I don't know why everyone is saying no to flash, I don't think that there's any restriction against it, I would still shoot this the old fashioned way, you just need a few minutes of preparation and then you shoot away. Get there a bit early, pick your shooting location, forget the tripod!, iso 400, set your camera to manual exposure, set your flash to auto, or manual if you know what you're doing, flash sync to 1/250th, set your camera to mf, af on those lenses is too slow for fast action, figure out your hyper-focal distance and set your f-stop, take a few shots to check your exposure, then do a custom WB so you don't end up with pink uniforms and sick looking people, final exposure check, fire away and enjoy!
Problem is with flash and white martial arts uniforms is it blows every detail out and can easily cause underexposure.Despite all the new technologies I still find this the case.
Syncing at 250 at 400 asa means a probability everything in the background will be black and you will need a reasonably small aperture and work with available light one way or the other. I try to work around f 5.6 - f8 to get a bit of focus depth, so higher iso is better as the camera can easily cope, flash or not and it is much less harsh as it lets in a little of the background lighting - even at 250th, of course if there is a good level of light you can go faster without flash. While flash is great to fill in a little and keep colours correct in the foreground it makes overall colour balancing the images later difficult if the other light is wildly different in colour temperature.
Flash can be very distracting for the competitors and you may be asked to stop if using it at a close range. I agree manual focus is better as it will be tricky focusing , but martial arts are pretty static as they don't move around a lot at a very fast rate, all the action tends to be in a very small area and the mf on even those lenses will easily keep pace, its the locking on of focus that may be tricky although Nikon has excellent predictive focussing( better than the Canons I use), come to that , make sure you are set on continuous focussing as the camera may lock up on the single focus setting at the most inopportune moments.
Do a few tests in the stadium before hand to get a feel for the light conditions, and if possible warm up a little in the early rounds, you will soon know if you are getting what you need on the screen. Conditions probably won't change during the course of the competition but you can continually fine tune as you work
