Continuum Drag
From Starbase 900 Wiki
Theorical force, from which is said that it stops ships from accelerating while they at warp. Continuum drag is comparable to air drag, but it's not air that influents movement, but the space-time continuum. However, there's no need for a force like that to explain why sustaining a warp field needs a constant influx of energy. This might be better explained by the fact that warp drive is non-Newtonian, and is thus not explainable with Newton's laws.