[Spartan]

Amandaa I think you're right about road running being harder on the joints, however I don't know how that would factor into cardiovascular demands? How does inreased pain and discomfort in your knees for example, make your heart work harder? Perhaps running outside is merely more painful, and thus perceived as a better cardiovascular workout?

Also, harder surfaces will deform less on impact, thereby conserving energy for the runner. Whereas softer surfaces like sand will absorb the kinetic energy from the runners stride - making the runner use up more energy. Thus running on harder surfaces, such as roads, takes less energy.