I (Lisa) feel it regularly when running races or during more difficult workouts - burning lungs, tired and throbbing legs, and constant mental input saying to slow down or better yet, walk.

Although I clearly feel that I cannot maintain the pace, and even question if I can continue, it was recently suggested to me that this fatigue is all in my mind. Is it possible that the cliché "mind over matter" and "no brain, no pain" actually have some merit? Is there a reservoir of deeply disguised energy that my brain is secretly hiding, while at the same time sending pain signals trying to get me to stop?

There is an idea called the Central Governor Theory that proposes while you're exercising your brain triggers fatigue signals well before your muscles run out of energy to trick your body into slowing down or even stopping. It suggests that your brain regulates your energy supply and when it feels you are in danger of running low, it triggers pain signals to be sent to your body.

Left alone it seems this governor can be quite conservative, initiating pain trying to get you to stop way before you are out of juice. Thus, fatigue may often be a mere emotional response that begins in your brain and tricks you into quitting.

The theory holds that physiological factors such as glucose levels, oxygen in the blood and lactic acid may not be the direct causes of fatigue. Rather, these are all just signals used in combination with other cues the brain takes into account to determine the limit. Accordingly, it is the brain that has the final say in endurance efforts, not the muscles, heart or lungs. This protective mechanism allows you to always have something left in the tank for an emergency or unexpected need.

This helps explain why interval training can be effective on more than just a physical level. Standard explanation for improvement in time performances is that changes occur in the body to increase its capacity to fight fatigue. Central Governor Theory says interval training works largely by teaching the brain that going faster won't do your body harm. By incrementally pushing past your perceived limit, you can train your brain to allow more access to your reserves. Each time you adapt to a new level of stress, your brain believes you can survive it so it allows you to go to that next step.

Additionally, this provides an explanation for race performances. When the finish line is in sight, it often becomes possible to sprint or at least increase your speed to the finish. When the muscles are closest to exhaustion, a finishing sprint doesn't make sense. But this theory shows that when knowing the end is near the brain slightly relaxes its control and allows access to these carefully hidden reserves.

Whether there really is a secret brain mechanism tricking you into slowing down or stopping while hiding extra energy is still unknown. But the idea could give you that mental edge to push through your current limit and reach your next fitness goal.