Lisa SH - going downstairs works a little better because it requires a different type of muscle contraction than going up. Muslce forces on the bone also act to improve bone density, as long as weight is being applied to the body. When you go down stairs the physical pull on the bone from the muscle action is greater. This is why it helps more.
This is also why weight training is important even if it is low impact.
Unfortunately, with road cycling, your body weight is supported and the muscle pull on the bone with each contraction is not that large, especially if you are spinning at a high cadence. That's why bone density can be a problem in cyclists.
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