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I've watched a few trials events near me and it's pretty sick! There was even a FIM world event here in RI back in 2000 I think it was?
Most of the guys do only wear a half helmet and gear was minimal surprisingly. Though maybe it's not to surprising as they really have to move around in odd ways on the bike to keep balance.
This is something I've been toying with as well for a few reasons.... cost is low, developing the balance skills that will improve enduro style riding, and I have some sick rock gardens/boulders/ledge on my own property