If someone asked for more challenging movements, what would those be?

Challenging in terms of flexibility, mayhaps? For example, by being required to place both legs behind the head, doing the splits, or folding the hands behind one’s back for prayer?

Challenging for strength, like handstand push-ups or one legged squats? Or challenging in terms of balance? For example circles of the pelvis and head, while standing on one leg?

Challenging for memory and muscle memory? Like a piano player being able to produce a difficult and long piece of sheet music from memory? Or a stage actor who faultlessly produces a long text from memory? A movement lesson with a hundred different moves?

Maybe challenging would mean the ability to stay focused, or maybe even awake? Or maybe, challenging for the ability to feel and sense small differences?

Challenging in cultural or ethical aspects? Movements that would seem indecent, like reaching down to touch ones sit-bones, or sliding the tongue in circles over the teeth or lips with an open mouth, or for example a twerk, movements of the buttocks, maybe even in a sensual fashion, that some would judge to be sexually provocative?

Or challenging in terms of pain, like finding a way to do joint movements without discomfort, for someone who is crippled by osteoarthritis or gout?

What is a challenge, to whom?