Hi,
First, welcome!
Second, you might find some more info by using the search feature with "bodybuilding".
Now, I assume you are going to be in the audience? I'm going to assume so.
If so, do you know if flash is going to be allowed? I'm going to assume not, in which case you are going to have to bump up the ISO and work with the available, somewhat tricky stage lighting. For that matter, you definitely don't want to use the built-in flash, and didn't indicate if you have an accessory flash.
It's tricky because the contestant might be brightly lit, but a large area of the stage around them dark, which can fool you camera's meter and cause a lot of over exposure.
It also might be even trickier, if they use colored lights. I'd just try to stick with shooting when it's only white light, and relatively steady, which I would guess will be while they are doing going through their poses.
So, I'm going to walk you through setting up a manual exposure, which won't change on you constantly like it would if you used any of the auto modes.
First I suggest you set the camera to shoot RAW. That way, if exposure or color temp is off a little, you can adjust it right before making JPEG conversions.
You'll probably use the 17-85 almost exclusively for this, probably out near the 85mm setting. Switch the camera to AV mode and set the aperture as wide open as the lens allows. Now set to spot metering and try to meter off one of the people on stage and in the white light, making a note of the shutter speed reading reading.
Do you feel comfortable using the camera on M mode setting? If not, you might want to practice. Switch to it and set both the aperture and shutter speed you used previously. Ignore what the meter is now telling you to do.
Now take some shots of the people on stage, and play back the images on the LCD. Ignore how the image looks, and switch to the histogram. Hopefully you will see a nicely balanced "curve" on it. With a lot of dark area on stage, there will be a big "clump" on the LH side, that can't be avoided. Mostly watch the RH side, that the "curve" is just barely reaching over and touching it. This is the highlights, and you don't want them going past the RH edge too much or they will get "blown out".
The only remaining tricky part of this is finding an ISO, so that your shutter speed is high enough to allow you to shoot by available light. If you have done all the above, and your shutter speed is 1/100, double your ISO (say from 400 to 800 or 800 to 1600) and you should be able to double your shutter speed, to make it easier to hand hold the shots.
Even if you have Noise Reduction enabled, if you shoot RAW images I think you will need to reapply it, while doing conversions from those files. If you instead choose to make JPEGs (or make RAW + JPEGS), you should check in the menu, under custom functions, that Noise Reduction is set to either On or Auto.
Be sure to do a bunch of test shots of other people's performance, and get set up well before your friend is on stage. That way you can be ready and just take as many photos as you want, while your friend is performing.
At least, those are the steps I'd take.
Now, I know it's hard to evaluate images on the LCD, you really can't trust what the image tells you and it takes some practice to learn how the histogram curve should look in different situations.
One thing that helps is if you can go to the location the day before and make some practice shots with different shutter, aperture and ISO settings, then evaluate them on your computer or even on prints. You have to be sure the lighting isn't going to change. If it's staying the same, you can then just set ISO, shutter and aperture the same as you best exposure, and you are all set to shoot.
Finally, there will probably be a professional or two photographers there, anyway, just in case you don't get any good shots. You can always order some from the pro!
In fact, be sure they even allow you to bring cameras in at all, in advance.
Hope this helps.
