You can make the stamina decrease when the 'MoveDirection' property changes and when the 'Jump' property changes from the humanoid object.
Then you can make it decrease a little more when the player jumps for example.
When the player moves then you can minus one from the variable called stamina. Then (I'm sure there is a more efficient way of doing this) get decals so
if stamina = 99 then load the decal.