Interossei muscle exercises?

[enharmonic]

Does anyone know any good exercises for strengthening my interossei muscles?

I want to strengthen those because I have a problem with tense wrists when playing repetitive things quickly, and interossei muscles apparently are the set of muscles that don't tense the wrist.
I've been reading this: https://www.jazclass.aust.com/piano/default.htm

[steven1a]

I've been strengthening my own interossei to good effect in my overall technique.  Simply using them by playing sideways--sweeping laterally across the keys as you play--is a decent way to strengthen them, but you could also wrap rubber bands around your fingers and spread them sideways.  Ask a physical therapist for more information on how to strengthen those muscles.  Rubber bands were recommended to me by a PT, as well as silly putty or play dough between the fingers. 

Let me know if I should elaborate further.

Dr. Steven P. Niles
Instructor of Music, Los Angeles Mission College