Cycling is considered a good, safe exercise because it does not cause much impact on sensitive joints such as the knees and the hips.

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But that quality also means that it does not stimulate bones to maintain their density, according to a report cited on RunnersWorld.com.

It said that the study showed that serious cyclists who rode seven to 22 hours a week for an average of nearly 10 years had lower body fat, more muscle and took in more calcium than a control group.

But they were also 2.5 to thee times more likely to have osteoporosis or near-osteoporosis.

The magazine suggested that adding some running and weight lifting would stimulate the bones to keep their strength up.