It's fucking physics too. A 200kg mass traveling at 1m/s (the descent phase) generates 100J of impact. Being pedantic, the effect on the knee is far greater given the multiplication of forces by the body's levers. The length of the force arm (in this case the femur) and the distance of the load from the pivot point (the knee) will vary between individuals.
We'll hold that multiplier as a constant for a given individual and now change the weight to 100kg but increase the velocity to 2m/s. There's 200J. And double the force experienced at the knee.