For anyone's who interested in the muscles that ACTUALLY support you while your riding, you need to check out Paul Chek!! He trains Ryan Hughes for one, the Russian weight lifting team, US Olympic team, etc etc... he knows his stuff!
The key to proper riding is postural strength... check out Ricky, Reedy and Bubba, they all look the same when they ride and that's down to musculature of the spine and abdominal wall. And no, it's not because of the flashy six pack (rectus abdominus) either, it's the entire area - obliques, TVA and illiopsoas muscles!!
The six pack (RA) is designed to stop your torso peeling backwards if you trip over and so contracts when you accelerate. Your torso and head want to go backwards while the bike shoots forward. Hit the throttle and your RA contract to keep the distance between your chin and pelvis constant. All good news...
Come into a corner seated though, lean to the side and correct your balance all the way through, then stand up on exit - this is NOT done by the six pack at all, but is controlled by the obliques. So forget crunches !
Land from a jump, stand up, sit down, ride a series of bumps and it's your TVA, obliques AND RA that work together to maintain a solid torso. But keep in mind that the TVA supports the lower vertabrae of the spine when contracted, and you'll realilse this is the baby you need to strenthen to protect your spine and hold a good posture. If you want a quick demonstration, take a breath into your diaphram (feels like your stomach's breathing), hold it and then pull your belly button in as hard as you can... that is the TVA, and that muscle is the one dirt bikers need to stregthen more than anything!
Turns out, crunches do NOT exercise obliques or the TVA at all, and the six pack is only partially stimulated by them either!!!! It's all to do with range of motion, but that;s a whole nother subject!!
Prove it to yourself!... next time you go riding, squeeze the tank with your legs, pull that belly button in hard and relax your hands on the bars.... suddenly you'll understand why your hands hurt! Because you're hanging on with your hands as your abdominal/postural muscles are too weak to support your torso!!! THIS is the key to bike control for the top guys, some of them just don;t realise they;re doing it as they've been riding longer than we can even imagine!
The point of all this rambling is, there are dirt bike specific exercises you can do, but be very wary of gym machines and instructors who don't understand the forces applied to the body on a dirt bike!
If anyone's keen I can try and describe a few? I usually charge for this, but since I'm a MXer, and you all ride too............ we'll call it even :nod: