The cleat position determines the ratio of muscle groups being used. For instance, if you have you cleat forward, you'll be using more of your calves during your stride, if you move it too far back you'll be using mostly quads. Neither extreme is good and results in unbalanced muscles and strains. Your cleat position should present a balance of these muscles such that you're using mostly quads as these are your biggest muscles.

Check out this chart to see what muscles are being used in what part of your pedal stroke.