There was an old Seinfeld episode - 'The Pez Dispenser' - where Jerry and company debated about how George's pianist girlfriend warmed up before a concert. Jerry quipped, "What, do you think they just crack their knuckles and come out?"
Actually, for some pianists, knuckle-cracking is part of a warm-up routine - not because it's beneficial to their playing, but because it has simply become habit. Not doing so would be a distraction. However, most pianists tend to rely on the old standbys:
Image © Brandy Kraemer