"Improve your marathon time while sitting at your computer" is the kind of claim you expect from an infomercial or a spam e-mail, not from the keynote speaker at an academic gathering.
"It sounds crazy," Samuele Marcora admitted during his talk at a conference on fatigue at Charles Sturt University in Australia last month, "but it's actually not."
Dr. Marcora, a professor at the University of Kent's Centre for Sports Studies in Britain, has spent the past few years unravelling the surprising links between tired brains and physical performance. His initial results suggest that what we perceive as physical limits are actually highly dependent on our levels of motivation and mental fatigue – and that we may be able to use this fact to our advantage.
Back in 2009, Dr. Marcora and his colleagues published a study in which 16 volunteers cycled to exhaustion after spending 90 minutes either watching "emotionally neutral" documentary movies or performing a demanding cognitive test called the AX-CPT, which requires "sustained attention, working memory, response inhibition and error monitoring."
Although the cognitive test didn't produce any physical fatigue, the volunteers gave up on the cycling test 15 per cent sooner when they were mentally fatigued compared to when they had simply watched the documentaries.
Dr. Marcora explains these results using a new "psychobiological" model of fatigue that views exercise limits as a balance between motivation and perceived effort: We stop not because our muscles are starved of oxygen or depleted of fuel, but because the effort it would take to keep going is greater than the rewards for continuing. In this picture, a tired brain and tired muscles are equally capable of increasing your perceived effort, and ultimately making you quit.
This principle doesn't apply only to endurance sports. At the fatigue conference last month, researchers from the University of Technology, Sydney, presented data on the effects of mental fatigue on intermittent sports such as soccer, which mix short bursts of intense sprinting with longer stretches of low and medium intensity.
Using the AX-CPT test to induce mental fatigue prior to a simulated 45-minute game, the researchers found that the short, high-intensity sprints – when motivation was maximal – were unaffected by mental fatigue.
But the volunteers covered less ground during the long periods of low-intensity activity, and found that maintaining any given speed felt harder when they were mentally fatigued, even though their heart rate and other physical indicators were unaffected – exactly as predicted by Dr. Marcora's model.
The findings have clear implications for athletes, since even low-intensity movement is crucial for maintaining good field position during a soccer game, the study's lead author, Mitch Smith, explained. "Practically speaking, we think it's important to avoid cognitively demanding activity prior to a match," he said.
For the rest of us, the findings about mental fatigue suggest that doing a workout after a demanding day at the office will feel tougher, even if you've spent the entire day comfortably sitting in your chair.
That's one reason many people prefer heading to the gym in the morning – but it doesn't necessarily trump competing factors like the stress-busting effects of post-work exercise or simple scheduling convenience. The motivational boost offered by group training or a personal trainer can also help offset the effects of mental fatigue.
The ultimate payoff of this research, Dr. Marcora hopes, will be ways of fighting off the effects of mental fatigue. One option, not surprisingly, is caffeine: Funded by the British ministry of defence, he's currently studying the use of high-dose caffeine gum, which is absorbed more quickly than caffeine pills or coffee. Initial results suggest that caffeine completely reverses the decline in physical performance caused by the mentally fatiguing AX-CPT task; Dr. Marcora speculates that caffeine blocks the effects of a brain chemical called adenosine that is linked to mental fatigue.
More intriguing is the prospect of training the brain to better withstand the effects of mental fatigue: no drugs needed, just a lot of hard thinking. Dr. Marcora is testing this idea by training a group of volunteers for a cycling test, with half of them training their brains for several hours a week using the AX-CPT and two other cognitively demanding tasks.
If it works, it will not only be a neat trick – "improve your marathon time while sitting at your computer!" – it will also offer powerful evidence for the psychobiological theory of fatigue, and provide a new way for athletes to improve their performance when they've maxed out the physical training their bodies can handle.
But don't make the mistake of thinking that this kind of brain training will offer any easy shortcuts to success, Dr. Marcora cautions.
"For the volunteers in these mental fatigue studies, it's not nice," he says. "It's really bad. People hate you at the end of the test."
Inducing mental fatigue
The AX-CPT is a "continuous performance test" that asks you to watch a series of cues (i.e. letters of the alphabet) and respond only after a specific sequence (i.e. A followed by X) appears. The test relies heavily on an area of the brain called the anterior cingulate cortex, which also happens to be the area of the brain related to perception of effort during physical exercise. As a result, prolonged use of this area of the brain makes exercise feel harder. You can read more about the AX-CPT, and try an online sample, at bit.ly/AXCPTdemo.
Alex Hutchinson blogs about research on exercise at sweatscience.com. His new book – Which Comes First, Cardio or Weights? – is now available.