On a hard surface our downward velocity quickly comes to zero, means the rate of change of velocity that is acceleration (in upward direction) is high. It is due to the stronger force exerted by the hard surface on our legs so they get injured. On the other hand if we jump on the soft surfaces like sand or pillow it takes more time to achieve zero velocity as our legs plunge to a distance. So rate of change of velocity ie deceleration is lower because the soft surface exert lesser force on our legs and they are not injured.
