Simple situation. Pressing space bar adds velocity to an entity using phent_addvelcentral(). The question is, how do I do this based on the condition that there isn't allready velocity along the same axis?

phent_addvelcentral(my_entity,vector(0,0,key_space * 10));

This will make the object "my_entity" move upwards as long as the space bar is held. What I want is for it to only add the velocity if there is no movement along the Z axis allready. In other words, it should make the object "jump" but not fly.