When I rode dirt bikes, I was instructed to endeavor to keep my elbows up, and thus maintain a grip on the throttle that is more like grasping a doorknob than holding onto a chinning bar, if that makes any sense. This allows the arm to flex extended and collapsed back and forth without necessarily torquing the throttle.
One must maintain a grip on the throttle such that being thrown all the way back with the wrist straight does not cause the engine to rev. In this position, the engine should be at an idle, and to accelerate, the throttle should be turned not by 'getting another bite' on the twistgrip in this same position, nor by dropping the elbow and trying to bend your wrist back and down, but by lifting the elbow and rotating the forearm, the doorknob way, which requires one to first scoot up and bend the elbow slightly. In that type of cycling it is not conceivable that you will have to apply throttle while your arms are fully extended. Your arms are always bent and you are foreward over the motorcycle when you are applying throttle.
It's a lesson you learn fast because you cannot let go of the twist-grip to adjust your 'null position' whenever you want, and nobody has enough strength to hold his wrist in a non-straight position while hanging on for dear life. With road bikes, though, I can see this being a real problem due to the handlebar and riding position geometries involved.