Toolbox: Can Thinking Make You Slow?

With the second week of the Tour upon us after a fair bit of shadow boxing, the pressure is immense on the leaders and the other pre-Tour favourites a bit further back. From sport psychology we know that pressure can affect us emotionally, but is there also a physiological link between mental stress and exercise capacity?

Rest day is a bit of a misnomer at the Tour. While there may be some physical recovery going on, the mental pressure only ratchets up to beyond fever pitch, thanks partly to there being no actual racing for the media to write and talk about. As a result, the riders are faced with a day full of worrying about how they have done so far, and also how they are going to tackle the remainder of the race.

We know from sport psychology that our mental and emotional state can be strongly affected by our surroundings and the pressure that is being placed upon us from various sources. The assumption has therefore been there is a direct link between this mental stress and our exercise capacity. How exactly this psychogiological link might work remains unclear. One idea is that there is a finite neural resource capacity within our central nervous system, and that an increased mental load impinges on the resources available to concurrently arouse our physical capacity. Another theory is that mental stress directly fatigues the motor cortex, thus decreasing our capacity to recruit and activate our muscles.

While this mind-body connection may seem to be a natural and logical argument, it is always nice to see actual experimental proof. Surprisingly, there has been minimal to date, but a new study in the Journal of Applied Physiology from the UK attempts to investigate the direct linkage between mental fatigue and exercise capacity.

Marcora et al. 2009
At the University of Bangor in Wales, Samuel Marcora’s research group took the experimental approach of testing exercise tolerance at a high workload after either performing a mentally fatiguing cognitive task or else watching a neutral visual documentary. Some of the relevant experimental notes:

• 16 subjects tested. They were 10 men and 6 women non-elite cyclists (peak power output 288 W and VO2max 52.8 mL/kg/min).

• The experimenters attempted to blind the subjects to the actual purpose and aims of the study by telling them that they were testing the effects of two stimuli on the physiological responses to exhaustive exercise rather than the actual tolerance to exercise itself. This deception is somewhat crude but is still essential, as most people would inherently assume that the mental task condition would make exercise more difficult.

• The cognitive task condition involved 90 min of a mentally demanding test. This was a variation of a test often used in experimental psychology to study brain function. This Continuous Probe Test (CPT) involved the continuous rapid presentation of letters on a computer screen, with the subjects having to click an appropriate button whenever the letter A was followed by a letter X, and another button when it was a combination of other letters. In addition, distractor letters in another colour were randomly presented. A monetary prize was used as motivation to maintain subject interest in the CPT task.

• The control condition involved watching a neutral television documentary with no requirement of feedback from the subject. How neutral? One was on “World Class Trains – The Venice Simplon Orient Express” and the other “The History of Ferrari – The Definitive Story”. Given the rabid subculture of trainspotting in the UK, this might actually be a bit stimulating to some!

• To quantify mental state, subjects, performance on the first and last 15 min of the CPT was compared. Subjects also completed questionnaires on mood states, motivation, and self-efficacy.

• The exercise task involved riding to exhaustion at a set intensity of 80% peak power output, during which physiological measures were also taken. To maintain subject motivation, a monetary prize for best performance was also provided.

The Answer Key
Having used this type of continuous probe task for some of my research, I can attest that even a 10 min segment seems like forever, so I have no qualms that 90 min of this task can be incredibly fatiguing. This was borne out by both physiological and psychological measures. Namely, heart rate during the mental task was slightly but significantly higher than during the neutral documentary condition, with no difference in blood glucose levels between conditions. The psychological questionnaires found decreased states of vigor in both conditions (see, TV does really turn you into a vegetable!), but mental fatigue was much higher following the CPT trial. This was supported by a marked drop in correct performance during the last 15 min of the CPT compared to the initial 15 min.

OK, so the experiment successfully created a state of mental fatigue. How did this affect exercise tolerance?

• No difference was found in the freely chosen cadence over the course of the test (83-84 rpm).

• Tolerance time was much shorter in the CPT condition (640 s) compared to the control condition (754 s). This decrease was supported by 13 of 16 subjects having a lower tolerance time with the CPT condition.

• When examined at a particular and consistent timepoint (e.g., at 5 min into the test), no differences were seen in physiological responses (e.g., HR) between the two conditions. However, heart rate and blood lactate were significantly lower in the CPT trial at the point of exhaustion compared to the control. No differences were observed during exercise or at exhaustion in other physiological variables such as blood pressure, cardiac output, or oxygen consumption.

• Another important finding was that the subjective perception of exercise, as measured using the standard Ratings of Perceived Exertion (RPE) scale was significantly higher throughout exercise following the CPT test compared to Control, both at a particular timepoint and also at the point of exhaustion.

Summary
This study was nicely designed and controlled, and provided very intriguing data about a direct link between mental stress/fatigue and our ability to exercise to our maximal capacity. Certainly, it appears that a state of high mental engagement and fatigue is contrary to our ability to maximally engage our physical capacity.

However, there are also lots of unanswered questions and further explorations to be done based on this data. That is because measures such as tolerance time to exhaustion and also RPE are fairly crude measures that really only get at the surface of the question at hand. That’s not to denigrate this well-done study, just that there remains lots of things we need to know to understand the details of this mind-body link better.

Notably, there is no way from this data to understand the actual mechanism behind any psychobiological link. That is because there were no actual brain monitoring such as EEGs (electroencelographs, which records the electrical patterns of the brain), nor were there tests on the brain’s direct capacity to recruit and activate muscles such as the interpolated twitch technique. So we now know that there is a direct link between mental fatigue and exercise capacity, but we do not know how this happens.

What is clear, though, is that we do want to keep our mental stress at a reasonable level if we want to achieve our peak performances. Therefore, don’t expect to set a personal best in a time trial immediately after a major presentation at work or a grandmaster chess match with Garry Kasparov.

An interesting “flip side of the coin” question is that we often look at exercise and cycling as stress relief, such as using it to unwind after a hard work day or to stimulate our mind and thinking (that’s my favourite excuse for hopping out for a ride or a squash game when I’m supposed to be writing a paper or a chapter of my latest book!). Does this converse link actually happen? Can exercise improve not just our mood but our actual cognitive functioning? We’ll explore this question in my next Toolbox article.

And as for the pros at Le Tour and also us recreational athletes, it pays to read Marvin’s excellent sport psych articles and get a handle on our psychological game to improve our cycling game!

Have fun and ride safe! I’ve just arrived in Australia as an invited speaker at a conference on exercise and fatigue hosted by Charles Sturt University, and will be exploring the riding scene here in the Blue Mountains region and also out west in Perth over the coming weeks!

Reference
Marcora, S.M, W. Staiano, and V. Manning. Mental fatigue impairs physical performance in humans. Journal of Applied Physiology. 106: 857-864, 2009.

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About Stephen:
Stephen Cheung is a Canada Research Chair at Brock University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at [email protected] .