When does your brain tell you to ‘take five’?

Just in case you were wondering, even while you’re lifting weights at the gym, your brain is still in charge. It’s the three-pound organ between your ears -- not the depletion of ATP in your muscles or a servomechanism in your heart -- that tells you to take a break before doing one more rep. A new study reveals how the brain decides to issue a “stop work” order, and what factors it takes into account in giving the go-ahead for hard physical labor to resume.

The brain’s bilateral posterior insula -- buried inside the organ a little above the ears -- registers and tracks the sensation of pain and its intensity and integrates that sensory information with motor function. Measurements performed there were found to be critical in the brain’s calls for a break or a resumption in physical work, the latest research, published Monday in the Proceedings of the National Academy of Sciences, has found.

The judgments issued by measurements taken there can be manipulated. And if you’ve ever watched a reality show or tried one of those “strong man” meters at the fair, the following will come as no surprise: Having a monetary incentive affects how high the signal will have to register there before the brain tells the body to take a break. Financial inducement will also shorten the recovery time necessary for an order to resume work to go out.

Without an incentive, the difficulty of the physical task alone dictated how soon a subject would take a break.

The French neuroscientists who conducted the research recorded the brain activity of 38 subjects while they squeezed a handgrip, sometimes with financial incentives, sometimes without. Their brain scans -- one that measures blood flow, another that measures electrical activity -- both showed a distinctive pattern of activity in the posterior insular cortex of both hemispheres during the squeezing of the hand grips: The strength of the signal grew as subjects squeezed and reached a peak as they neared a break point in their efforts. When a subject stopped, the signal could be seen dissipating until it reached a point at which the subject would routinely start again.

And when monetary incentives made subjects work longer and rest less, the ebb and flow of the distinctive “work capacity” signal matched their behavior.

The researchers cautioned that they don’t know whether the distinctive pattern of signals they observed reflected a person’s subjective experience of fatigue or pain. They proposed next to try similar experiments that would alter those sensations with analgesics to see how long and hard a subject would work before taking a break.