For cardiovascular exercise, weight-bearing, higher-impact activities such as walking, jogging and aerobics are better than nonweight-bearing or lower-impact exercise such as swimming, cycling and elliptical machines, which put less strain on your muscles and bones.

CNN: How much weight is needed to strengthen my bones?

The British Medical Association is also concerned that making cycle helmets compulsory would put many people off cycling, and that the resulting lack of exercise would be more dangerous than the risk of head injuries.

BBC: Cyclist