In Madden NFL 08,on Xbox 360, why when you are doing forty yard dash does the player sometimes stumble?

When I do NFL Superstar I can't get a good rating because of itIn Madden NFL 08,on Xbox 360, why when you are doing forty yard dash does the player sometimes stumble?
Your timing is off. I know it's hard to get good timing, just try pulling down on the thumbsticks with some rythm. Your time will improve with practice.In Madden NFL 08,on Xbox 360, why when you are doing forty yard dash does the player sometimes stumble?
cause ur doing it wrong