Mental Fatigue and Strength Training: How Cognitive Load Kills Performance
You see it all the time—athletes grinding through heavy lifts, pushing through fatigue, and maxing out their physical limits.
You see it all the time—athletes grinding through heavy lifts, pushing through fatigue, and maxing out their physical limits. But here’s the thing: athletes don’t break down physically first. They break down mentally.
A new study has just made this brutally clear.
Researchers set out to test how mental fatigue (MF)—not physical exhaustion—impacts weight lifting, resistance training, and endurance cycling. And the results?
Cognitive fatigue doesn’t just make training feel harder—it actively destroys performance.
Let’s break it down.
Mental Fatigue Increases Perceived Effort—Even When the Body Is Fresh
In this two-part study, researchers first tested how pure cognitive fatigue (without physical exertion) impacted strength training. Sixteen athletes were asked to complete a grueling 90-minute cognitive task designed to exhaust their brains. Then, they were put through a standard weightlifting protocol, testing lifts at 20%, 40%, 60%, and 80% of their 1-rep max (1RM).
What happened?
Even though their bodies were 100% fresh, the athletes’ perception of effort skyrocketed. Lifting the same weights felt 7% to 15% heavier than normal. That means a weight that felt easy when mentally fresh suddenly felt like a grind—without any actual physical fatigue involved.
This proves a critical point: fatigue isn’t just in your muscles. It’s in your brain. And when your brain is tired, everything feels harder, even when you’re physically capable.
Mental Fatigue Before Training = A Harder, Worse Session
In the second phase of the study, researchers tested what happens when cognitive fatigue is mixed into a typical training session.
Here’s how it worked:
- Control group: Standard strength training session—6 exercises, 3 sets of 12 reps at 70% 1RM, followed by a 20-minute cycling time trial.
- Mental fatigue group: Same exact training session, but with cognitively demanding tasks inserted between exercises and during rest periods.
The outcome?
- Sessional RPE (how hard training felt) was 22% higher for the mentally fatigued group.
- Perceived effort was significantly worse across all lifts.
- Cycling performance plummeted, with fatigued athletes producing lower power outputs and covering less distance in the time trial.
Again, no actual physical disadvantage—just mental load crushing their ability to push.
This isn’t just a minor issue. If you’re training while mentally fatigued, your performance is suffering—guaranteed.
Why This Matters for Strength, Power, and Endurance Athletes
This research reveals a brutal truth for athletes, coaches, and high-performance teams:
- Mental fatigue can increase perceived effort—even when there’s no actual physical fatigue.
- If your cognitive load is high before or during training, your performance will take a hit.
- Mental fatigue doesn’t just affect endurance—it impacts resistance training, lifting efficiency, and power output.
For years, cognitive load has been ignored in training, but this study makes it clear: if you’re serious about performance, mental fatigue monitoring should be as standard as tracking reps, weights, and heart rate.
How to Fix It: Practical Takeaways for Athletes and Coaches
If mental fatigue is the silent killer of performance, how do you beat it? Here’s what the best athletes are already doing:
✅ Monitor Cognitive Load – Track screen time, social media use, and demanding mental work before training. If you’re doing mentally taxing tasks pre-session, expect your RPE to increase.
✅ Reduce Cognitive Load Before Training – Minimize distractions, unnecessary meetings, and decision-making stress before heavy training days. Don’t overload your brain before a max-effort session.
✅ Use Brain Endurance Training (BET) – Controlled cognitive training can actually increase mental resilience and decrease the negative effects of MF. This is why elite teams are now incorporating cognitive priming sessions.
✅ Sleep and Nutrition Matter More Than You Think – Sleep debt amplifies mental fatigue, and low-carb states can accelerate cognitive exhaustion. Dial in recovery and fuel properly.
✅ Be Strategic About Cognitive and Physical Training – If an athlete has mentally fatiguing tasks (film review, meetings, study), schedule them AFTER the most demanding physical sessions—not before.
Final Thought: This Is the Next Performance Edge
For years, teams have tracked everything—heart rate, GPS data, force plate metrics. But until now, cognitive fatigue has been completely overlooked.
This study makes it obvious:
Mental fatigue isn’t just an annoyance—it’s a performance killer.
And in a world where inches, milliseconds, and reps separate the elite from the average, ignoring cognitive fatigue isn’t just a bad strategy—it’s a liability.
The best athletes train their brains like they train their bodies. The question is—are you?