Issue #22: How Mental Fatigue Limits Physical Performance

🥞 Fueling for Performance

👨‍💻 Introduction

Topic: Mental Fatigue Impacts Endurance Performance

We often think of fatigue as something that happens in the muscles: lactic acid buildup, depleted glycogen, or an elevated heart rate that won't come down. But what if your brain can make you quit before your body truly needs to? This week we’re exploring a study that I found while re-reading the book, Endure (very good read and on kindle unlimted!): Samuele Marcora’s 2009 paper Mental Fatigue Impairs Physical Performance in Humans. It has been cited over 2000 times in other research and has influenced how coaches and athletes think about endurance limits, shifting the conversation from physiology to psychology. If you’ve ever felt like a session was harder than it should be after a long workday or stressful morning, this might explain why.

🔍️ Deep Dive

Study Overview

The researches wanted to test if mental fatigue, without any physical effort, could actually reduce endurance performance. Sixteen trained men and women completed two lab sessions:

1. Mental Fatigue group: 90 minutes of a demanding attention and memory task (AX-CPT).

2. Control group: 90 minutes of watching emotionally neutral documentaries.

Afterwards, each participant performed a cycling test to exhaustion at 80% of their peak power output while the researchers monitored physiological and psychological responses.

Key Findings

In short: mental fatigue didn’t affect the physiological markers like cardiovascular or muscular function, it simply made the effort feel harder. Participants hit their subjective limit faster, even though their bodies could have gone longer.

Potential Mechanism Proposed from the authors

Long and demanding cognitive work activates and fatigues the anterior cingulate cortex (ACC), a brain region linked to effort perception and decision-making. When the ACC is mentally drained, it interprets normal effort signals from the body as more intense. That amplified perception of effort becomes the limiting factor, not muscle oxygen or fuel supply.

Why It Matters

This study reframed endurance performance as psychobiological (the study of mental functioning and behavior in relation to other biological processes), not purely physical. It helps explain why:

• Tough workdays or poor sleep make training feel disproportionately hard.

• Managing mental fatigue before a race can be just as important as tapering training volume.

• Mental freshness is a performance variable, not a bonus.

🧪 Lab Notes

1. Protect your brain before key sessions.
Avoid heavy screen time, decision-making, or mentally draining work before threshold or race-pace workouts. Even 60-90 minutes of cognitive fatigue can reduce your endurance capacity.

2. Train your perception of effort.
Occasionally train under mild mental fatigue. For example, do intervals after a long workday or mentally engaging task. This builds tolerance to discomfort and teaches you to hold pace when RPE climbs early.

3. Track your effort perception.
Compare RPE to heart rate or power across similar sessions. If RPE rises while HR stays steady, it may be mental fatigue, not physical overtraining.

4. Recover the mind like the body.
Just like your body needs rest, your brain likely does too.

5. Apply it to race week.
If possible, Taper physical load and mental load. Reduce screen time, decision-making, and high-stress work before competition.