Lavin was competing in a Dew Tour event in Las Vegas -- when he hit a dirt ramp, lost control, and slammed down onto the ground. Friends and crew workers immediately rushed to Lavin's side, but he didn't appear to be moving.
Lavin was rushed to nearby University Medical Center hospital ... where he was placed into a medically induced coma.
We're told Lavin is also experiencing brain swelling and is listed in critical condition.
UPDATE: A rep for MTV tells us, “Our thoughts are with TJ. He is an important part of MTV’s extended family and we’re all praying for him."
As for Lavin's condition, a rep for the Dew Tour tells us, "he is in the intensive care unit and is currently stabilized."