rest. Over time the amount of creep adds up. If the creep doesn't fully recover from one work day to another, then the workers starts the new workday with too much creep in the tissues. Creep causes damage to the tissues. The body tries to heal itself and sets up a cycle of inflammation. Continued motion keeps the spine from healing. The added creep results in chronic inflammation and disability.
Scientists are trying to find out just how many repetitions are needed to cause damage and injury to the spine. The goal is to give workers a work-to-rest ratio for safety in the work place.