We know Newton's second law of motion which describe relation between force and motion.
The rate of change of momentum is defined as force.
When object move it has some velocity and its mass as well.
we define now momentum as mv ,where v is velocity and m is mass of the object.
When bycle rider falls , velocity comes to zero
i.e rate of change of momentum = m(v-0) =mv
which is force. It puts great impact on rider's body.
Rider not falls vertically downword but it describe circular parts,
So another factor comes into effect ti.e angular momentum as well.
To reduce this impact of the force, helmet is designed for bicyle rider.