The reason for the bending of the legs when jumping is the phenomenon of inertia.
Inertia is the phenomenon of the body preserving its state.
When jumping, a person moves down at a certain speed. When the feet touch the surface of the earth, the human body continues to move downward by inertia. The human body by inertia acts on the legs with a certain force. The legs of a person under the influence of this force bend at the knees.
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