A stress reaction sometimes called a stress fracture is an overuse injury to the bone. It occurs as a result of strain on the bone. Damage occurs from the strain of repeated load cycles. The amount of load to cause a stress reaction is much lower than the stress required to fracture the bone from a single traumatic event.

Stress reactions are linked most often with vigorous exercise. Exercise with repetitive, weight-bearing loads, like running or marching cause bone reactions in the pelvis, hip, or leg. Repeated rotation of the spine is linked with stress reactions of the spine.

Stress reaction is a better term than stress fracture because no fracture line is seen on X-ray. The bone reacts to the increased load by trying to remodel itself. The bone can handle a small amount of remodeling.

Too much repeated stress speeds up the remodeling too fast. In an attempt to make better bone, some bone cells are absorbed to be replaced by newer, stronger units. But the resorption process is faster than the building or remodeling. The bone gets weak and the affected person experiences pain. With enough stress or increased load, the weak bone can fracture.