The Fatigue Factor

Fatigue is an unavoidable part of the marathon. In a sense, marathon running is like a contest pitting your physical strength and spiritual will against the multi faceted attacks of fatigue. Fatigue can take many forms. The type of fatigue that affects you depends upon how fast and how far you are running.

Phosphocreatine Depletion

A sprinter must deal with depletion of phosphocreatine, the source of short term, high intensity energy. This system produces energy for short, intense bursts. The supply of phosphocreatine will be depleted within 5 to 20 seconds. This is the primary cause of fatigue when performing an all out sprint but is of little concern or interest to a marathon runner. You may use this system when sprinting to the finish line or when you accelerate to pass a competitor, but the fatigue caused by this system should really not concern you during a marathon.

Blood Acidity

When you are racing at distances from 800 meters to 10 kilometers another type of fatigue rears it’s ugly head. Remember earlier I said that a by-product of anaerobic energy production is lactic acid? In the past it was commonly believed that lactic acid was the cause of muscle fatigue when running at more intense race paces. Now we know that not only does lactic acid not cause fatigue, but is a valuable source of energy production.

Lactic acid accumulation may indirectly contribute to fatigue at fast paces. As you increase your pace, lactic acid begins to pile up faster than your body can process it for energy. The accumulating lactic acid goes through a conversion that results in a build up of hydrogen ions. The hydrogen ions cause a condition called acidosis. This condition causes your body to release buffers that reduce the effect of the hydrogen ions. The end result is that the buffers cause a decrease in the muscle pH, which causes a decrease in the ability of your muscles to do work.

Some exercise physiologists believe that this process is caused by your body trying to protect itself. If the acids in your muscle were allowed to continue to rise, serious muscle damage could be the result. The buffers that decrease your muscles ability to work could be a self protective mechanism that keeps you from damaging your body by continuing to run at an intense pace when your muscles are already too acidic.

Your marathon race pace is slow enough that lactic acid accumulation does not take pace, so blood acidity is not a direct major contributor to marathon fatigue.

Potassium Buildup Fatigue

A recent and interesting finding is that a buildup in potassium can also be a major contributor to running fatigue. Your muscles depend upon an exchange between potassium and sodium to fire a muscle contraction. During high intensity distance running, potassium is released faster than it can be used. This can cause an interruption or weakening of the ability of your muscles to contract - or in simpler terms, it causes fatigue. This potassium build up takes place at roughly the same pace and at the same time as the hydrogen ion build up, so it is difficult to determine whether the hydrogen ion build up or the potassium is causing the fatigue. The most recent research seems to point to potassium build up, but both probably play a role.

Just as blood acidity, potassium build up only occurs at speeds at or near your 10K race pace, so this type of fatigue is not a major concern when running at marathon pace.

Hypoglycemia

Phosphocreatine build up, blood acidity and potassium accumulation are all causes of fatigue when running near or faster than 10K pace, but are not usually a problem for a marathon runner. Marathon pace is slow enough that your body is able to clear the accumulating lactic acid and potassium. If you are running your marathon fast enough to encounter a significant rise in blood acidity or potassium you are headed for certain disaster before you reach the half way point! The more moderate marathon pace brings its own types of fatigue that you must do battle with. The first and most common face of marathon fatigue is hypoglycemia, which is more commonly known as carbohydrate or glycogen depletion. When you hear the term “running into the wall”, most runners are referring to hypoglycemia.

When you eat foods containing carbohydrates, your digestive system converts the various sugars in the carbohydrates into glucose. This conversion is necessary to make the carbohydrates usable by your body. When the glucose is released into your bloodstream a hormone called insulin is also released. Insulin tells your body’s tissues to store some of the newly released glucose. This is stored in your muscle and liver as glycogen.

Glycogen depletion is one of the main causes of fatigue for marathon runners. During marathon pace runs, your stores of glycogen will be depleted at around 18 to 21 miles (29 to 35 km). This point is referred to, by marathoners, as “hitting the wall”. Much of this sensation is caused by a nearly complete depletion of glycogen.

Central Nervous System Fatigue

Your brain is a remarkable organ. It toils endlessly in directing all of the physical, mental and emotional processes needed to keep your body running efficiently. As much work as the brain does on a daily basis, it has been estimated that most of us only use about 10% of our brains total capacity. That leaves a lot of power that is going untapped. As a runner your brain may be performing a service that you are not even aware of.

One of the most recent theories concerning the causes of running fatigue involves your brain and your central nervous system (CNS). Your brain controls every action that takes place in your body, both conscious and sub-conscious. It controls your breathing, your heart rate and is responsible for sending signals to your muscles to contract. It is now believed that your brain may be a major if not the primary cause of running fatigue in not only the marathon but also shorter running events.

Your brain and your spirit are sometimes at odds. Your brain wants to keep your body in a state of balance. It wants to maintain your ATP levels and your stores of glucose. It wants to protect your muscles and organs from damage. Your spirit is more concerned with successfully completing your goal. One of these times is during running events. Your spirit wants to run faster and further than your brain considers to be safe. When you run long distances your body begins to run low in energy providing glycogen. Your muscles also begin to suffer some minor damage and trauma from the repeated strides and contractions. According to this latest theory your brain takes over at that point. In effect, your brain tries to over-ride your spirit. Since you won’t consciously stop running or slow your pace, your brain begins to decrease the signals to your working muscles. You feel this in the form of fatigue and pain. It becomes much more difficult to maintain your pace.

While this theory is relatively new and unproven, it does explain why many athletes are able to increase their pace during the final miles of a marathon. If the cause of the fatigue were purely physical, increasing pace at that point should be impossible. One of the benefits of training may be that your brain begins to learn and understand that your muscles are able to withstand longer and longer distances or faster paces. As a result you are able to run longer and faster before your brain steps in to slow you down.

Continue to The Goals of Marathon Training

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