A shake of a leg must be a measure of time. Idiomatically. But why would that be used to measure-?

"I was going to spend some time in the lab today," she said, chasing the ridiculous problem out of her mind. "I'm running a series of basic behavioral simulations. And where do you expect to go today?"

He seemed off his balance and that was disconcerting. It made her feel awkward, as if something about her, something unperceived, made her suitable to gape at. She lifted her shoulders up and back, squaring off against the entirely imagined disdain.