However, whereas walking is a low-impact activity because one foot is always in contact with the ground, in jogging and in running, impact is introduced. This is a high-impact exercise that places strain on the body, notably the joints of the knee.
Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as x-ray photographs. Radiation can damage cells and make them cancerous.
In 1965 the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered.