The short answer to the title is, it depends on your fitness level.
Lactic acid has been blamed for fatigue, delayed-onset muscle soreness, and cramps. A closer look at the research has shown that this is not the case at all. In fact, once lactate or lactic acid are metabolized, they are actually a preferred fuel source for quick and intense activities that we perform during intense exercise.
Exercise and Lactate
Larger than normal amounts of lactate are produced during the metabolism of glycogen (called glycolysis) primarily when insufficient oxygen is present (i.e. when you are huffing and puffing, trying to catch your breath!). Lactate is constantly being produced by our muscles and organs, but in very small amounts. Here is how it happens:
Glucose enters a cell and gets broken down via several enzymatic steps and electron carriers into pyruvate. One enzyme, lactate dehydrogenase (LDH), can convert pyruvate into lactate in one easy step with or without oxygen. The concentration of oxygen will determine how easily the lactate molecule will be produced into energy. In the presence of oxygen, a lot of lactate can be converted into energy. Without oxygen, lactate may still be converted into energy, but it is more dependable on several other factors (listed below). Because of how energy efficient lactate formation occurs, there is a lot of potential for lactate to be a preferred energy source.
During mild exercise, sufficient oxygen is available to the cells and the rate of lactate removal equals its rate of formation, so that there is no accumulation of lactate. This is dependent upon a number of factors including:
•    the type of muscle fiber that is exercising (more lactate is produced in fast-twitch muscles during intense exercise than slow-twitch muscles)
•    the concentration of monocarboxylate transporters (which play a major role in the regulation of intracellular pH and lactate concentration during intense muscle activity)
•    the concentration and isoform of LDH
•    the overall ability for tissues to create energy from other energy systems, such as the oxidative system.
Contrary to popular belief and older textbooks, lactate is not a toxic byproduct or waste-product of metabolism accelerated by exercise. As mentioned earlier, lactate is produced at rest in a number of organ systems and can serve as a valuable source of energy. When sufficient oxygen becomes available via rest or a decrease in exercise intensity, lactate is reconverted to pyruvate for use as energy. In addition, lactate and pyruvate formed in muscle during exercise can be used to manufacture glucose by a process called gluconeogenesis. This “extra” glucose supplements any existing blood glucose and muscle glycogen.
Lactate and Fatigue
Muscle fatigue is not due to “lactic acid build-up” in a muscle like motor oil in a car. During the formation of lactate (or lactic acid), there is an increase of positively charged hydrogen ions from the breakdown of the energy-producing molecule called adenosine triphosphate (ATP). These electrical charges, which increase the acidity of the blood, can interfere with the muscle contraction process and the efficiency of the enzymes involved in energy production (certain enzymes are more efficient at a better pH).
Lactate is not the cause of muscle soreness, since blood lactate returns to normal within an hour post-exercise. Delayed-onset muscle soreness is caused by the mechanical injury and secondary post-exercise inflammation.
Most muscle cramps, on the other hand, are caused by muscle nervous receptors that become overexcitable with muscle fatigue.
Lactate and the Athlete
Lactate does not accumulate significantly until exercise intensity reaches about 55% of the healthy, untrained subject’s maximum oxidative capacity. The rate of lactate accumulation now exceeds the rate of removal and fatigue is initiated. The intensity of exercise has to be decreased if the activity is to continue. This threshold is called the blood lactate threshold or OBLA (Onset of Blood Lactate Accumulation).
Many tissues, particularly skeletal muscles, continuously produce and use lactate. Blood levels of lactate reflect the balance between lactate production and use. An increase in lactate concentration does not necessarily mean that the lactate production rate was increased. Lactate may increase because of a decreased rate of removal from blood or tissues.
Lactate production is proportional to the amount of carbohydrates broken down for energy in the tissues. Whenever you use carbohydrates, a significant portion is converted to lactate. This lactate is then used in the same tissues as fuel, or it is transported to other tissues via the blood stream and used for energy. Rapid use of carbohydrate for fuel, such as during intense exercise, accelerates lactic acid production. Temporarily, lactic acid builds up in your muscles and blood because it can’t be used as fuel fast enough. However, if you slow down the pace of exercise or stop exercising, the rate of lactate used for energy soon catches up with the rate of lactate production.
Elite level endurance and intermediate-anaerobic athletes are able to perform at such a high intensity level because:
•    Their OBLA occurs at a higher percentage of their aerobic capacity.
•    They can remove lactate more rapidly or convert it more efficiently to glucose.

Proper training programs can speed lactic acid removal from your muscles while exercising. This can be achieved by combining high intensity, interval, and over-distance training.
To improve your capacity to use lactate as a fuel during exercise, you must increase the lactic acid load very high during training. Training with a lot of lactic acid in your system stimulates your body to produce enzymes that speed the use of lactic acid as a fuel.
High-intensity interval training will cause cardiovascular adaptations that increase oxygen delivery to your muscles and tissues. Consequently, you have less need to breakdown carbohydrates to lactate. Also, better circulation helps speed the transport of lactate to tissues that can remove it from the blood.
Because resistance training utilizes carbohydrates primarily as energy, high-intensity resistance training that produces high levels of lactate, such as certain Crossfit programs, are an effective way for you to improve your OBLA, ability to utilize lactate for energy and improve your overall fitness level.
Long-distance or endurance training causes muscular adaptations that speed the rate of lactate removal. Adaptations of endurance training increase the blood supply and the mitochondrial capacity of a trained muscle. Mitochondria are structures within the cells that process fuels, consume oxygen, and produce large amounts of ATP. A larger muscle mitochondrial capacity increases the use of fatty acids as fuel, which decreases lactate formation and speeds its removal.
Thus, many long-distance training programs incorporate different training modalities throughout their overall training program in order to allow the body to develop a better ability slow lactic acid production from carbohydrates and to enhance tissues ability to use lactic acid as fuel.
Lactate is an important fuel for the body during rest and exercise. It is one of our most important energy sources.
In order for you to improve your fitness leve in the shortest amount of time, you must train at a high intensity. This is uncomfortable because it will involve training at a low-oxygen availability. However, this will improve your body’s ability to use lactate as an energy source under strenuous exercising conditions. Therefore, get the most out of your time for exercise with the most benefits by training hard!