Science has introduced a lot of terminology into the vernacular of endurance sports. Words like "anaerobic" and "VO₂max" are batted around in magazine articles, chat forums and training groups. But more than a few endurance athletes are a little unclear about their meaning. So here's everything you wanted to know about some of the most common endurance sports terms (but maybe were afraid to ask):

Aerobic. A process whereby oxygen breaks down carbohydrates, fats and some amino acids inside muscle cells. The resulting energy release causes muscle cells to contract so you can swim, bike, run, etc. Training produces a number of physiological adaptations that increase your ability to use oxygen to produce muscle energy. For example, it increases the pumping capacity of the blood, the number of oxygen-carrying proteins in the blood, and the number of mitochondria ("aerobic factories") inside the muscle cells.

Aerobic capacity. The maximum rate at which your muscles can use oxygen to release energy. One of the main goals of endurance training is to increase aerobic capacity.

Anaerobic. Various processes by which energy is released in muscle cells without the help of oxygen, producing energy quickly but inefficiently. The faster you move, the more your muscles rely on anaerobic processes to meet their energy requirements. Therefore, anaerobic power is typically relied on for short bursts of high-intensity work such as short intervals or a sprint to the finish.

Anaerobic threshold. The intensity level at which the byproducts of anaerobic metabolism begin to accumulate in your muscles and blood, hastening fatigue. The greater your aerobic capacity, the faster the pace of running, cycling, etc., you can maintain before reaching this threshold.

Economy. The relative energy efficiency of your running, cycling, etc. The goal is to use as little energy as possible to move at any given speed. The more time you spend practicing a sport, the more your economy will improve.

Fast-twitch. A broad class of muscle fibers that tend to be faster and stronger, but also fatigue more quickly than slow-twitch muscle fibers. Muscle fiber characteristics depend on genetics and training. Some athletes are born with more slow-twitch fibers, others with more fast-twitch. But certain fast-twitch fibers can become more like slow-twitch fibers as a result of training, and vice versa.

Fat-burning zone. A range of exercise intensity in which your muscles burn mainly fat as opposed to carbohydrates. The largest amount of fat burning occurs at moderate intensities, whereas carbohydrates are burned at high intensities. However, there is no evidence that exercising in the fat-burning zone results in faster or greater overall body fat loss than exercising at higher intensities. This is probably because high-intensity exercise reduces fat storage between workouts more than moderate-intensity exercise does.

Glycogen. Form in which the simple sugar glucose (carbohydrate) is stored in your muscles and liver. Glycogen is the most important fuel for moderate- to high-intensity exercise.

Intensity. How hard you're working relative to your own personal maximum effort. You can gauge intensity by speed, pace, heart rate or power output.

Lactate. A byproduct of anaerobic metabolism. During high-intensity exercise, lactate accumulates outside muscle cells. While lactate does not cause fatigue, it is an indicator of other factors that do, such as increasing acidity in the muscles and potassium buildup.

Lactate threshold. Another term for anaerobic threshold.

Lactic acid. A preceding form of lactate. When lactic acid is produced in the muscles, it loses a hydrogen ion and becomes lactate. These hydrogen ions cause the muscles to become acidic and to fatigue.

Slow-twitch muscles. A general class of muscle fibers best suited to aerobic (slow) versus anaerobic (fast) metabolism.

Target heart rate. The specific heart rate or range that indicates you're training at the appropriate intensity for your individual abilities and goals. Use a heart rate monitor to measure this intensity.

VO₂max. Another term for aerobic capacity, VO₂max indicates maximal oxygen uptake.