Not true stop action, photographer is following reider and the background id blurred as a result. If the background had been stopped it would be excellent. The wheels still in motion add to the interest of this photo.
A fill flash would have worked well here to get more detail in the rider and under the bike. The dark rider against the bright sky / bright bike against the dark hills works real good.
Excellent shot. If you'd been lower (if you could ) you could have taken this against the sky and it would have been even stronger, rather than against the fairly 'muddy' background.
I think that this is a very good interpretation of the "stopped motion" challenge. I also like the composition of this photo. I like action shots and this is a great example of that. Your depth of field nicely blurs out the background and brings the subject of the photo to my immediate attention. There are two things that would make this an even better photo. The tires on the ATV are not frozen in time. A faster shutter speed or possibly a flash on a shot like this would improve that. A flash may have also improved the detail of the rider. The rider is a little dark. Excellent shot!