It all started with an innocent race once around the track in sixth grade. Midway through the final curve, I felt something, something that would change my life. Her name, I discovered later, was lactate. As I continued to run, she teased me with her power, drawing on the reigns, gently at first, then harder with each passing moment. Harder. Harder. By the time I had reached the finish line, she had taken control of my whole body with her rapture. I could no longer move. It was love at first sight.
First discovered in 1780 in sour milk, lactic acid (or lactate, as she is known at the pH of body fluids and to her friends) is produced in a metabolic pathway known as glycolysis. Her mother, pyruvic acid, also known as pyruvate and herself a product of glycolysis, is converted into lactate when oxygen is not supplied fast enough to meet the needs of the cell. This happens a lot during intense exercise because the muscle cell's need for energy (ATP) is too immediate to wait on oxygen, who left pyruvate standing alone at the altar (the entrance to the Krebs cycle) for his duties as the patriarch of metabolism. "I'm oxygen," he says to the muscle cell, with more than a hint of superiority. "I can give you a lot of ATP, but you will have to wait for it." Oxygen knows that he is worth the wait, as he controls the fate of endurance (not to mention that he is the sustenance of life). Therefore, as it is well known, there is an accumulation of lactate in the muscles and blood during intense exercise. And from the time I first experienced her caress in sixth grade, I was hooked. I still regularly sneak away from home to go to the track, just so I could be near her and feel her embrace.
It wasn't until years later, when I began my graduate work in exercise physiology, that I learned the extent to which lactate really is misunderstood. And it was then, when I finally understood what was misunderstood by so many, that our love affair blossomed.
Fatigue's Faulty Scapegoat
Fatigue is a difficult thing to pin down. Because there are so many things happening simultaneously inside muscles when they are working hard, it is difficult, if not impossible, to determine the exact cause of fatigue. It's like trying to find out what causes cancer. Fatigue, like cancer, has many different faces. The fatigue associated with the marathon is not like the fatigue associated with the 800 meters, any more than breast cancer is like prostate cancer.
Scientific inquiry typically begins with the formation of a hypothesis and the design of a research study to test that hypothesis. One of the key attributes of a well-designed study is the controlling of confounding variables, things that can interfere with the outcome. It is only when these confounding variables are controlled that a scientist can determine if the observed outcome is an effect of the treatment that was given. It is similar to determining why you ran well or poorly on a given day. After all, there are many things that influence athletic performance. Things like the weather, the training program, the athlete's level of fatigue, the pacing of the race, the athlete's degree of anxiety or nervousness, stress from other areas of the athlete's life, these all could have influenced the athlete's performance on Tuesday. But how does the coach know which is the cause? Such is the case with determining the cause of fatigue.
From the time Nobel Prize winners A.V. Hill and Otto Meyerhof discovered in the 1920s that lactic acid is produced during fatiguing muscle contractions in the absence of oxygen, lactic acid has been the exercising community's scapegoat for fatigue. But why? Why does lactate get all the blame? There has never been any experimental evidence that has shown a cause-and-effect relationship between lactate production and fatigue. While lactate increases dramatically during intense exercise, so do other metabolites, most notably hydrogen ions, which are considered the major threat to the muscle's acid-base balance. Lactate doesn't even reveal all of herself unless the exercise uses anaerobic glycolysis as the predominant metabolic pathway. So in events like the 100 meters or a marathon, speaking about lactate is like speaking about your mistress in the presence of your wife. When anaerobic glycolysis is the predominant energy system being used, hydrogen ions, like lactate, accumulate in muscles and blood. However, it is the accumulation of hydrogen ions, which are produced from the breakdown of ATP during muscle contractions and from other chemical reactions of glycolysis, that decreases muscle pH, causing metabolic acidosis and, ultimately, fatigue.
But even hydrogen's role in fatigue has been questioned by some scientists, who lay the blame on yet other metabolites. Because of lactate's concomitant increase with hydrogen ions and the simple method of measuring her concentration, blood lactate is used by scientists only as an indirect measure of acidosis. Although it has been widely accepted by the scientific community for a long time that lactate is innocuous and is not the cause of fatigue during intense exercise, lactate is still regarded by runners as the enemy. Scientific terminology is, unfortunately, slow to change, and lactate has suffered the most.
Lactic Acid is Burning Me
Jane Fonda may have been the first to popularize muscle burning during exercise, asking her exercise video audience to "feel the burn." As a result, there have been many misconceptions about the nature of the muscle burn, including the wrong assertion that lactic acid is the cause, possibly due to the connotation of the word "acid" and its association with burning. However, lactic acid is a weak acid and, as already discussed, is not the cause of acidosis. No physiologist has ever burned himself when taking a blood sample from a subject containing a high blood lactate concentration. "Burning" may even be the wrong term to use when describing how muscles feel during intense exercise, since the sensation is certainly not the same as putting your hand over a fire or pouring hydrochloric acid on your skin. (Now that's burning!) No one seems to know exactly what causes the sensation of muscle burning, but it is possible that it is nothing more than the increase in muscle temperature that accompanies intense exercise.
Will Lactic Acid Massage My Sore Muscles?
Many athletes, coaches, fitness professionals and the general public think that lactic acid is also the cause of muscle soreness. However, muscle and blood lactate return to pre-exercise levels within 30 to 60 minutes after exercise, so any extra lactate is long gone by the time soreness develops. Muscle soreness is rather the result of microscopic tears in the muscle fibers, causing an initial mechanical injury (which may be related to the contractile proteins, actin and myosin, pulling apart) and a delayed biochemical injury, which usually brings about the perception of soreness. The soreness typically worsens during the first 24 hours after exercise, peaks from 24 to 72 hours and then subsides within five to seven days as the muscles heal.
Oh, Lactate! How I Need You!
Not only does lactate not cause fatigue, her production in muscle is vital during intense exercise, as she serves a number of roles. Lactate production maintains the ratio of certain biochemical molecules, supporting the continued ability of glycolysis to keep working. Lactate is also used as a fuel by the heart, is used by the liver to make new glucose (blood sugar) by a process called gluconeogenesis and is converted back into glycogen (the stored form of carbohydrate) by a reversal of the chemical reactions of glycolysis. Both the new glucose and glycogen are then themselves used as fuels by muscles so exercise can continue at the desired intensity. So much for lactate being a waste product.
Lactic Acid Whispering Sweet Nothings in My Ear
As a mirror to what is taking place in the muscle during exercise, the lactic acid concentration of the blood, which is typically obtained from a prick to the finger or ear, tells us the changing relationship between effort and speed. My first finger prick came in the fall of 1995 in the Human Performance Laboratory at the University of Calgary in Alberta. A number of finger pricks generates a graph like the one shown below. At slower speeds, lactate increases slowly, while at faster speeds, lactate increases rapidly. But why the tease? Why does lactate change the rate at which she reveals herself? At slower speeds, lactate is removed from the muscles as quickly as she is produced. At faster speeds, however, when there is a greater reliance on anaerobic glycolysis for energy, lactate removal starts lagging behind lactate production, and lactate begins to accumulate in the muscles and blood.
Think of a bucket with a hole in it that sits out in the rain. When it's drizzling, the water that fills the bucket empties through the hole. But when it's pouring, water fills the bucket faster than it empties through the hole, and water accumulates in the bucket. To take the analogy further, there is an intensity of rainfall at which the amount of water emptying the bucket is just enough to keep up with the amount of water entering the bucket so that the level of water reaches the top of the bucket but does not overflow. If the rainfall is heavy enough, the bucket will eventually overflow. The point at which lactate quickly accumulates (the overflowing bucket) is an important marker in physiology and is affectionately called the lactate threshold. With an increase in aerobic fitness, the "lactate curve" shifts to the right (the dotted curve in the graph) because there is less lactate accumulation at the same submaximal speeds. With training, the lactate threshold occurs at a faster speed. This happens because endurance training improves the ability to remove lactate (someone cut a larger hole in the bucket). The lactate threshold could just as easily be called the acidosis threshold since, as already discussed, the accumulation of lactate is only a reflection of the state of muscle and blood acidosis.
Above, the rightward shift of the lactate curve indicates an improvement in endurance.
Biochemically, a lactic acid value indicates the status of pyruvate metabolism, with a high value indicating conditions that favor the conversion of pyruvate to lactate instead of its transportation into the Krebs cycle. Athletes who achieve high maximal lactate values (the highest point on the graph's curve) do so because they have many fast-twitch muscle fibers that use anaerobic glycolysis as their primary energy system. Being able to increase an athlete's maximal lactate value through training would help performance in those events that rely on anaerobic glycolysis and therefore result in high lactate values, such as 400 and 800 meters. Being able to produce lots of lactate is a good thing.
Lactic acid is also used in the clinical setting, where a high resting lactic acid value may indicate liver disease or hypoxemia (deficient oxygenation of the blood). Since the liver uses lactic acid to make glucose, a high lactic acid value may indicate liver dysfunction. Alternatively, a high lactic acid value may indicate hypoxemia since, in the absence of oxygen, pyruvic acid will be converted to lactic acid.
Although my intellectual love affair with lactate over the years has sometimes been put on hold to study other things, our physical love affair has always remained, rekindled every time I go to the track. Perhaps, someday, she won't be misunderstood, and she can be admired for what she truly is.