The best runners in the world are, from the wait down, relaxed and fluid. The only way they can run in such a fashion is to know how to take the weight of your lower body (your legs!!!)
Your pelvis is design to absorb this pressure, however if you have poor posture and you relax your abdominals, all of the forces and weight of running (roughly 2.5X body weight) is placed on your legs - making it harder to push away from the ground; making runners run heavy and really have to push off hard with each stride.
This video discusses ways to utilise posture to ensure you can run with all of the weight taken off your lower body, allowing your to run relaxed and take all of the forces of running off your joints, too!!!!!!
If you learn one thing as a runner. This should be it!!!