Exercise intensity refers to how much energy is expended when
exercising
Exercise is a body activity that enhances or maintains physical fitness and overall health and wellness.
It is performed for various reasons, to aid growth and improve strength, develop muscles and the cardiovascular system, hone athleti ...
. Perceived intensity varies with each person. It has been found that intensity has an effect on what
fuel
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
the body uses and what kind of adaptations the body makes after exercise. Intensity is the amount of physical power (expressed as a percentage of the
maximal oxygen consumption) that the body uses when performing an activity. For example, exercise intensity defines how hard the body has to work to walk a mile in 20 minutes.
Measures of Intensity
Heart Rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
is typically used as a measure of exercise intensity. Heart rate can be an indicator of the challenge to the
cardiovascular system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
that the exercise represents.
The most precise measure of intensity is oxygen consumption (VO
2). VO
2 represents the overall metabolic challenge that an exercise imposes. There is a direct linear relationship between intensity of aerobic exercise and VO
2. Our maximum intensity is a reflection of our maximal oxygen consumption (
VO2 max). Such a measurement represents a cardiovascular fitness level.
VO
2 is measured in
METs
The New York Mets are an American professional baseball team based in the New York City borough of Queens. The Mets compete in Major League Baseball (MLB) as a member of the National League (NL) East division. They are one of two major league ...
(mL/kg/min). One MET, which is equal to 3.5 mL/kg per minute, is considered to be the average resting energy expenditure of a typical human being. Intensity of exercise can be expressed as multiples of resting energy expenditure. An intensity of exercise equivalent to 6 METs means that the energy expenditure of the exercise is six times the resting energy expenditure.
[Vehrs, P., Ph.D. (2011). Physical activity guidelines. In Physiology of exercise: An
incremental approach (pp. 351-393). Provo, UT: BYU Academic Publishing.]
Intensity of exercise can be expressed in absolute or relative terms. For example, two individuals with different measures of VO
2 max, running at 7 mph are running at the same absolute intensity (miles/hour) but a different relative intensity (% of VO
2 max expended). The individual with the higher VO
2 max is running at a lower intensity at this pace than the individual with the lower VO
2 max is.
Some studies measure exercise intensity by having subjects perform exercise trials to determine
peak power output, which may be measured in
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
s, heart rate, or average
cadence (cycling). This approach attempts to gauge overall workload.
Intensity Levels
Exercise
Exercise is a body activity that enhances or maintains physical fitness and overall health and wellness.
It is performed for various reasons, to aid growth and improve strength, develop muscles and the cardiovascular system, hone athletic ...
is categorized into three different intensity levels. These levels include low, moderate, and vigorous and are measured by the
metabolic equivalent
The metabolic equivalent of task (MET) is the objective measure of the ratio of the rate at which a person expends energy, relative to the mass of that person, while performing some specific physical activity compared to a reference, set by convent ...
of task (aka metabolic equivalent or METs).
The effects of exercise are different at each intensity level (i.e.
training effect
Exercise physiology is the physiology of physical exercise. It is one of the allied health professions, and involves the study of the acute responses and chronic adaptations to exercise. Exercise physiologists are the highest qualified exercise ...
). Recommendations to lead a healthy lifestyle vary for individuals based on age, weight, and existing activity levels. “Published guidelines for healthy adults state that 20-60 minutes of medium intensity continuous or intermittent aerobic activity 3-5 times per week is needed for developing and maintaining cardiorespiratory fitness, body composition, and muscular strength.”
Fuel Used
The body uses different amounts of energy substrates (
carbohydrates or
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
s) depending on the intensity of the exercise and the
heart rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
of the exerciser.
Protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
is a third energy substrate, but it contributes minimally and is therefore discounted in the percent contribution graphs reflecting different intensities of exercise. The fuel provided by the body dictates an individual's capacity to increase the intensity level of a given activity. In other words, the intensity level of an activity determines the order of fuel recruitment. Specifically, exercise physiology dictates that low intensity, long duration exercise provides a larger percentage of fat contribution in the calories burned because the body does not need to quickly and efficiently produce energy (i.e.,
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
) to maintain the activity. On the other hand, high intensity activity utilizes a larger percentage of carbohydrates in the calories expended because its quick production of energy makes it the preferred energy substrate for high intensity exercise. High intensity activity also yields a higher total caloric expenditure.
This table outlines the estimated distribution of energy consumption at different intensity levels for a healthy 20-year-old with a
Max Heart Rate (MHR) of 200.
These estimates are valid only when glycogen reserves are able to cover the energy needs. If a person depletes glycogen reserves after a long workout (a phenomenon known as "
hitting the wall
In endurance sports such as road cycling and long-distance running, hitting the wall or the bonk is a condition of sudden fatigue and loss of energy which is caused by the depletion of glycogen stores in the liver and muscles. Milder instances ...
") or during a low carbohydrate diet, the body will shift into
ketosis and use mostly fat and muscle for energy.
Intermittent fasting
Intermittent fasting is any of various meal timing schedules that cycle between voluntary fasting (or reduced calorie intake) and non-fasting over a given period. Methods of intermittent fasting include alternate-day fasting, periodic fasting, a ...
can be used to train the body to shift easily into ketosis.
See also
*
Exercise physiology
*
Human power
Human power is work or energy that is produced from the human body. It can also refer to the power (rate of work per time) of a human. Power comes primarily from muscles, but body heat is also used to do work like warming shelters, food, or o ...
*
Bioenergetic systems
Bioenergetic systems are metabolic processes that relate to the flow of energy in living organisms. Those processes convert energy into adenosine triphosphate (ATP), which is the form suitable for muscular activity. There are two main forms of syn ...
References
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Physical exercise