Have you ever pushed through a tough workout, only to feel your muscles burning and giving out? That's muscle fatigue in action—a common experience for athletes, weekend warriors, and fitness enthusiasts.
But what causes it, exactly?
Muscle fatigue isn't just about "getting tired” -- it's a complex physiological process involving your body's energy systems, nerves, and even your brain. In this blog post, we'll break down the science behind muscle fatigue, explore its main causes, and offer some tips to delay its onset.
Let's get started!
What is Muscle Fatigue?
Muscle fatigue is defined as “a transient decrease in the capacity to perform physical actions”.[1] It can happen during intense exercise, like sprinting or weightlifting, or even prolonged low-intensity activities, such as hiking for hours. While it might feel like your muscles are simply “tapped out”, the root causes behind muscle fatigue are numerous, stemming from both peripheral (muscle-level) and central (brain and nervous system) factors.
Scientists have long debated the exact mechanisms, but research points to a combination of metabolic buildup, energy depletion, ion imbalances, and neural signals. Contrary to popular belief, lactic acid isn't always the villain—though it plays a role in some cases. Instead, factors like inorganic phosphate accumulation may be more significant in limiting muscle performance.
Metabolic Causes: When Your Muscles Run Out of Fuel
At the heart of muscle fatigue are metabolic changes within the muscle cells themselves. During exercise, your muscles rely on adenosine triphosphate (ATP) for energy. When ATP stores deplete, or when by-products accumulate, performance drops.
ATP and Glycogen Depletion
Muscles use glycogen (stored carbs) to produce ATP. Prolonged activity can exhaust these reserves, leading to fatigue. This is often seen in endurance sports where energy demands outpace supply.
Accumulation of By-Products
As muscles work anaerobically (without oxygen), they produce metabolites like hydrogen ions (H+), which causes the pH to drop in the muscle and ultimately inhibit muscle contraction.
The more intense your workout, the more quickly these metabolites accumulate since oxygen delivery can’t keep up with demand. This forces muscle cells to rely on less efficient energy pathways.
Neural and Central Factors: The Brain's Role in Fatigue
Producing force or power depends on the contractile mechanisms within muscle fibres. Before that can occur, though, a chain of processes in the nervous system must instruct the individual fibers to contract. A disruption at any point in this pathway could also impair force or power generation.
Nervous System Exhaustion
Repeated muscle contractions can lead to impaired signaling from the brain or spinal cord to the muscles. This might involve neurotransmitter depletion (such as acetylcholine, which is required for muscle contraction) or feedback from fatigued muscles signaling the central nervous system to slow down.
Motivational Decline
We’ve all reached that point in a workout when we feel we can’t do another rep. This loss of motivation is a form of central fatigue, influenced by psychological factors and brain chemistry, such as a drop in dopamine or an increase in brain serotonin.
Other Contributing Factors: Ions, Blood Flow, and Lifestyle
Beyond metabolism and nerves, several other factors can accelerate the onset of muscle fatigue:
Ion Imbalances
Exercise disrupts the balance of ions like potassium, sodium, and calcium inside and outside muscle cells. For instance, potassium buildup outside muscle cells can depolarize membranes, making it harder for muscles to contract properly.
Impaired Blood Flow
Restricted oxygen and nutrient delivery due to vasoconstriction (narrowing of blood vessels) or dehydration can hasten fatigue.
Lifestyle and Training Influences
Overtraining, undertraining, or injuries can lead to short-term or chronic fatigue. Factors including poor nutrition, lack of sleep, or certain medications can exacerbate it.
How to Combat Muscle Fatigue
While you can't eliminate fatigue entirely—it's a natural protective mechanism—you can delay it with the following tips:
- Get enough sleep
- Stay hydrated
- Have carbs pre workout
- Don’t neglect rest days
- Limit alcohol intake
You can also use certain supplements to help you train harder, last longer, and delay the onset of fatigue. Some of our favorite supplements for improving endurance and stamina are:
- Creatine: improves the body’s ability to regenerate ATP
- Betaine: enhances endurance by improving cellular hydration through its role as an osmolyte, which helps maintain optimal fluid balance and reduces fatigue during exercise
- Caffeine: research-backed CNS stimulant that improves fatty acid oxidation (thereby boosting energy production) and delays the onset of central fatigue
- Tyrosine: supports dopamine levels in the body and helps improve resistance to psychological stress
The Bottom Line on What Causes Muscle Fatigue
Muscle fatigue is a complex physiological process driven by a mix of metabolic waste, energy shortages, neural signals, and ion shifts. Being aware of these factors can help you plan your diet and workouts to train smarter and push your limits safely.
For those of you looking to take your workouts to the next level, we suggest the following 1UP supplements:
- Pre Workout: Contains betaine, tyrosine and many other research-backed ingredients to help increase workout performance, enhance hydration and delay the onset of fatigue
- Creatine: King of sports nutrition that’s been studied thousands of time and repeatedly shown to be safe and effective for enhancing performance, stamina, and results
- Hydration Plus: Supplies essential electrolytes that can get depleted during strenuous workouts to help maintain fluid balance and support proper muscle contraction and relaxation
References
- Enoka RM, Duchateau J. Muscle fatigue: what, why and how it influences muscle function. J Physiol. 2008 Jan 1;586(1):11-23. doi: 10.1113/jphysiol.2007.139477. Epub 2007 Aug 16. PMID: 17702815; PMCID: PMC2375565.
- Taylor JL, Amann M, Duchateau J, Meeusen R, Rice CL. Neural Contributions to Muscle Fatigue: From the Brain to the Muscle and Back Again. Med Sci Sports Exerc. 2016 Nov;48(11):2294-2306. doi: 10.1249/MSS.0000000000000923. PMID: 27003703; PMCID: PMC5033663.
