As funny as it sounds, you learn how to crash well, absorbing impact and stuff. Also, I'm sure they stretch, and work out in order to be tough and flexible thereby preventing injury.
Air sense also helps. There is a difference between landing back seat, and landing on your head. As long as your feet get back under you, usually you won't be too bad.
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When you assume, you make an ass out of u and me.(think about it)
'My knee hurts' (Jeff Merat after grinding a lunch table for an hour instead of sitting in the ski patrol shack for his torm mcl and acl which he got earlier that day.)
'I'm not asking for any help, just mabe for you to get off your ass.'(my dad)