I watch as Christina controls Prof Haggard's fingers like a marionette. The mechanical nature of it is unsettling. A graph on a screen shows his muscle activity plotted by time; 20 milliseconds after she clicks the button, it depicts an elegant leap and drop, like a heartbeat on an ECG. That 20 milliseconds is how long it takes for the signal to travel down his nerves. "The conduction time would be less from my jaw muscles, more from my leg muscles," he says. And as many of us will recognise, the process gets less effective as we age: "As I get older, the curve will move slowly to the right on the graph."