Fatigue takes many different forms. From a physiologist’s perspective, it’s the inability to maintain or repeat a given level of muscle force production, resulting in an acute impairment of performance – or much more simply put you slow down. Runners will know that the fatigue associated with an 800 metre race is not like the fatigue associated with the marathon.

However, all causes of fatigue have one thing in common, they lead to a decrease in effective muscle force production.

Fatigue is not something specific to slow or average runners. Even world record holders fatigue; they just do it later in a run and at a much faster pace than the rest of us. Indeed, fatigue is necessary to protect our bodies from damage. Paradoxically, the only way to get faster is to cause some damage so the fatigue occurs at a faster pace.

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To do that you must repeatedly threaten the body’s survival with training stimuli so that it adapts and physiologically overcompensates. So when the same stress is encountered, it does not cause the same degree of physiological disruption.

Central nervous system

Although some research supports that the central nervous system (CNS) can cause fatigue by reducing the commands it sends to our muscles – the prevailing evidence and thought among scientists is that fatigue, at least during short-term exercise, is not caused by the CNS but rather by changes occurring in muscle.

The CNS seems to play a greater role in fatigue during prolonged exercise, like marathons and ultramarathons, where there are changes in the levels of the brain neurotransmitters, which increase the perception of effort and lead to feelings of tiredness and lethargy.

Since the CNS command to the muscles can be reduced by the CNS being inhibited directly from changes in brain neurotransmitters or from feedback from the muscles’ metabolic condition (i.e. in response to the chemical reactions taking place within them) it’s possible that under conditions that represent a risk to organs, especially the heart, inhibitory signals may be sent to the brain.

In response to these signals, the neural command is dampened and the muscles’ power output consequently declines in an attempt to prevent damage to the heart. This may partially explain why maximum heart rate is lower at altitude, since the lower oxygen availability would put the heart at risk at a lower heart rate than at sealevel.

Acknowledging the potential, but unresolved role of the CNS, I’ve provided the main causes of fatigue for different races, with advice on how to combat them.

800m/1,500m/mile

Middle distance atigue is primarily caused by a high rate of anaerobic metabolism – this occurs when running faster than your heart can supply oxygen to your muscles. To provide the energy for muscle contraction, a high-energy chemical compound called adenosine triphosphate (ATP) is broken down into its constituents—adenosine diphosphate (ADP) and inorganic phosphate (Pi).

Since our muscles don’t store much ATP, we must constantly resynthesise it. Running faster relies on increasing the rate at which ATP is resynthesised, so that it can be broken down to liberate energy for muscle contraction.

When you exceed your aerobic metabolic capacity to resynthesise ATP, muscles lose their ability to contract effectively. This is because of an increase in hydrogen ions, which causes the muscle pH to decrease – a condition known as ‘acidosis’.

Acidosis has a number of nasty side effects:

(1) it inhibits the enzyme that breaks down ATP inside muscles, which decreases muscle contractile force

(2) it inhibits the release of calcium (the trigger for muscle contraction) from its muscle storage sites

(3) it inhibits ATP produced by glycolysis (a metabolic pathway, that literally means the burning of glycogen), by inhibiting the key enzyme involved in glycoysis.

In addition to hydrogen ion accumulation, other metabolites create specific problems inside muscles, from inhibition of specific enzymes involved in muscle contraction to interference with muscles’ electrical charges, ultimately leading to a decrease in muscle force production and your running speed.

While the effects of anaerobic metabolism cause that heavy, dead-legged feeling when racing the mile or running 400-metre repeats at mile race pace, limitations in aerobic metabolism also cause fatigue in the middle distances by limiting the pace that you can maintain aerobically. Your legs feel like lead during these short races because you’re not getting enough oxygen to them.

That’s why it’s so important to run lots of miles even if you’re a middle-distance runner—you have to develop yourself aerobically to delay the acidosis and the accumulation of metabolites. Here’s how to combat fatigue in the middle distances:

Short intervals

Short intervals (45 seconds to about 2 minutes) run at 800 to 1,500-metre race pace improve your ability to buffer acidosis and increase anaerobic capacity by increasing the number of enzymes involved in anaerobic metabolism. For specific workouts, see Workouts to Combat Fatigue (below).

3,000 to 10,000 metres

The 3,000 to 10,000 metres are primarily aerobic races. So, limitations in aerobic metabolism, due to inadequate blood flow to and oxygen use by the muscles, are the major causes of fatigue.

However, since any race that is run faster than lactate threshold pace includes a significant anaerobic contribution, metabolic acidosis and accumulation of metabolites also contribute to fatigue in these longer distances because they are run faster than lactate threshold pace.

That’s why it’s important to also do anaerobic work for these longer distances Lacate threshold refers to the fastest speed which you can manitain
aerobically. Here’s how to combat fatigue in the 3,000 to 10,000 metres:

Long intervals

Long intervals (3-5 minutes) increase the heart’s stroke volume (amount of blood pumped by the heart per beat) and cardiac output (amount of blood pumped by the heart per minute), leading to an increase in VO2max, (the maximum volume of oxygen your muscles consume per minute). Research has shown that high-intensity training (@ 95 to 100% VO2max) is the optimal stimulus for its improvement.

While long intervals are the most potent stimulus because you repeatedly reach and sustain VO2max during the work periods, short intervals (<1 minute) can also improve VO2max, as long as you run them at a high intensity and with short, active recovery periods to keep VO2 elevated throughout the workout.

Regardless of the length of the intervals you choose, you should run them at the speed at which VO2max occurs (referred to as ‘velocity at VO2max’ or ‘vVO2max’), which is approximately 3,000-metre race pace for highly trained runners. If you run 3,000 metres in longer than about 10 minutes, your vVO2max will be between mile and 3,000- metre race pace (see Workouts to Combat Fatigue).

If using heart rate as a guide, you should come close to reaching your maximum heart rate by the end of each interval.

Tempo runs

combat fatigue2

Tempo runs improve your lactate threshold above which acidosis occurs. Lactate threshold is the best physiological predictor of distance running performance. Increasing your lactate threshold pace allows you to run faster before you fatigue, because it allows you to run faster before oxygen-independent metabolism begins to play a significant role.

The longer the race, the more important it is to train your lactate threshold because the more important it becomes to be able to hold a hard pace for an extended time. So, for the marathon and half-marathon lactate threshold training should be the focus of your workouts.

I typically use three types of lactate threshold workouts with my athletes:

(1) Continuous runs at lactate threshold pace

(2) Intervals run at lactate threshold pace with short rest periods

(3) Shorter intervals run at slightly faster than lactate threshold pace with very short rest periods. Lactate threshold pace is about 10 to 15 seconds per mile slower than 5K race pace (or about 10K race pace) for runners slower than about 40 minutes for 10K.

For highly trained and elite runners, the pace is about 25 to 30 seconds per mile slower than 5K race pace (or about 15 to 20 seconds per mile slower than 10K race pace). The pace should feel ‘comfortably hard’.

High mileage

High mileage seems to improve running economy – the oxygen cost of maintaining a given pace. Research has shown that runners who perform high volumes of endurance training tend to be more economical, which has led to the suggestion among scientists that running high mileage (greater than 110K) improves running economy.

Economy is improved largely from increases to capillary and mitochondrial density, the former increasing the speed that oxygen can diffuse into your muscles, and the latter increases your muscles’ capacity to use oxygen. Both of these are enhanced with high mileage. It is also possible that the countless repetition of the running movements results in optimised biomechanics and muscle fibre recruitment latterns.

Additionally, economy may be improved by the weight loss that usually accompanies high mileage, which leads to a lower oxygen cost; the hypertrophy (growth) of slow-twitch skeletal muscle fibres (which are more suited for aerobic metabolism) and a greater ability for tendons to store and utilise muscle’s elastic (propulsive) energy.

Because it’s hard to prove cause and effect, it is not entirely clear whether high mileage runners become more economical by running more miles or are innately more economical and can therefore handle higher mileage without getting injured.

Marathon

If you’ve ever run a marathon, you know that the feeling of fatigue is quite different from the middle distances or even a 10K. That’s because marathon fatigue is primarily due to running out of fuel rather than the by-products of metabolism.

Training to combat fatigue_3

You have enough stored carbohydrate (glycogen) to last slightly more specifically to last for more than two hours of sustained running at a moderate intensity. So, unless you plan on running as fast as Paula Radcliffe over 42K, you’re going to run out of fuel. Glycogen depletion and the accompanying low blood sugar levels (hypoglycemia) coincide with hitting the infamous wall.

Given the length of the marathon, there are other things that cause fatigue that don’t play a major role in shorter races. When you sweat dehydration occurs – this causes a decrease in the plasma volume of blood, which decreases the heart’s stroke volume and cardiac output. When this happens oxygen flow to muscles is compromised and running pace slows.

Relentless pavement pounding causes muscle fibre damage and this reduces muscular force. Muscles produce heat when they contract and this mean that running for long periods of time is a threat to your body temperature.

The resulting ‘hyperthermia’ decreases blood flow to the active muscles (since more blood is directed to the skin to increase convective cooling), reducing the ability to regenerate ATP via aerobic metabolism. Finally, running for long periods can cause psychological or neural fatigue – the latter of which results from changes in the levels of brain neurotransmitters. Here’s how to combat fatigue in the marathon:

Run long

Repeatedly running for long periods of time (longer than two hours) or running for slightly less time but with a portion at lactate threshold pace, during which carbohydrates are used at a faster rate presents a threat to the muscles’ survival by depleting their storage of glycogen.

Given adequate ingested carbohydrates following the long run, the skeletal muscles respond very beneficially to the ‘empty tank’ by synthesising and storing more glycogen than usual, thus increasing endurance for future efforts.

Consume 1.0 to 1.5 grams of simple carbohydrates (e.g., glucose) per kilogram of body weight within 30 minutes after long runs and lactate threshold/long slow distance combos to maximise the rate at which glycogen is stored and continue to consume 1.0 to 1.5 grams per kilogram every two hours for four to six hours afterward.

Ingest carbs during the race

Muscles prefers carbohydrate as fuel during exercise and research examining supplementation with carbohydrate during prolonged exercise has shown that fatigue can be delayed. Begin ingesting glucose about 30 minutes before you hit the wall so the glucose has time to be absorbed into your blood where it can be used for energy.

Drink fluids with sodium

Since your sweat rate exceeds your ability to ingest fluid while running, dehydration is difficult to prevent. However, since endurance performance declines with only a two to three percent loss of body mass due to fluid loss, it’s important to minimise its effects. During the marathon, drink fluids with sodium – this is because water goes wherever sodium goes, specifically meaning more water is conserved by the kidneys.

Run long on pavement

Unless you’re planning on running a trail marathon, do your long runs on pavement to prepare for the muscle fibre damage you’ll sustain in the race. To preserve your legs do your other runs during the week on softer surfaces.

Acclimatise to the heat

Climate has a greater effect on the marathon than it does on any other race. If your marathon is going to be in hot, humid weather, prepare yourself by acclimatising to those conditions. While cardiovascular adaptations to running in the heat are nearly complete within one week, the sweating response takes about two weeks, so give your self at least two weeks of slowly introducing yourself to the heat.

The next time you train for a big race, ask yourself what causes fatigue at that distance and then do the right training to combat it. To survive the threat you present it, your body will improve its endurance.

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