[scp3125]

This is because maintaining inertia is efficient, despite the stress on the system. This is the reason High Intensity Interval Training and High Intensity Resistance Training work at all, because you're making a conscious effort in your exercise methodology to eliminate momentum and acceleration from the movements, which increases the work done within the context of your body, reducing the exerted energy lost to mechanical inefficiency of repeated movements.

You don't even need to stop moving while walking to see this effect in action. Just try walking so slow that you've eliminated almost all the momentum from the motion of walking. Just try it for a minute straight and see how it feels. (Forewarning: It's going to look ridiculous, like you're walking in slow motion.)