Stiff muscles and joints may have various causes. Overtaxing is the most common one, e.g. by working out too heavily while untrained, or doing jobs around the house which overburden the body. The stiffness is caused by an accumulation of metabolic products in the muscles. Joints may become stiff from overtaxing, but also as the result of a fall. Even more often rheumatic disorders are involved, such as joint inflammations, wear and tear of joints, and muscle cramps in the lower back.