Mental or Muscular fatigue? Why sometimes your workout is harder than it should be.

Mental or Muscular fatigue? Why sometimes your workout is harder than it should be.

Fatigue is my worst enemy.

Begrudgingly, as I sip coffee to combat the deep fatigue setting into my hamstrings after this mornings indoor trainer session (which included 5x 30s @120% FTP and 5x 30s @110% FTP during time trial simulations), I recognize that this muscular sensation will affect my performance in other tasks today:

  • getting up out of this chair
  • walking
  • squatting
  • commuting on my bike, etc

As athletes, we are all likely familiar with this sensation of deep muscular fatigue.


But, what about mental fatigue?

Researchers have been studying the effect of mental fatigue on physical performance for several years now (1–8). What they’re tracking is to what degree cognitive fatigue, a subset of mental fatigue, impacts our ability to perform in aerobic endurance sports, such as cycling and running.

Enter, the Psychobiological Model of Endurance Performance.

Using the framework of the Psychobiological Model researchers hope to understand the connection between work performed in the cognitive domain and the influence it exerts on the physical. The Psychobiological Model itself rationalizes that “endurance performance is determined by a decision-making process based on perception of effort and potential motivation.” (7)


Within this model, cognitive fatigue is described as a “psychobiological state caused by prolonged periods of demanding cognitive activity and characterized by subjective feelings of “tiredness” and “lack of energy.” (1)

But– what does that mean?

Well, theoretically, if your brain is tired, it’s ability to exert effort is limited by its own perception of fatigue. So, if your brain gets tired, the perception of effort is higher, and even if you’re just as motivated as you were last week, the amount of energy required to overcome this fatigue sensation is GREATER because your brain is just too darn tired.


So, what have researchers found to support or contradict this argument?

Check out the next post “Shut Up Legs, or Shut Up Brain?” to find out!




  1. Marcora SM, Staiano W, Manning V. Mental fatigue impairs physical performance in humans. 2009;857–64.
  2. Marcora SM, Bosio A, de Morree HM. Locomotor muscle fatigue increases cardiorespiratory responses and reduces performance during intense cycling exercise independently from metabolic stress. AJP Regul Integr Comp Physiol [Internet]. 2008;294(3):R874–83. Available from:
  3. McCormick A, Meijen C, Marcora S. Psychological Determinants of Whole-Body Endurance Performance. Sport Med. 2015;45(7):997–1015.
  4. 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:
  5. Boksem MAS, Tops M. Mental fatigue: Costs and benefits. Brain Res Rev [Internet]. 2008;59(1):125–39. Available from:
  6. Van Cutsem J, Marcora S, De Pauw K, Bailey S, Meeusen R, Roelands B. The Effects of Mental Fatigue on Physical Performance: A Systematic Review. Sport Med. 2017;47(8):1569–88.
  7. Blanchfield A, Hardy J, Marcora S. Non-conscious visual cues related to affect and action alter perception of effort and endurance performance. Front Hum Neurosci [Internet]. 2014;8(December). Available from:
  8. Smith MR, Marcora SM, Coutts AJ. Mental fatigue impairs intermittent running performance. Med Sci Sports Exerc. 2015;47(8):1682–90.