Physical exercise is critical not only for a healthy body, but also for a properly functioning brain. The evidence is so convincing that, about a year ago, it finally triggered an end to my rather comfortable but sedentary office-chair existence.
I started running every other night or so. And while the first week or two involved a lot of roadside wheezing and sore muscles, running is now one of my favourite activities. But one thing has never really changed: To this day, I still rarely want to go out and do it.
In fact, trying to convince myself to lace up and go out can be downright painful. It's a puzzle. How can the thought of running be so aversive when the act itself - a stretch of time with no other demands than deciding which route to take and which music will keep me moving - is usually so rewarding?
One possibility is that the human brain has evolved to discourage us from expending energy needlessly. Our ancient ancestors existed in unpredictable environments with scarce resources. An aversion to unnecessary physical activity would have helped themconserve energy for situations in which it was really needed, such as finding food or fleeing from danger.
Indeed, recent neuroimaging research suggests that the striatum, a part of the basal ganglia near the centre of each of the brain's hemispheres, may be critically involved in making a choice away from actions that entail greater physical effort.
When subjects were considering whether to perform a given action, neural activity within one part of the striatum, the putamen, was found to decrease with the amount of physical effort the action would require. And those actions requiring greater exertion were typically chosen less often than the easier options.
By helping to produce an aversion to unnecessary physical activity, the striatum may be partly to blame for the growing number of couch potatoes in Western societies. But this bias against physical exertion is not all-powerful. A number of studies indicate that increasing the reward associated with an effortful action can lead to its being chosen over an easier option. And brain scans show that the size of such a reward is associated with activity in the nucleus accumbens, which is another part of the striatum linked to motivation.
The eat-or-be-eaten nature of our evolutionary history meant that there were often immediate and sizable rewards after physical exertion. You got to live. Between the competing signals in the brain, the incentive to act in these situations would have been much stronger than any default aversion to expending energy.
In an era of central heating and drive-thrus, the motives for engaging in physical activity are not nearly as compelling. And many of the rewards - better physical and emotional health plus enhanced memory, cognitive control and decision-making - take time to emerge.
Even worse, many people have yet to learn about the remarkable benefits of physical exercise for neural and cognitive functioning, which further diminishes the weight of these rewards. This combined with the comforts of modern living can make it particularly difficult to overcome our brain-based bias against physical exertion.
Fortunately, by learning and thinking about exercise-related rewards we can strategically increase the incentive value of physical activity. This may explain why being reminded of such benefits, and how I always feel better after running than before, is so effective at getting me out the door.
Moreover, exercise is associated with physical changes within parts of the basal ganglia involved in cognitive control. This suggests that the more we exercise the easier it will be to overcome our sedentary impulses.
Mark Fenske, co-author of The Winner's Brain: 8 Strategies Great Minds Use to Achieve Success, is an associate professor in neuroscience at the University of Guelph.