Brian Peters/19 Aug 12 - 11:24 AM
"Playing the anglo standing up without support...requires mostly the muscles of the wrist.. in pulling the hand backwards aginst the strap".

RE:Right! Such muscular activity is necessary to get stability enough both for pumping and to find the buttons.The resulting hand position is dysfunctional however for these reasons:a) it causes extreme extension in the wrist which is unsuitable for the joint structures b)it causes static muscular effort which always is harmful c)it limits the reachable keyboard area

BP:" This certainly needs practice!"
RE: The question is whether it is fruitful trying to "strengthen" the forearm extensor muscles by practise or exercise with the object to manage this ergonomically inappropriate situation better, or if a more stable connection between the playeer and the instrument along with more physiological playing habits might be more rational and efficient ways to deal with it.

BP:" My anglo weighs just over 1400 grams, by the way".
RE: It shouldn't matter in reality whether 1000 or 2000grams.Your combined extension power in the wrists likely is 10-20kg (or more) and the force fraction used to stabilize the connection with the instrument some 100grams - not the entire weight of the instrument.
Nevertheless *static* effort is a villain and should be eliminated if possible