Key Finding: Caffeine and a short burst of exercise both raised participants' heart rates and blood pressure, but neither was effective at reducing the mental fatigue caused by a demanding cognitive task.
Study Overview
A recent study published in PLoS One set out to determine if common strategies—specifically caffeine consumption and short aerobic exercise—could effectively combat mental fatigue.
Researchers recruited 26 healthy adults (aged 18-30) who were regular caffeine consumers (150-500 mg daily).
Each participant completed three separate experimental sessions:
- Acute aerobic exercise: 20 minutes of cycling.
- Caffeine: A dose of 2.5 mg per kg of body weight.
- Placebo: A cornstarch solution.
In every session, mental fatigue was induced using a Stroop task—a cognitive test that requires focus and conflict resolution. The team then measured fatigue levels using both subjective self-reports and objective performance on a psychomotor vigilance task (PVT).
Results
Physiological indicators (heart rate and blood pressure) rose as expected in both the caffeine and exercise conditions, but not in the placebo group.
However, the core findings regarding fatigue were clear and consistent:
- All participants reported a significant increase in subjective mental fatigue after completing the Stroop task, regardless of which treatment they had received.
- Crucially, no significant differences in either objective or subjective mental fatigue were observed between the three groups. Neither caffeine nor exercise outperformed the placebo in countering the fatigue.
Limitations
The authors note several important caveats that affect how these results should be interpreted:
- No post-treatment measures. The study did not measure mental fatigue after the caffeine or exercise interventions, only during them.
- Single cognitive task. Only the Stroop task was used to induce fatigue. The study also did not assess EEG brain activity or heart rate variability.
- Dietary control. Nutritional intake was only loosely controlled, and participants self-reported their caffeine consumption and sleep patterns.
- Narrow sample. The participants were young, highly educated, and physically active, which limits the generalizability of the findings to the broader population.