This is a good question. Finding the optimal work-to-rest ratio needed to prevent soft tissue injuries would save many workers from discomfort and lost wages. Some engineers are studying the spine's response to repeated loads.
t's been proven that repeated motion does put the soft tissues at risk for injury and trauma. It's not clear exactly how many movements and what conditions lead to soft tissue damage.
Another factor scientists are looking at is a concept called creep. All soft tissues that have some elasticity can stretch or "give" a little. They can also contract or tighten up. The amount of elastic movement of the soft tissues is referred to as creep.
Repeated motions cause the tissues to lose their creep. The muscles around the joints contract to increase stiffness. This is a way to protect the joints from too much load in a loose position. Unfortunately the body doesn't always get the creep back it needs with rest.
A recent study using cats showed that even after seven hours of rest, creep wasn't restored. The cats started the next day with less creep than they had the day before. It's possible in the human body that the more creep is lost, the greater the chances of a disabling injury.
More studies are needed to fully answer these questions in humans.