Graham, I would stay away from both of those shooting sports. As Dave pointed out 3D uses color so on a football or soccer field the AF system can be distracted by the same color (think another player in the same uniform) moving within the frame. 21pt dynamic is usually a little too large an area as well and sometimes the camera will pick out the wrong player to focus on. That can still happen in 9pt dynamic but is less likely than when using 21 pts. Most of the people in the sports forum use single point for football and soccer as well. After much experimentation with all of those settings they have found those two settings (single point & 9 point dynamic) best (with AF-C). Personally I use single point the vast majority of the time.
There is quite a good deal of information about autofocus at this link and although some of the info only pertains to the D4 and its AF settings the D7000 shares many of the same settings and as such much of the information on these pages is helpful.
You may also find it helpful to review some of the posts in the sports forum as most people include their settings when they post images. Good luck.
Marc There are always two people in every picture: the photographer and the viewer.-Ansel Adams