The different types of fatigue

As a coach I often hear the word, “fatigue”. It’s something that all runners get to experience in their careers. But, did you know that there are different types of fatigue? For example, the fatigue we feel after a speed workout is different than the fatigue we feel after a high mileage week of running. Fatigue can fall into one of four different categories. Muscular, metabolic, energy depletion, and central nervous system fatigue. Below I’m going to explain each of these four types of fatigue and how to combat them.

• Muscular fatigue. This type of fatigue results in micro trauma to our muscles. It’s that soreness and weakness we feel in our legs after a tough speed session. This micro trauma that causes muscular fatigue is also the cause of delayed onset muscle soreness (DOMS). This damage is what impacts our performance because it decreases strength and power. There are some things you can do during or after a workout or race to reduce damage to the muscles. Running on a softer surface could reduce micro trauma to muscles by reducing the impact and spreading out the impact that is absorbed over a longer period, but this isn’t an instant fix. Most strategies for decreasing muscular fatigue involve limiting the damage after a workout ends. Ice baths, compression socks, massage, or putting your legs up against a wall. What we want to do is limit inflammation. Inflammation occurs for two reasons. First, the damaged muscles leak their contents out into their surroundings. Next, the body rushes blood and lymph fluid to the damaged areas to repair them. Repair may seem like a good thing, but the combination of the cellular fluids leaking out from the damaged muscles and the fluids rushing in from elsewhere in the body can increase pressure to the point where further damage is done. So overall, we want to do our best to reduce inflammation.

• Metabolic Fatigue. This is the buildup of acid in the muscles and blood during a hard interval or race. Or Metabolic Acidosis. The common term for this is lactic acidosis buildup. Although this term is outdated as an anaerobic effort results in an increased amount of protons in the muscles and blood, which causes a drop in blood pH, which then irritates the nerves and causes pain. In other words, lactate buildup. Traditional anaerobic training improves our resistance to metabolic fatigue because we’re building a tolerance for it. However, doing too much of it can be a bad thing. Too much of it will decrease our aerobic development and cause overtraining. The only two ways to prevent metabolic fatigue are to improve efficiency and to increase aerobic fitness. Being a more efficient runner will allow us to hold a certain pace while burning less energy. There’s less anaerobic respiration happening which results in less metabolic fatigue. Then, being more aerobically fit will allow us to maintain a certain pace using an increased proportion of aerobic energy, which decreases metabolic fatigue.

• Energy Depletion. This type of fatigue occurs when our glycogen stores run out. This usually happens around 90 minutes of running which forces a big change in our energy usage and causes a drop in performance. Marathoners refer to this as, “hitting the wall”. As most know, the main source of energy during an aerobic effort is glycogen. This is a stored form of sugar. However, fat is also used during aerobic training, though it is a smaller source of energy than glycogen. The good thing about burning fat is that we’ll pretty much never run out of it while running. Unfortunately, relying on fat reserves for most of our energy necessitates a slower pace like with ultra runners. The main limitation is not aerobic capacity, but fuel utilization rate. However, if we can change the ratio of our energy sources at a given pace, we will be able to last longer running that pace without running out of fuel. There is plenty of evidence out there that states training in a low glycogen state can encourage the body to burn more fat and also allow us to be more efficient burning glycogen. So when stores are topped off on race day your efficiency will be at its highest. Something else to consider is that we can begin to experience fatigue from energy depletion before we actually run out of energy. The brain is trying to tell us that the fatigue is right around the corner. So, if a runner runs out of fuel after about 90 minutes of running, it wouldn't be surprising to see a slow down after 60 minutes in or earlier. Which in this case, eating carbs in a race lasting as short as an hour does have its benefits.

• Central Nervous System Fatigue. This is a type of fatigue that causes a big variability in running on any given day. It is majorly affected by mental factors such as mood, anxiety, excitement, along with other factors. Physical factors also play a role such as hormone changes, infections, or illnesses. Then there’s chronic fatigue syndrome. Runners diagnosed with this can be compared to CNS fatigue because even though their physiology is identical to healthy runners there is a significantly higher rate of perceived effort while running. It’s a controversial topic because some believe it’s a real physical issue while others claim it’s an imaginary mental issue. But it’s clear that a runners mental state can play a physical role. What evidence does this show us? There’s interaction with brain factors, like mood or anxiety, and the central nerve system. We shouldn’t be surprised it has an affect on running performance. There are also some external factors that can cause CNS fatigue. For example, CNS is usually the reason for overtraining. Not enough recovery throws off the biochemical and hormone balance of blood, which reduces the function of CNS. We can stimulate the CNS with both external and internal factors. For example, drinking caffeine can decrease our perceived effort and boost running performance. With internal factors, having a focused and determined mindset can allow us to have that increased energy at the end of a tough race which allows us to block out the other forms of fatigue. Of course there are limitations to this, like if you’ve bonked at the end of a marathon. There’s pretty much nothing you can do in this situation.

In the end, fatigue is broken up into four categories like explained above. Each of these have their own separate causes and fixes. Fatigue from a long run is usually muscular fatigue. Fatigue from a high mileage week is usually caused by CNS fatigue. Fatigue from a speed workout is usually metabolic fatigue. And then energy depletion happens when our glycogen stores are low. I hope all this information can be useful whenever you find yourself trying to figure out what’s going on with your body when you’re feeling worn down.