Are you talking about acceleration on a platform character?
You can always tie the animation speed to the x velocity of the character - like so :
Say animation is 24fps and the walk cycle syncs up with the characters top speed of 330pps.
Then each tick, while walking animation is playing - set anim speed to:
It's not that, its the animation of the character. Example: i have animation1, animation2, animation3. If "Metros"<150 player use animation1,If "Metros"<1000 player use animation2, If "Metros">1000 player use animation3 (this animation doesnt display, i dont know why)