Playing repetitive fast sequences for lengthy periods will cause muscle spasms regardless of how you sit /Play and given Francis's Age i expect he would suffer from it quite a bit given the years he probably played keys,try playing the Chronlogie 6 sequence for the whole track and see if your hand aches afterwards....Even with strong joint and muscles this can be quite tiresome for a few minutes
I'd imagine he'll suffer from an Arthritic condition in his hands as he gets older,Jarre too,I get muscle spams from time to time if i play too hard and too fast for long periods and sometimes get a burning sensation in the hands,if I've overdone it for too long which is similar to arthritic pain,and I'm almost half there age,as with every joint ,if you overuse it without actually caring for it it will eventually wear out and cause you serious pain,being a Player also requires proper warmup and rest of the joints in the hands.not just correct posture,in any case most keyboard players tend to have quite strong Muscles in there hands especially if they've been brought up on a real Piano,but even the strongest hands will suffer from Muscles spasms from time to time.
Shaking his hands just loosens the muscle tightness and fatigue.