With some practice, it's not that hard to balance a baseball bat on the end of your finger. But try to poise a pen or a short stick, and the task becomes rather difficult. That's because a smaller object moves more quickly--at speeds that approach the time required to carry out corrective motions. Now findings published in the October 7 issue of Physical Review Letters suggest that random movements induced by the nervous system can help keep a stick balanced on a fingertip.

Juan L. Cabrera and John G. Milton of the University of Chicago placed reflective tape on the ends of a rod and filmed a person trying to balance it. By analyzing how light bounced off the bar, the scientists determined that the stick's quivers lasted between 10 milliseconds and one second. But 98 percent of the subject's adjusting movements occurred faster than the average reaction time for such a task, suggesting that the person wasn't controlling the stabilizing actions. When Cabrera and Milton devised a mathematical equation to describe the motion, complete with a random component, it fit well with their observations of a stick on the verge of falling over. Scientists have introduced such random movement into high-performance aircraft to increase stability. The fact that humans benefit from randomness too, the authors say, suggests that researchers might want to incorporate it into designs for more lifelike walking robotics.