When you race, you run hard. The goal is to get to the finish line as quickly as possible and to beat as many people as you can. However, we’ve all inevitably hit a point of running fatigue during a race, and most likely during training as well. When your legs refuse to work any harder, your brain cells seem to go numb, and all you feel is pain. The finish line eventually comes into sight and you feel like you’re sprinting! But times just seems to slow as your legs sloppily drag themselves across the ground.

You probably wonder what causes running fatigue. What can you do to prevent it? Or even simply delay the inevitable appearance of running fatigue. If you’re running to the best of your ability, you can be sure to experience it, but hopefully, this article can help you to understand what you can do to help combat running fatigue so you can run faster and further.

7 Causes Of Running Fatigue

Glycogen Depletion

Your muscles store glycogen which is converted into glucose during exercise. An adult weighing about 70kg can store around 100g – 120g glycogen in the liver and about 400g in the muscles. During exercise, your body uses an enzyme known as glycogen phosphorylase to break down the glycogen into glucose.

The glucose is then used during respiration to release energy for muscular contractions. If your body runs out of glycogen to use (typically called ‘hitting the wall’ during a marathon) then your muscles will struggle to contract due to a decreased amount of energy being released.

You are more prone to glycogen depletion if you do not eat enough carbohydrates (particularly if you exercise a lot). Untrained runners usually run out of glycogen during a marathon. However, if you consume fast-absorption carbohydrates during a marathon, this can prevent glycogen depletion.

Interestingly, in people who follow a high-fat, low-carb diet, some become so efficient at using fat and breaking it down into glycogen and glucose that they can run with less risk of running out of glycogen since they also have fats as a fuel source which they can breakdown easily.

Lactic Acid Accumulation

When you exercise your body releases energy for muscular contractions using oxygen – this is aerobic respiration. At higher or maximal intensities, to release as much energy as possible, your body will start releasing energy without oxygen – anaerobic respiration. The problem with this is anaerobic respiration creates a byproduct known as lactic acid.

Lactic acid is removed from the blood during exercise, but if too much lactic acid accumulates it starts to make the body’s cells more acidic and lowers their pH. Enzymes cannot function well in acidic environments so the enzymes denature and are rendered useless. This prevents the body from releasing as much energy for muscular contractions and this irritates nerve endings causing fatigue and pain.

Reduced Rate Of ATP Synthesis

ATP is the molecule that is broken down to release energy. Our body synthesises ATP so it can be broken down again to release energy. When ATP and PC (phosphocreatine) stores are depleted, there is not enough ATP to carry out muscular contractions. Phosphocreatine is usually broken down into phosphorus and creatine, and the phosphorus is used to create more molecules of ATP.


Water is lost through sweating, and if it is not replaced we begin to dehydrate. Dehydration causes our blood to become thicker (more viscous) which stops it from flowing to the working muscles as easily. This means oxygen is arriving at the muscles at a slower rate, so less energy (from ATP) is being released per minute.

This also causes a loss in electrolytes, which are salts like potassium and sodium which your body uses for muscular contractions. Blood pressure decreases, as a result, so blood flow is slower. To prevent further water loss, your sweating rate is lowered which causes your body temperature to rise more quickly.

Reduced Levels of Calcium

Calcium is an electrolyte required for muscular contractions. This is because for muscles to contract, calcium has to be released. Insufficient calcium in the diet can be one reason for this type of muscular fatigue. Another reason for this muscular fatigue is due to an increase in hydrogen ions (which can come from lactic acid accumulation). Hydrogen ions increase the acidity of the body’s cells and reduce the amount of calcium released.

Reduced Levels Of Acetylcholine

Choline is a nutrient synthesised by the body in small amounts and is used to create the neurotransmitter, acetylcholine. Because only a small amount of choline is synthesised within the body, we must consume choline within our diet from foods such as eggs.

Acetylcholine helps nerve impulses to travel through our body and initiate muscular contractions. When levels of acetylcholine are low, our body cannot cause muscular contractions as easily and our muscles get fatigued.

Muscle Damage

During vigorous and repetitive exercise, your muscle fibres can become torn and damaged over time. Whilst in this damaged state, your muscle fibres are unable to contract as powerfully and are more susceptible to fatigue. This is one of the reasons why rest and recovery is so important to perform at your maximum.

Did you know about these causes of running fatigue? If you found this article useful please share with your fellow runners and let me know what you think of it down below!