That is decidedly untrue. The big problem with cycling and swimming is that they are low impact and don't stress your bones an joints enough. Your joints and bones are just like any other tissue in your body. They respond to stress by getting stronger and when they are not stressed they get weaker. As in all things you can of course over do it, but the amount of running the average person would do (15-20 mpw or less) is very beneficial to bone an joint health.