A glossary of commonly heard endurance-related terms

What exactly does "anaerobic threshold" mean? Is it important to know your heart-rate reserve? Is VO2max testing worthwhile for the average age-grouper?

This brief glossary will answer many questions you've been afraid to ask about the intensity-related terminology commonly used by endurance-sports coaches and others.


This term usually refers to aerobic metabolism, which involves the release of energy from carbohydrates and fats for the kinds of muscle contractions athletes use in endurance activity.

Oxygen is used to help in this energy-release process, hence "aerobic" (occurring in the presence of oxygen). The process happens inside organelles called mitochondria located within muscle cells.

More than 95% of muscle energy is produced aerobically in all endurance sports activities lasting more than a few minutes.

Aerobic capacity

Also called VO2max, aerobic capacity refers to the maximum rate at which an athlete can use oxygen for aerobic metabolism.

Aerobic capacity varies widely among individuals. Factors that affect it are the size and power of the heart, the density of oxygen-carrying compounds in the blood, the density of capillaries and mitochondria in the muscles, and the activity of aerobic enzymes.

Proper training enhances all of these factors and thereby increases aerobic capacity.


There are actually three distinct types of anaerobic metabolism, but only one of the three -- anaerobic glycolysis -- matters much to endurance athletes.

In anaerobic glycolysis, carbohydrates are broken down to release energy without the help of oxygen (see "Aerobic" above). This process is less efficient than aerobic metabolism (it yields less energy per glucose molecule), but it's also 2 to 3 times faster.

Therefore, the faster you go in any activity, the more anaerobic metabolism is called upon to supplement aerobic metabolism in supplying the total energy needs of the working muscles.

These two energy systems also complement each other in the sense that the aerobic system is able to take up and use a certain key intermediate product of anaerobic glycolysis: namely, pyruvate.

Anaerobic threshold

Also known as the "lactate threshold," this is the intensity level above which a certain intermediate product of anaerobic metabolism -- pyruvate -- is produced faster than the aerobic system can absorb and use it. Unused pyruvate quickly splits into lactate and positively charged hydrogen ions, which are acidic.

At exercise intensities above the anaerobic threshold, these ions begin to accumulate in the muscles, increasing their acidity to a point that causes exhaustion. The burning feeling you get in your muscles at high exercise intensities is caused by this phenomenon, called acidosis.

Critical speed

Critical speed is the maximum speed a swimmer, cyclist, runner or other endurance athlete can maintain without experiencing fatigue caused by muscular acidosis. At critical speed or less, an athlete can continue usually until muscle glycogen depletion ("bonking") or accumulating muscle damage causes exhaustion after a few hours.

Cyclists with power meters on their bike can also measure critical power in watts, which is essentially the same thing.

Heart rate reserve

Heart rate reserve is the difference between your resting heart rate and your maximum heart rate. For example, if your resting heart rate is 60 beats per minute and your maximum heart rate is 190 BPM, then your heart rate reserve is 130 BPM. This variable is used in some formulas designed to produce customized target heart rate training zones.


Intensity is an athlete's current work rate as a percentage of his or her maximum work rate in a given activity. The absolute measurement of intensity is power output. Only cyclists can conveniently monitor their power output through the use of power meters on their bikes, but runners and other athletes can use speed is a close equivalent.

It is also possible to measure intensity in physiological terms, specifically as a percentage of VO2max or as a percentage of maximum heart rate.

Lactate testing

Also called "lactate profiling," lactate testing is used to determine the speed, power output level, and/or heart rate at which an athlete's anaerobic threshold currently lies. Tiny blood samples are taken from an athlete at several points during a graded exercise test, in which speed is increased every few minutes until the athlete becomes exhausted.

While lactate testing provides useful information, athletes can usually find their anaerobic threshold pretty accurately by other means. For example, for runners the lactate threshold corresponds to 10K race place plus 1 - 12 seconds per mile.

Lactate threshold

See anaerobic threshold above.

Maximum heart rate

Maximum heart rate is genetically determined, decreases slowly with age, and is task-specific (i.e. your maximum heart rate in swimming is not the same as your maximum heart rate in cycling or running). Training does not increase maximum heart rate, and knowing your maximum heart rate is of little value except when used to determine target heart rate training zones.

Maximum lactate steady state

This is the speed or power output level an athlete can maintain without experiencing a steady increase in blood lactate levels leading to exhaustion. (Even though it's the accumulation of acidic hydrogen ions in the muscles that causes exhaustion, blood lactate levels are a reliable indicator of muscle acidity.)

In most athletes, the lactate value associated with this variable is about 4 mmol/L. Maximum lactate steady state is basically equivalent to critical speed.


Power is work performed per unit time, or work rate. It is usually measured in watts. As mentioned above, cyclists are the only endurance athletes who can conveniently monitor their power output level during training and racing, through the use of power meters (different brands of which use different measuring mechanisms).

Power output is an ideal intensity measurement because it is absolute. Speed is not absolute because, for example, speed decreases on hills even as the work rate stays the same or increases. Heart rate is not absolute because it is affected by a variety of factors, including stress and dehydration.

Target heart rate

There are several formulas endurance athletes can use to estimate appropriate training intensity zones based on heart rate. The most accurate formulas involve zones based on percentages of heart rate reserve. Here's one example:

Intensity: target heart rate range (low fitness level) / (high fitness level)

Recovery: 50-60% / 60-70%
Aerobic: 60-70% / 70-80%
Anaerobic threshold: 70-80% / 80-90%
VO2max: 95-100+% / 90-100+%

Ventilatory threshold

At low to moderate exercise intensities, oxygen consumption increases linearly as speed increases. However, at a point very close to the anaerobic threshold in most athletes, oxygen consumption begins to increase geometrically.

The ventilatory threshold can be pinpointed in a graded exercise test wherein the athlete breathes into a tube that captures and measures exhaled gases. There is no practical value in knowing your ventilatory threshold.


Also called "aerobic capacity," VO2max is the maximum rate of oxygen consumption (usually adjusted for body weight) an athlete can achieve during exercise.

All else being equal, the higher an athlete's VO2max is, the higher his or her anaerobic threshold will be and the faster her or she can go in endurance races without becoming fatigued.

Training can increase VO2max by up to 20%. VO2max can be determined through a graded exercise test; however, it is only of academic interest because oxygen consumption cannot be monitored during workouts.

Matt Fitzgerald is the author, most recently, of the "Runner's World Guide to Cross-Training" (Rodale, 2004).

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