There are two medical problems here...

One is numb skin on the bottom... Cause is usually just unrelieved pressure, no blood supply to the skin, loss of feeling where there is pressure. Solution is a seat which allows several positions and different parts of the skin to be pressed on at different times.

The other problem, Numb leg syndrome likely has more to do with pressure on the illiac nerve as it goes through the muscles of the bottom. The result here is numbness of the whole leg or a big part of it. Solution is similar, but is related to getting weight off the compartment through which the nerve passes. Best solution is to have a part of the seat so that you can put weight on the upper leg instead of the bottom all the time. This requires a shaped seat instead of just a pad.

Similar but different problems with a single cause... local disruption of the blood supply to nerves which allow normal feeling.