When bones see increased amount of stress, such as an increase in running when preparing for a marathon, they respond by putting more bone in the areas of the bone that are seeing more stress. However, if the area of bone sees stress too fast, the bone will actually begin to fail. As the fracture worsens, the pain continues after exercise. This may lead to a limp and even pain at night. If untreated the bone can actually break, which can be a severe injury depending upon which bone is involved.
Cartilage also needs stress applied very gradually.
Cartilage is the white tissue on the ends of the bones at the joint that allows the bones to glide and move smoothly over one another. As a person matures, it is common for the cartilage to see some wear and tear so that it is not perfectly normal. When the cartilage sees too much stress too rapidly, it first causes pain in the joint and secondarily fluid in the joint. Swelling in a joint is a bad sign and means that the cartilage is not happy. If the joint is not rested, the pain and swelling can increase and cause an inability to use the joint for not only exercise but for activities of daily life.