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Just for reference, I'm 5'3" and 113 lbs and I've found that to be my ideal racing weight. I do lift weights so have a fair amount of muscle mass. I believe that you can be too this, too light. If I go down to below 110 I do not have the strength that I need to compete on a mtbike... but that's just me.
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http://www.cyclingnews.com/fitness/?...4/letters05-17