You are just not used to placing a good majority of your weight onto your fanny while riding your MTB. You get used it after a few days of riding.
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"No drug, not even alcohol, causes the fundamental ills of society. If we're looking for the source of our troubles, we shouldn't test people for drugs. We should test them for stupidity, ignorance, greed, and love of power." -P.J. O'Rourke