Shut Up Legs, or Shut Up Brain?

Shut Up Legs, or Shut Up Brain?

Last post talked about the Psychobiological Model of Endurance Performance and what it means to your brain (whether you know it or not). So, this post, let’s take a look at what some of the research in the area is saying about how this theory applies to actual performance!

In studies using cycling as their endurance task, with both trained and untrained individuals, results have found up to a 15% decrease in performance (1,2), in another study using highly trained field sport athletes in an intermittent running protocol, results found a decrease in performance by only 2.8%, however, this was found to be a result of ONLY decreased performance in the aerobic portion of the intermittent test protocol1 (3). The distance covered during short sprint efforts was unchanged, only the long endurance efforts changed, meaning the total distance covered by players was decreased only partly here, but why?


Here’s where it gets interesting, despite the decrease in performance in each of these studies, none of the athletes display any physiological characteristics2 indicating fatigue. So why is their performance decreasing?!


Here’s the thing, each of these aerobic tasks is a self-motivated task, Time To Exhaustion (TTE) trials are like a mini race, only you’re given a specific workload that is based on your maximal capacity, and you’re told to hold it for as long as you physically can. This is a task that requires immense amounts of motivation. To control for this, in part, there is usually some cash prize provided to participants based on their performance, this keeps people motivated. Who would have thought?

Mental Fatigue and TTE Performance measures [1]
So, in these TTE trials, as participants approach a certain time (call it, T1) those who have received the cognitive fatigue intervention generally have a greater perceived exertion at T1 than those who didn’t receive the cognitive fatigue task. All it boils down to in these TTE trials then, is how hard you feel like you’re working. If the effort feels harder you’re likely to give up sooner!

Why does this happen? Well, that’s the question research is trying to address. So far, we know it happens. There are plenty of theories out there, but there’s lots to sort through.


In the meantime, you can take this knowledge and apply it to your own racing and training schedule by trying not to schedule difficult workouts immediately after a cognitively demanding task. Don’t plan to complete a work project the day before your 10km, marathon, or Ironman races and you hopefully won’t be too affected by this phenomenon! At least until we figure out how to mitigate these negative effects, that’s the best we can do!

1 – note that overall performance was decreased, due to the inclusion of both high and low intensity (ie. Sprint and running efforts), whereas research, including this study, show no effect of mental fatigue on sprint performances

2 – physiological characteristics included heart rate, blood lactate, and VO2 measurements




  1. Salam H, Marcora SM, Hokper J. The Effect of Mental Fatigue on Critical Power during cycling exercise. Eur J Appl Physiol Occup Physiol [Internet]. 2017;0(0):0. Available from:
  2. Marcora SM, Staiano W, Manning V. Mental fatigue impairs physical performance in humans. 2009;857–64.
  3. Smith MR, Marcora SM, Coutts AJ. Mental fatigue impairs intermittent running performance. Med Sci Sports Exerc. 2015;47(8):1682–90.