I'm watching The Theory of Everything and I think I finally figured out whats wrong with Knee's dancing.
It's called motor neuron disease. It's a progressive neurological disorder that destroys the cells in the brain that control essential muscle activity such as speaking, walking, breathing, swallowing. The signals that muscles must receive in order to move are disrupted, cut off. The result is, gradual muscle decay, a wasting away. Eventually, the ability to control voluntary movement is lost entirely.
