Answer:
mass is not eager to be accelerated or in this case slowed down.
Step-by-step explanation:
In general, Newton's 1st law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.
What this means is that a mass is not eager to be accelerated or in this case slowed down. It will require an external force (muscle power) to get the mass of a running man to slow down. It is not instant also, because it takes time to slow down.
EXTRA
Supose the running man would try to stop instantly by purposely running against a brick wall. Even in that case, there is a small amount of time needed ( a fraction of a second perhaps) in which the moving mass of the man is forced to stop moving. That force is usually so huge, that it will injure someone in case of a crash, which might result even in death!
The purpose of an airbag or wearing a helmet, is to extend the time during a crash, FROM 0 to close to zero.
The layer of foam inside a helmet, protects the head in case of a crash, because it takes an amount of time for the head to squash the foam inside the helmet because the mass needs to be stopped ALMOST instantly. That difference between zero seconds and maybe 1 second, is sometimes enough to be able to survive a crash.