Diving as well as other water activities does place added demands on our bodies. Immersion in water, even without additional activity, causes stress on the heart and lungs. To dive safely, we all must maintain a level of physical fitness that is beyond sitting in a chair watching television. Even a small amount of regular exercise will help you in the water. However, a person who runs regularly may not necessarily be in the best shape for water activities. Different muscles are used, and each of us must adapt to the physical challenges diving brings on us. In very general terms, persons over age 40 should not start an exercise routine, begin a new sport, or take up scuba without consulting a physician to be sure you can proceed safely.