How does joint stress change when braking and when not braking.
Updated: Nov 8, 2019
When the leg reaches ahead of the body and the heel contacts the ground, the energy absorbed by the leg is a result of the combination of the small downward velocity due to gravity and the impact due to the horizontal velocity. To visualize the horizontal motion, it may be helpful to think of the ground moving under the body rather than the body moving forward.
At a 6-minute mile pace the horizontal speed is about 14 ft/sec or 10 miles per hour. The downward velocity is negligible compared to the horizontal velocity. The distance that the body falls in each stride should be minimal. Falling 2 inches takes 0.01 seconds and results in a vertical velocity of -0.32 ft/sec. Falling 3 inches takes 0.016 seconds and results in a vertical velocity of -0.48 ft/sec. If the foot is not moving relative to the body (not rotating forward or backward) when it contacts the ground, then it would absorb part of the energy created by the horizontal velocity of 14 ft./sec. A worse case than this is when the foot is still moving forward when it contacts the ground. In this case, the velocity of the foot relative to the ground is greater than 14 ft./sec creating a greater energy to be absorbed by the leg.
The energy due to the horizontal velocity can be eliminated by accelerating the foot backward before contacting the ground. If the foot is moving backwards at the same speed that the ground is moving under the body, then there is no energy from the horizontal velocity to be absorbed by the foot and leg. From a health perspective, this reduces stress on the joints. From a performance perspective, this eliminates braking. When the horizontal velocity is neutralized, the energy that the muscles use will be propelling the body forward rather than slowing it down.