A new fitness function for sensorimotor discovery: Seek transitions in the function linking muscle force to muscle displacement.

Q. Frida appears to like being held in a standing position so some weight is put on her feet. Why?
A. Perhaps it is intrinsically interesting to apply pressure to a muscle, while it does not change position? 

Q. What are the consequences for motor discovery of applying this fitness function during action selection? 

It is this kind of question which can be addressed in the NAO robot. One can request the muscle to go to a particular location, and if it does not go there, one has a measure of the current required to try to make it go there. The regions and circumstances in which such resistance is noted are informative.