Mental effort increases physical fatigue, reduces HR variability


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- Alex Hutchinson (@sweatscience)


A pretty neat study just appeared online at European Journal of Applied Physiology, looking at the links between mental effort and physical fatigue. This is a topic I’ve touched on previously, and find really interesting. The new study, from researchers at Michigan Technological University and Virginia Tech, adds some new wrinkles.

The protocol is quite complex, but basically a bunch of volunteers did fatiguing shoulder exercises while doing mental arithmetic (“Here’s a number, multiply it by three… now multiply it by three again…” etc.). The researchers measured how quickly the subjects’ shoulders fatigued, and how quickly they recovered and returned to full strength in the 15 minutes after the exercise bout. As you can probably guess, the subjects doing mental arithmetic lost strength and reached failure more quickly than the controls.

Why does this happen? Well, the researchers discuss some previous work suggesting that mental activity triggers stress which triggers low-level muscular contractions, which can lead to premature fatigue. But I actually find another explanation more convincing:

It has been shown that fatiguing contractions require high attentional demands due to changes in the excitability of motor cortex. As such, it could be argued that additional mental demand in the current study may have reduced available attentional resources needed to increase the drive to motor neurons to maintain the required force levels, resulting in early task failure (i.e., shorter endurance times).

In other words, it takes focus and mental effort to push to your limits, and those are finite quantities that can be squandered thinking about other things. That seems like the simplest explanation to me, and it would fit with the research by Samuele Marcora that I mentioned above.

A neat additional observation: the mental arithmetic resulted in lower “heart rate variability” (HRV). Basically, you measure the time between successive heart beats — if that time is always identical, you have low HRV; if it fluctuates, you have higher HRV. This tells you something about the balance between sympathetic and parasympathetic nervous systems; when you’re under stress, the sympathetic system ramps up and release norepinephrine (aka noradrenaline), which elevates your heart rate but reduces heart rate variability. The result: it takes longer for your heart rate to settle back to normal — which is exactly what the researchers observed in the subjects doing the mental arithmetic.

11 Replies to “Mental effort increases physical fatigue, reduces HR variability”

  1. Thank you for that post (and very interesting blog by the way!).

    This is a cool study. Funny that mental workload had no impact on rates of perceived exertion… I would have thought the contrary in light of Marcora’s results on perception of effort.

    Also, mental work decreases cardiac parasympathetic modulation as well. This could contribute to the lowered heart rate variability observed with physical and mental workload (although parasympathetic modulation is already lowered by exercise alone).

    However, recovery of parasympathetic modulation is probably to quick to contribute to the reduced heart rate variability reported after concurrent physical and mental processing vs. the control condition. The slower HR recovery could indeed be attributable to higher levels of norepinephrine…

  2. Very interesting post. I wonder if a practical implication is that wasted mental efforts should be avoided during competition? Devoting mental energy to calculating pace and goal time or thinking about competitors and race strategy requires mental resources that impact physical performance. That’s why it’s best to run your own race and mentally prepare your pre race strategy so you can put yourself on autopilot – less to think about during the race.

  3. Seems like a reasonable explanation as to why following a pacemaker is ‘easier’ despite there being little in the way of evidence regarding any ‘drafting’ benefit at typical running speeds.

    I have always found it easier to tune out and follow in workouts & races at paces that seem much harder when leading/ running alone. The mental effort of worrying about pace may explain that.

  4. It basically fits in with how I see workout beginners gain strength and exhaust themselves: After a workout, for them it is more or less not the trained muscles that are fatigued, but the brain, that is trying to coordinate those muscles’ fibers into action.

  5. Interesting post, Alex. I wonder how these findings relate to what happens when you “bonk”. When you run low on glycogen, one of the first things that happens is that your brain doesn’t have the fuel it needs and you have trouble thinking clearly. This can make even the simplest mental tasks seem difficult and may create a feedback loop that increases the effect on heart rate, fatigue, and heart rate variability, further accelerating glycogen depletion.

  6. I would be interested to see whether or not the shoulder fatigue is associated with tissue oxygen index i.e. if fatigue onset coincides with muscle deoxygenation. Did the same participants perform all conditions i.e. 15%, 35%, and 55% with and without mental tasks?

  7. The answer to this question is found in the book “Your Brain at Work” by David Rock. Try this simple experiment: start to run up a steep hill at a good pace, then see what happens when you try to multiply a number like 32×46.

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