Braking is the highest ground force you will come across in the running world.
It occurs when your foot touches the ground ahead of your center of gravity.

Runners complain that running downhill hurts. That is because they are braking. Running downhill is when you feel braking the it most.

Land ahead of yourself -> Overstride -> Brake -> High Impact -> Trouble.

Shorten your stride, land under your center of gravity: Running downhill will be very FAST and then you know you’re in the right path.

–Running Biomechanics Coach, Master Your Running
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Running Biomechanics: Occupy Braking