To get the sharpest images, you’ll almost always want to stop down one or two f-stops from the widest available aperture your lens provides. That is, an f2.8 lens often produces its best results at f5.6. Unfortunately, with action photography, a high shutter speed is often desirable, so you’ll find yourself shooting wide open more than you’d like.
In that case, the maximum aperture of your lens can be important when shooting action in low-light situations. A “slow” lens can limit the maximum shutter speed you can use, thus affecting your ability to freeze action. For example, if your lens opens no wider than f8 (a common limitation for longer lenses and zoom settings), the best you can do with your camera set to ISO 100 in full daylight is 1/500th second at F8. The Nikon D90 has a 1/1,000th or briefer setting, but you can’t use it without increasing the ISO setting to 200 or higher, thus increasing your chances of detail-robbing noise in your photos. If daylight is waning or you’re shooting indoors, an f8 lens may limit you to sluggish 1/250th or 1/125th second speeds.
So, a larger maximum aperture is better, assuming that the lens performs well wide open; an optic that is a bit fuzzy at its maximum aperture is no bargain. Keep in mind that the maximum opening of some zoom lenses varies, depending on the focal length setting. That is, a lens that rates an f4.5 aperture at the 28mm setting may provide only the equivalent of f6.3 or slower when zoomed all the way to the telephoto position.