I have also heard that kneeling on pencils is not very comfy also.

If you want other positions that are uncomfortable to hold for long I have a few for you. not sure if this is what you are looking for, but thought I would share them. These are some of the positions that Master has me practice every day. There are also different variations on these positions also. Just do a google search on gorean positions.

Half Bow:
Starting from kneeling you are to lean back aching over until the top of your head rests on the ground as you look behind, your hands slightly out to each side of your thighs with the palms up, your whole bottom half stays in nadu ( kneeling), it can be a rather painful position to hold properly for any real long length of time but it is a very sexy position.

Full Bow:
or (Gorean bow) From the half bow you move your hands up and back palms on the floor to either side of your head elbows pointed in the direction you are facing away from. your knees and pull up with your tummy slowly rising up your back arched acutely on your hands and feet into the shape of a bow,this position is more difficult to hold then the half bow and is sometimes used as a punishment position. It is very much like a back bend. It is very hard to hold, if you want uncomfortable, this is it. lol.

Reverse Bow:
lay on your belly, reach back and grab your ankles and pull arching your back until only your belly and lower chest are touching the floor, your hips and thighs should not touch the ground. This is also used as punishment, it is hard to hold for long.

enjoy!