The kick-off event of the 2012 AMA Motocross Championship season happens this weekend at the Hangtown Motocross Classic in Sacramento, Calif. And it’s an Outdoor season chock-full of sub-plots and intrigue ready to play out over the course of a grueling summer series.

The Outdoor season is unique in that it’s a battle of attrition rather than a sprint for the finish like the Supercross series.

Outdoor races take place in triple-digit heat over long, difficult tracks set up to take the meaning of “arm pump” to hellacious new levels. Fatigue and injury are a factor at every stop on tour, and the season is a real test of fitness for some of the fittest, toughest athletes on the planet.

2011 AMA Motocross Champ Ryan Villopoto hurt his knee at the Seattle Supercross race and will not be around to defend his title (he just won the 2012 AMA Supercross title, too) against the likes of past champions Chad Reed and Ryan Dungey and young upstarts like Justin Barcia, Brett Metcalfe and Mike Alessi.

Just to keep things interesting is the news that 2008 Outdoor Champ James “Bubba” Stewart will return to race the entire ’12 Outdoor series. If Bubba’s return to the Outdoor isn’t enough to make things interesting, following the last race of the 2012 Supercross season he broke his contract with Joe Gibbs Racing/Yamaha and picked up a brand new bike sponsor in the form of Yoshimura Suzuki Racing. That’s right, he’s on a Suzuki now and he’s riding the Outdoor series essentially for free. His contract with Yoshimura Suzuki doesn’t technically begin until 2013.

Check out the 2011 season premiere of FUEL TV’s excellent “The Moto: Inside the Outdoors” for a primer on Hangtown and the Outdoor series: