The Science of Fatigue in Running: How Central and Peripheral Fatigue Jointly Determine Your Limits

Introduction

In the later stages of a run, your legs feel heavy, your pace drops, and you feel like you “can’t go on.” Where does this feeling of “can’t go on” come from? Is it that your muscles have truly run out of energy, or is your brain setting a “protective threshold” for you? The answer from modern sports science is: both, and they interact with each other. Understanding the difference between central fatigue and peripheral fatigue is an important foundation for breaking through personal limits and optimizing race strategy.

Peripheral Fatigue

Peripheral fatigue occurs at the muscle level, with the main mechanisms including:

Accumulation of metabolic byproducts:

• Accumulation of lactate and hydrogen ions reduces muscle fiber contractile capacity and pH levels
• Accumulation of phosphate (Pi) in the sarcoplasm interferes with the formation of cross-bridges between myosin and actin

Energy depletion:

• Muscle glycogen depletion leaves insufficient energy substrate for high-intensity output
• ATP resynthesis rate cannot keep up with the rate of consumption

Calcium regulation dysfunction:

• After prolonged contraction, the efficiency of calcium release from the sarcoplasmic reticulum declines, leading to reduced muscle contractile force

Central Fatigue

Central fatigue is defined as: a decline in the brain’s (anterior cingulate cortex, prefrontal cortex) ability to drive motor neurons, meaning that even if the muscles still have residual capacity, the nervous system’s “recruitment signal” is also decreasing.

Key mechanisms of central fatigue:

1. Serotonin Hypothesis: After prolonged exercise, the tryptophan/BCAA ratio in the blood rises, leading to increased serotonin synthesis in the brain, causing feelings of fatigue and drowsiness

2. Dopamine/Norepinephrine System: In hot environments or fatigued states, dopaminergic drive capacity declines, and the rating of perceived exertion (RPE) rises disproportionately

3. Central Governor Model: Proposed by Tim Noakes, the brain actively “protects” the body from overexertion, reducing output before the true physiological limit is reached—this “protective threshold” can be adjusted through training and race strategy

The Interaction Between Central and Peripheral Fatigue

The two types of fatigue do not operate independently but influence each other:

• Peripheral fatigue (signals from muscle receptors) → transmitted to the brain → amplifies the sensation of central fatigue
• Central fatigue (reduced neural drive) → decreased utilization of peripheral muscles → residual muscle strength is not recruited
• Psychological expectation (proximity of the finish line) → influences the setting of the central protective threshold → alters the force that can be output

This explains a common phenomenon: in the final kilometer of a marathon, even when the body is exhausted, runners can suddenly accelerate to the finish line—because a “visible finish” changes the brain’s protective calculations.

Training Strategies for Central and Peripheral Fatigue

Strategies to reduce peripheral fatigue:

• Improve aerobic base (higher lactate threshold)
• Adequate glycogen stores (carbohydrate loading before the race)
• Timely carbohydrate intake (60-90g per hour during exercise)

Strategies to enhance resistance to central fatigue:

• Training under fatigue (fatigue tolerance training): Add tempo runs to the later stages of long runs to train the brain to maintain output under fatigued conditions
• Pacing strategy: Negative splits allow the brain to reserve “central reserve” in the early stages
• Mental techniques: Break down goals (only run to the next landmark), reducing the psychological load of perceived exertion
• BCAA supplementation (limited research support, but some runners report benefits): Competes with tryptophan for entry into the brain, potentially delaying serotonin-mediated central fatigue

Practical Recommendations

1. Learn to distinguish between “feeling fatigued” and “true physiological limits”: Feeling very tired does not necessarily mean the muscles are truly powerless; try “challenging the brain’s protective threshold” at the end of training sessions
2. Add “end-of-fatigue accelerations”: Complete the final 3-5 kilometers of long runs at goal pace or slightly faster to train the central nervous system’s ability to maintain neural drive under fatigue
3. Carbohydrate loading before the race: Reduces peripheral fatigue from muscle glycogen depletion in the later stages of a marathon, indirectly reducing the triggering of central fatigue
4. Practice negative splits: Consciously control early pace so the brain “believes” there is reserve left for the later stages, helping to prevent the central protective threshold from activating too early

Conclusion

Fatigue is a story written jointly by the body and the brain—it is not only the muscles that are speaking. Understanding the interaction between central fatigue and peripheral fatigue allows you, in the final kilometers of your next race, to tell your brain with greater confidence: “There is still strength ahead—let’s keep running.”

Related Reading

• Central Fatigue and Peripheral Fatigue: Decoding the Dual Mechanisms of Exercise Exhaustion
• Brain Activity During Running: Central Fatigue Theory and the Runner’s Peak Experience
• Fatigue Mechanisms in Swimming: Central Fatigue vs. Peripheral Fatigue in Long-Distance Swimming
• Neuromuscular Fatigue in Running: The Science of Recovery After Long-Distance Runs