Cycling is good, but all around physical activities is better. Swimming is good for low impact, but you still need to do other things. I have been told that for years, and I am still trying to keep with that. I like almost all sports, so that is a good thing for me.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun