Because your question was very vague, I will make an untra-cool-Petey-post to cover every thing you could theoretically have meant.
Position names:
There are
3 main positions on the speedball field, each with different focuses and skills required to play effectivley.
The first, is the
BACKMAN
Backmen are generally taller, louder, and are more often than not umm... 'supplied with ample girth' shall we say. The backman's job is to fire off break to attempt to shoot the other team's players, and fire pretty much constantly during the game. The backmen/women generally have a very clear view of the game and should ALWAYS be communicating with their mids and fronts. Backmen also have the job of closing the key lanes, which means keeping a constant stream of paint in the lanes between key bunkers, so if an opposing layer runs through the 'closed' lane, he/she will be running through a steam of paint.
MID
Mid players will have a much clearer view of the field than front players, but still not as clear as the backmen. Mid players generally will fire off break, but still go up a few bunkers. They should always be talking to their back players and frontmen, because the front guys will not always be able to hear the backmen and visa versa. Being a mid requires speed, and the ability to play both back and front positions when it is required. It is also a good idea for mids to fill the spot where a front player was when he moved so the opposition may not notice the move, which isn't key, but can't hurt.
FRONT
This position is the most physically demanding, as it requires players to be able to play all sorts of bunkers from the size of standups to shoes. They should be flexible, and fast. I'm a frontman, and i'm 5'5" which helps me alot. Being short finially has it's advantages! w00t! Also: I have pretty nice abs goin because of the position I play. I snap shoot, bend over backwards, do superman dives, etc. which helps keep me in shape- but that may not vbe good enough for some people. My body will react to changes very fast. If I do crunches for a week, i'll see changes. I suggest frontmen do spring and stretching excercises because playing the front means making 50's off break and running throigh lanes, fast enough to catch only 1 er 2 shots if any.
It helps if you're as magical as I am, too- I always get bounces, it's great
i'm pretty skinny and even without umm... 'biological padding'

I still get a ton of bounces because i'm magic!