Overall fatigue can be thought of as the sum of four distinct specific fatigues--muscular fatigue, metabolic fatigue, energy depletion, and central nervous system fatigue.
Muscular fatigue results from microtrauma to muscles--the soreness and weakness in your legs 30 minutes after a hilly run are an example of this. It is thought that the microtrauma that causes muscular fatigue is also what causes delayed-onset muscle soreness (DOMS). This damage to the muscles directly impacts performance by reducing strength and power output. Interestingly, it seems that DOMS is brought on exclusively by eccentric movements, which is why a middle-distance runner gets sore calves and hamstrings after a race, but usually not sore shin muscles or quadriceps. The quadriceps do work eccentrically when running downhill or over very soft terrain, which is why hilly or mushy runs seem to beat up your quads.
There is little you can do during a workout or race to reduce the eccentric activities that damage muscles. Running on a softer surface may reduce microtrauma to muscles by reducing the peak impact loading rate and spreading out the impact that must be absorbed over a longer period, but there's nothing more than anecdotal evidence for this. Most strategies for decreasing muscular fatigue involve limiting the damage after a workout ends. Icebaths, compression socks, putting your legs up against a wall, and Alberto Salazar's much-mocked cryosauna all attempt to decrease inflammation in the legs and increase the efflux of fluids from the muscles back into the blood and the rest of the body. Inflammation occurs for two reasons: 1) the damaged muscles leak their contents out into their surroundings and 2) 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 and internal fluid leakage occurs. In cases of major internal damage, such as surgery or bodily trauma, the swelling in response to the injury can cause life-threatening complications, and compression, elevation, and ice effectively combat this swelling. There's no reason to worry, however--if internal bleeding and swelling from surgery is a fire hose, post-workout microtrauma is a faucet drip.
There is some evidence that moderate-temperature "ice" baths will reduce soreness and increase performance in the days following a hard workout. Interestingly, most studies use 12-15°C (54-59°F) water baths--rather puny compared with training-room tough guys who like to crank it down to 50°F or colder. But there's no evidence colder is better! In fact, some people speculate that very cold temperatures can actually increase damage. I like to think that the hydrostatic pressure from the weight of the water itself is a significant contributor to the recovery boost. Standing in an ice bath with 24" of water provides a pressure differential of 45 mmHg between your feet and your knees--that's more than compression socks would provide!
Likewise, compression wear also has a body of evidence supporting its use. Compression wear was actually invented to control swelling and reduce the possibility of deep-vein thrombosis in hospital patients after surgery. Veins have a series of one-way valves that are supposed to allow blood to flow only one direction (back towards the heart). But the veins in your legs are susceptible to pooling, where blood stagnates because gravity is preventing the blood from achieving a high enough pressure differential for the valves to operate properly. Compression counters the hydrostatic pressure inside your body due to gravity, allowing better bloodflow. Compression wear has been getting a lot of attention recently because it might also increase performance during exercise via better bloodflow, increased proprioception, or decreased muscle vibration.
Fortunately for the impoverished runner, the evidence for these approaches is equivocal--some studies find no difference in recovery betwen groups who use compression or ice-bathing and those who do not. If there are benefits, they are relatively small. Further, leg elevation is a low-tech approach that likely has many of the same effects as compression and ice-bathing (namely, increasing fluid efflux from and reducing fluid influx to the lower body). My entire high school team used to put our legs up against a wall for 5 minutes or so following our cool-downs.
Take that, cryosauna!
Metabolic fatigue is the familiar "burn" from metabolic acidosis--the buildup of acid in the muscles and blood after a hard interval workout, a race, or a set of pushups. "Lactic acid buildup" is a common tern for this condition, although this has been shown to be a misnomer. In truth, the fatigue from a hard anaerobic effort like the 400m dash probably results from the buildup of protons (H+) in the muscles and blood, which causes a drop in intramuscular and blood pH, which in turn irritates nerves (causing pain) and disrupts muscular sensitivity to calcium ions, impeding the muscles' ability to contract. Lactate, a negatively-charged ion produced by a completely different biochemical process, closely mirrors the concentration of protons, but not exactly. There is a small window when an athlete just exceeds the anaerobic threshold called the isocapnic buffering zone in which blood lactate is increasing but blood pH is not. The reason the pH is not dropping is because the natural buffering agents in the muscles and blood (bicarbonate, phosphate, and others) are doing their job--buffering the acid and not allowing it to accumulate.
As an interesting aside, the isocapnic buffering zone allows for some highly specialized workouts which can target a few physiological processes by exploiting the fact that lactate is rising but blood pH is not dropping. These workouts are sometimes called "crest-load" workouts and involve briefly exceeding the AnT, then backing off and recovering before blood pH drops and you begin to struggle. This is repeated multiple times just like a regular interval workout. The difference is that you are (in theory) training your body to process lactate but not submitting yourself to the damaging effects of a drop in blood pH. These "crest-load" workouts should have a strong effect on the expression of monocarboxylate transporter (MCT) proteins--these proteins latch on to lactate and (as the name suggests) transport it across cell membranes. This allows the MCT proteins to shuttle lactate from one muscle to another via the blood. The MCT proteins have a very high affinity for transporting lactate to one muscle in particular: the heart. Since all of the heart's energy comes from aerobic processes, it is an excellent place for the body to burn lactate for energy. Moving lactate around and using it as energy should, in theory, take some of the strain off the muscles under the most metabolic stress. So greater expression of MCT proteins should increase performance. I've added a great number of qualifiers here (should, in theory, probably) because MCT proteins are very poorly understood and "crest-load" workouts still remain in the esoterica of applied exercise physiology. They are not widely-used and well-understood workouts, but perhaps in the future we'll know more about where they belong in a training cycle, how often you need to do them to reap the benefits, etc.
We may hear more about these proteins soon, as MCT proteins are also under scrutiny by cancer researchers. Fast-growing cancer cells use them the same way muscles do, shuttling around lactate to assist with cellular metabolism. There's one other example I can think of where oncology and exercise physiology unexpectedly cross paths, and it is hypoxia-inducible factors. These are partially responsible for the performance boost from high-altitude training and also induce tumor growth (though surprisingly, people who live at high altitude have lower cancer mortality rates). It's fascinating that some of the same factors that influence athletic performance also influence the progression of cancer.
Anyways, getting back on topic, more traditional hard, anaerobic training (like interval workouts) improve your resistance to metabolic fatigue by a more blunt mechanism: increasing your tolerance. Just like a heroin addict shooting up, repeated exposure to anaerobic workouts over time increases your ability to deal with metabolic fatigue. However, doing too much too soon can land you in a world of hurt. Okay, so maybe you can't overdose and die on anaerobic training like you can on heroin, but too much anaerobic training will hamper your aerobic development and cause overtraining.
The only real ways to prevent metabolic fatigue at a given speed are to improve efficiency and to increase aerobic fitness. You ought to have a broad idea of how to go about doing this (and if not, fear not, yet another topic for yet another post!). Becoming a more efficient runner will allow you to maintain a given speed for less energy, so less anaerobic respiration will be required, and hence less metabolic fatigue. Becoming more aerobically fit will allow you to maintain a given speed using a higher proportion of aerobic energy, which does not increase proton buildup and does not increase metabolic fatigue.
Moving on, fatigue from energy depletion is only occasionally encountered by high school and collegiate athletes, but is a familiar feeling to marathoners. This occurs when glycogen stores run out after 90-120 minutes of continuous running, forcing a drastic change in energy pathway usage and usually a large drop in performance (colloquially known as "hitting the wall"). A lot of cutting-edge marathon training schemes today focus on shifting the balance of energy pathways before glycogen stores run out. As you're probably aware, the chief source of energy in a high-end aerobic effort (like a marathon) is glycogen, which is just a stored form of sugar. However, fat is also used during aerobic exercise, though it is a smaller source of energy than glycogen. The upside of burning fat directly is that you'll never run out of it during an athletic effort. While most athletes have enough glycogen to fuel them for 90-120 minutes of high-end aerobic running, even the skinniest of runners have enough fat for hundreds of kilometers. Unfortunately, relying on fat reserves as the source for most of your energy necessitates a fairly slow pace (hence the slow pace of ultramarathon records--the chief limitation is not aerobic capacity, but fuel utilization rate). However, if a runner can change the ratio of his or her energy sources at a given pace, he or she will be able to last longer running that pace without running out of fuel. There is evidence that training in a low-glycogen state can encourage the body to burn more fat at a given effort. Below, world-famous Italian coach Renato Canova describes how athletes need to attain the optimal ratio of fuel sources:
However, any training in a glycogen-depleted state will be of lower quality, since your power output will be limited (by your lack of fuel, of course). So the benefits of shifting fuel sources needs to be weighted against the benefit of a high-quality workout."The real difference in Marathon Training between the recent past and today, is the different interpretation we have about how to build the right fuel. The secret of top Marathon runners is the ability in changing your engine. As my English is not very fine, may be that is difficult to explain the idea, but I try.
We have a table. On this table there are 3 different containers : one is full of FAT, another is full of GLYCOGEN, and a third is empty. The athlete must learn to create the optimal mixture, taking part of it from one container, part from the other."
Additionally, the body is a little more complicated than a car--it won't run at full-throttle until the gas tank is empty. The paper I referenced above alludes to a "throttling" effect where the brain will actually slow down the body before glycogen runs out. The brain senses that glycogen stores are dwindling, and forces a slow-down in order to avoid a crash. This is similar to what happens with regard to exercising in the heat--the brain slows the body down before temperatures rise above normal levels; in other words, it anticipates the threat to homeostasis and proactively moves to counter it.
The upshot of this is that you can begin to experience fatigue from energy depletion before you actually run out of energy. So a problem that seems to belong solely to marathoners and ultramarathoners starts to creep into the realm of middle and long-distance track training. If a normal athlete runs out of fuel after about 2 hours of running, it wouldn't be surprising to see detrimental effects 90 minutes in or earlier. Indeed, research has shown a boost in performance when ingesting carbohydrates in aerobic events as short as one hour. Now, it probably isn't necessary to refuel halfway though every single easy run. You won't notice or reap the benefits of a few percentage points of improvement on your daily 8-mile route. But refueling during long (90+ minute) runs should be seriously considered. I hate to rely on anecdotes, but I find that refueling about halfway through a long run provides an almost-immediate lift in how I feel. This is even more relevant if you have a tendency to "hammer" the end of your long runs--it'll allow you to squeeze down the pace faster with fewer problems. The best source of sugar on-the-run will vary from individual to individual. I've tried everything from honey to gatorade to smarties candy, but research shows that 1) you should take anything you consume with water and 2) most people can tolerate a solution of 5-6% sugar in water. Higher concentrations cause gastric problems in some, but not all people. Gatorade is 5.8%, Powerade is about 7%. Fruit juice should generally be avoided because its sugar profile can cause an upset stomach (this is partially why marathoners in the '70s used watered-down, de-fizzed Coco-Cola); a mixture of fructose and glucose is also absorbed faster. When starting out, I've found straight Gatorade is too strong. Watering it down 1:1 or 2:1 will make it more palatable.
How much to drink? Surprisingly, much of the benefit of refueling mid-workout has nothing to do with actually burning the sugar ingested. Indeed, swishing with a carbohydrate solution and immediately spitting it out provides a boost to performance! This meshes well with the idea that the brain "throttles" the body based on carbohydrates in reserve. Once it senses that more carbohydrates are being ingested, it allows the body to return to full function since new fuel is on the way. If you want a number, however, most research indicates that the body can absorb no more than 60g of carbohydrate per hour (equivalent to about 32 ounces of Gatorade). But you really only need to concern yourself with such high intake levels during a very long, high-quality workout (like a 13+ mile marathon-pace tempo or similar). Taking a few swigs of a watered-down sports drink halfway through your long run will be enough for most people to avoid energy depletion in everyday training.
Finally, there is central nervous system (CNS) fatigue. It is the least-understood component of fatigue, but plays a major role in any athletic endeavor. CNS fatigue is likely what causes the large day-to-day variability in performance in any given athlete. It is heavily affected by mental factors like mood, excitement, pressure, anxiety, etc., as well as physical factors (both external and internal): hormones, infections and diseases, and even time of day. Sports psychologists focus on getting athletes "primed" to avoid CNS fatigue--too little anticipation causes a lifeless, time-trial-like performance, while too much causes an athlete to "choke." People diagnosed with chronic fatigue syndrome are an interesting case to examine in relation to CNS fatigue: even though their physiology is usually identical to healthy individuals, patients with chronic fatigue syndrome report significantly higher rated perceived exertion (RPE, a measure of "how hard" a physical effort is) than healthy subjects, and elect to stop tests to exhaustion much earlier. Chronic fatigue syndrome is a controversial topic; some researchers believe it's a real, physical illness, while others peg it as a psychosomatic illness--a physical manifestation of something "imaginary." But it's clear from other widely-accepted diseases that mental states can have physical manifestations--clinical depression can cause increased fatigue, altered sleeping patterns, headaches, and cramps, among other things.
What does this tell us? There's a big interplay between brain factors (mood, mental state, disposition) and the central nervous system, and we shouldn't be surprised that it has an affect on athletic performance. In one study, swimmers classified as "pessimistic" swam worse in a race simulation when researchers falsely gave them slow split times. Swimmers classified as "optimistic" continued to hit their splits despite getting falsely slow split times. From a purely physical standpoint, negative psychological feedback decreased athletic performances in the pessimists but not in the optimists. Did the pessimistic swimmers who slowed down feel more tired when they heard their (falsely) slow splits? Or did they decide (subconsciously perhaps) to decrease the stimulation of their CNS? The interface between the brain and the rest of the CNS is very poorly understood. How do negative thoughts cause poor athletic performance? How does that mental state translate into lower physical stimulation of the nerves? It probably has to do with levels of different neurotransmitters, but we are venturing far, far away from my areas of expertise. I simply don't know enough about the neurology and physiology at play to answer these questions. Hopefully I'll do more reading and will post again on this topic!
What I do know, however, is that there are some external factors that can modulate CNS fatigue. For one, the cause of overtraining is usually chalked up to central nervous system fatigue. Insufficient recovery throws off the biochemical and hormonal balance of the blood, which depresses the function of the nervous system. In contrast, the CNS can be stimulated by caffeine, which increases athletic performance and decreases rated perceived exertion. Additionally, internal factors can stimulate the CNS too--a focused burst of mental energy at the end of a race ("kicking") allows the CNS to temporarily overcome the other three forms of fatigue. Even though all of the physiological factors are stacked against you: blood and intramuscular pH is rising, muscles are damaged, and in a marathon, glycogen stores are depleted. And yet an enormous mental effort can recruit and fire more motor neurons and tremendously increase your speed at the end of a race. There are limitations, though--if you've ever found yourself doing the "walk of death" at the end of an 800, you know well that no amount of mental effort will overcome the state of metabolic fatigue you've worked yourself into.
So, summing things up, fatigue can be broken down into four components: muscular fatigue, metabolic fatigue, energy depletion, and central nervous system fatigue. Each has separate causes and remedies, and in specific situations, one type of fatigue is usually dominant over the others. Fatigue at the end of a long run is mostly muscular and energetic in origin, fatigue in the middle of a high-mileage week is mostly caused by CNS fatigue, and fatigue during an interval workout is mostly metabolic fatigue. Cold-water immersion, compression wear, and leg elevation might be able to reduce muscular fatigue after a hard run or workout, though the science isn't bulletproof. Improving your tolerance to metabolic fatigue through interval workouts will increase your ability to perform under anaerobic stress, but improving efficiency and increasing aerobic fitness are the only ways ways to significantly reduce your reliance on anaerobic respiration, the root cause of metabolic fatigue. Energetic fatigue happens when glycogen stores run low. Though full-on glycogen depletion is only a serious concern for marathoners and ultramarathoners, the brain's efforts to throttle performance to conserve glycogen might affect high school and college athletes on their long runs. Refueling with a diluted sports drink can hold off energy depletion. Finally, central nervous system fatigue has numerous causes and effects. It can be modulated by various internal and external factors, but we do not have a comprehensive understanding of how it works.