Interval training is a method of training where one increases and decreases the intensity of his workout between aerobic and anaerobic training (KRAEMER & RATAMESS, 2004; Talanian, Galloway, Heigenhauser, Bonen, & Spriet, 2007). Interval training in Sweden, where some say it originated, is known as fartlek training (Swedish for “speed play”)(Ferraz et al., 2010). The protocol for interval training is a well-known method for improving fitness(Dunham, 2010). Technically, it is defined as high-intensity intermittent exercise. In an interval session, high-intensity periods of work are interspersed with rest intervals. The objective of this training is to improve muscle performance (speed, strength, and endurance)(Buchheit et al., 2009).
Interval training has been the basis for athletic training routines for years(Suh, Rofouei, Nahapetian, Kaiser, & Sarrafzadeh, 2009). The first forms of interval training, called “fartlek” involved alternating short, fast bursts of intensive exercise with slow, easy activity(KRAEMER et al., 2004).
Interval training works both the aerobic and the anaerobic system. During the high intensity effort, the anaerobic system uses the energy stored in the muscles (glycogen) for short bursts of activity(Wells, Selvadurai, & Tein, 2009b). Anaerobic metabolism works without oxygen. The by-product is lactic acid, which is related to the burning sensation felt in the muscles during high intensity efforts(Muscle & Production, 2009). During the high intensity interval, lactic acid builds and the athlete enters oxygen debt. During the recovery phase the heart and lungs work together to “pay back” this oxygen debt and break down the lactic acid. It is in this phase that the aerobic system is in control, using oxygen to convert stored carbohydrates into energy(Wells et al., 2009b).
Aerobic exercise and fitness can be contrasted with anaerobic exercise, of which strength training and short-distance running are the most salient examples(Delextrat & Cohen, 2008). The two types of exercise differ by the duration and intensity of muscular contractions involved, as well as by how energy is generated within the muscle. Initially during aerobic exercise, glycogen is broken down to produce glucose, which then reacts with oxygen (Krebs cycle) to produce carbon dioxide and water and releasing energy. In the absence of these carbohydrates, fat metabolism is initiated instead. The latter is a slow process, and is accompanied by a decline in performance level (Mansouri, 2009).
Aerobic exercise comprises innumerable forms. In general, it is performed at a moderate level of intensity over a relatively long period of time(Reid et al., 2010). For example, running a long distance at a moderate pace is an aerobic exercise, but sprinting is not. Playing singles tennis, with near-continuous motion, is generally considered aerobic activity, while golf or two person team tennis, with brief bursts of activity punctuated by more frequent breaks, may not be predominantly aerobic.
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The word ‘anaerobic’ literally means without oxygen. Anaerobic exercise means you’re working at such a high level of intensity, that the cardiovascular system can’t deliver oxygen to the muscles fast enough. Muscles, trained using anaerobic exercise, develop differently compared to aerobic exercise, leading to greater performance in short duration, high intensity activities, which last from mere seconds up to about 2 minutes. Any activity after about two minutes will have a large aerobic metabolic component(Scott, 2008).
To understand the physiological differences between aerobic and anaerobic exercise, one should be familiar with fuel sources in the body(Meckel, Machnai, & Eliakim, 2009). Carbohydrate, including sugars, starches, and fibers, is the preferred energy source for the body, is the only fuel capable of being used by the central nervous system, and is the only fuel that can be used during anaerobic metabolism(Hargreaves, 2008; Dosil & Crespo, 2008). Carbohydrates are converted to glucose and stored in muscle cells and the liver as glycogen, with approximately 1,200 to 2,000 kcal of energy stored in the form of carbohydrate. Each gram of carbohydrate ingested produces approximately 4 kcal of energy(Hargreaves, 2008).
Fat can also be used as an energy source and is the body’s largest store of potential energy, about 70,000 kcal in a lean adult. However, the basic storage form of fat useful as an energy source, triglyceride, must be broken down into free fatty acids (FFA) and glycerol before FFAs can be used to form ATP by aerobic oxidation. The process of triglyceride reduction, termed lipolysis, requires significant amounts of oxygen, thus carbohydrate fuel sources are more efficient than fat fuel sources and are thus preferred during high-intensity exercise. From each gram of fat 9 kcal of energy is produced(Hargreaves, 2008; Dosil et al., 2008).
Protein is used as an energy source in cases of starvation or extreme energy depletion and it provides approximately 5% to 10% of the total energy needed to perform endurance exercise. Protein yields approximately 4 kcal of energy per gram and is not a preferred energy source under normal conditions(Hargreaves, 2008; Dosil et al., 2008).
In human body there are three metabolic pathways to produce energy(Wells, Selvadurai, & Tein, 2009a).
The first pathway is anaerobic, meaning that it does not require oxygen to function, although it also can occur in the presence of oxygen. This pathway is called the ATP-PCr system, where PCr stands for phosphocreatine or creatine phosphate. Similar to ATP, PCr is a high-energy compound found in skeletal muscle cells that functions to replenish ATP in a working muscle, extending the time to fatigue by 10 to 20 seconds. Thus energy released as a result of the breakdown of PCr is not used for cellular metabolism, but rather to prevent ATP levels from falling. One molecule of ATP is produced per molecule of PCr(Barrett, 2009). This simple energy system can produce 3 to 15 seconds of maximal muscular work and requires an adequate recovery time, generally three times longer than the duration of the activity.
The production of ATP during longer bouts of activity, such as that required to address aerobic capacity impairment, requires the breakdown of food energy sources. In the glycolytic system, or during anaerobic glycolysis, ATP is produced through the breakdown of glucose, obtained from the ingestion of carbohydrates or from the breakdown of stored liver glycogen. Anaerobic glycolysis also occurs, without the presence of oxygen, but is much more complex than the ATP-PCr pathway, requiring numerous enzymatic reactions to breakdown glucose and produce energy (Bhise, 2008). The end product of glycolysis is pyruvic acid, or pyruvate which is converted to lactic acid in the absence of oxygen, and the net energy production from each molecule of glucose used is two molecules of ATP, or three molecules of ATP from each molecule of glycogen. Although the energy yield from the glycolytic system is small, the combined energy production of the ATP-PCr and glycolytic pathways enables muscles to contract, without a continuous oxygen supply, and thus provides an energy source in the first part of a high intensity exercise until the respiratory and circulatory systems catch up to the sudden increased demand placed on them. Further, the glycolytic system can only provide energy for a limited time because the end product of the pathway, lactic acid, accumulates in the muscles and inhibits further glycogen breakdown and eventually impedes muscle contraction(Barrett, 2009).
The production of ATP from the breakdown of fuel sources in the presence of oxygen is termed aerobic oxidation or cellular respiration. ATP is produced in the mitochondria, cellular organelles conveniently located next to myofibrils, the contractile elements of individual muscle fibers. The oxidative production of ATP involves several complex processes, including aerobic glycolysis, the Krebs cycle, and the electron transport chain(Bhise, 2008).
Carbohydrate or glycogen is broken down in aerobic glycolysis, Similarly to the breakdown of carbohydrate in anaerobic glycolysis, but in the presence of oxygen pyruvic acid is converted to acetyl coenzyme A (acetyl Co-A). Acetyl Co-A undergoes a number of complex chemical reactions in the Krebs (citric acid) cycle, producing two molecules of ATP (Fig. ). The end result of the Krebs cycle is the production of carbon dioxide and hydrogen ions, which enter the electron transport chain, undergo a series of reactions, and produce ATP and water.
Difference between aerobic and anaerobic exercise can be summarized as below:
The literal meaning of aerobics is oxygen. Hence, aerobic exercise can be defined as the one, which involves the use of oxygen to produce energy, whereas anaerobic exercise makes the body to produce energy without using oxygen.
Anaerobic exercises are high intensity workouts that are performed for a short time. On the contrary, aerobic exercises generally simple exercises and are performed for a longer time, at moderate intensity.
A person doing aerobic exercises requires more endurance, because unlike anaerobic exercise (which is done for a short period), aerobic exercise is done for a long time.
Generally, aerobic exercise is performed for about 20 minutes or more. On the other hand, the duration for an anaerobic exercises is two minutes, which can be only sustained for a longer time through proper training.
The metabolic processes used by aerobic and anaerobic exercises differentiate them from each other. Although both aerobic and anaerobic exercises produce energy through glycolysis (conversion of glucose into pyruvate), the substance used to break down glucose is different. While oxygen is used to break down glucose by aerobic exercise, the anaerobic exercises make use of phosphocreatine, stored in the muscles, for the process.
Aerobic and anaerobic exercises are done to accomplish individual goals. Aerobic exercises concentrate on strengthening and the muscles involved in respiration. It improves the circulation of blood and transportation of oxygen in the body, reduces blood pressure and burns fat. On the other hand, anaerobic exercise helps build strength and muscle mass, stronger bones and increases speed, power, muscle strength and the metabolic rate as well. It concentrates on burning the calories, when the body is in rest.
When one performs aerobic exercise, s/he will notice an increase in the heart beat rate and the rise in his/her level of respiration. Energy is provided by carbohydrate and fats, when he works out the muscles. On the other hand, the sources of energy during anaerobic activity are adenosine triphosphate (ATP) and creatine phosphate.
There are some acute and chronic changes following aerobic exercise. Repetitive form of training leads to the adaptation response. The body begins to build new capillaries, and is better able to take in and deliver oxygen to the working muscles. Muscles develop a higher tolerance to the build-up of lactate, and the heart muscle is strengthened. These changes result in improved performance particularly within the cardiovascular system.
The ability to sustain aerobic exercise depends on numerous cardiovascular and respiratory mechanisms aimed at delivering oxygen to the tissues. The following changes would be expected during aerobic exercise and would be considered normal responses.
There is a linear relationship between heart rate (HR), measured in beats/min, and intensity of exercise, indicating that as workload or intensity increases, HR increases proportionally. The magnitude of increase in HR is influenced by many factors, including age, fitness level, type of activity being performed, presence of disease, medications, blood volume, and environmental factors such as temperature and humidity.
The volume or amount of blood ejected from the left ventricle per heart beat is termed the stroke volume (SV), measured in mL/beat. As workload increases, SV increases linearly up to approximately 50% of aerobic capacity, after which it increases only slightly. Factors that influence the magnitude of change in SV include ventricular function, body position, and exercise intensity.
The product of HR and SV is cardiac output (Q), or the amount of blood ejected from the left ventricle per minute (L/min) (Q = HR X SV). Cardiac output increases linearly with workload because of the increases in HR and SV in response to increasing exercise intensity. Changes in Q depend on age, posture, body size, presence of disease, and level of physical conditioning.
Arterial-Venous Oxygen Difference
The amount of oxygen extracted by the tissues from the blood represents the difference between arterial blood oxygen content and venous blood oxygen content and is referred to as the arterial-venous oxygen difference (a-vO2 diff), measured in mL/dL. As exercise intensity increases, a-vO2 diff increases linearly, indicating that the tissues are extracting more oxygen from the blood, decreasing venous oxygen content as exercise progresses.
The distribution of blood flow (mL) to the body changes dramatically during acute exercise. Whereas at rest, approximately 15% to 20% of the cardiac output goes to muscle, during exercise approximately 80% to 85% is distributed to working muscle and shunted away from the viscera. During heavy exercise, or when the body starts to overheat, increased blood flow is delivered to the skin to conduct heat away from the body’s core, leaving less blood for working muscles.
The two components of blood pressure (BP), systolic (SBP) and diastolic (DBP) pressure, respond differently during acute bouts of exercise. To facilitate blood and oxygen delivery to the tissues, SBP increases linearly with workload. Because DBP represents the pressure in the arteries when the arteries at rest, it changes little during aerobic exercise, regardless of intensity. A change in DBP of less than 15 mm Hg from the resting value is considered a normal response.
The respiratory system responds during exercise by increasing the rate and depth of breathing in order to increase the amount of air exchanged per minute (L/min). An immediate increase in rate and depth occurs in response to exercise and is thought to be facilitated by the nervous system, initiated by the movement of the body. A second, more gradual, increase occurs in response to body temperature and blood chemical changes as a result of the increased oxygen use by the tissues. Thus both tidal volume, or the amount of air moved into and out of the lungs during regular breathing, and respiratory rate (RR) increase in proportion to the intensity of exercise.
As a result, aerobic exercise can reduce the risk of death due to cardiovascular problems. In addition, high-impact aerobic activities (such as jogging or jumping rope) can stimulate bone growth, as well as reducing the risk of osteoporosis for both men and women.
In addition to the health benefits of aerobic exercise, there are numerous performance benefits:
- Increased storage of energy molecules such as fats and carbohydrates within the muscles, allowing for increased endurance
- Neovascularization of the muscle sarcomeres to increase blood flow through the muscles
- Increasing speed at which aerobic metabolism is activated within muscles, allowing a greater portion of energy for intense exercise to be generated aerobically
- Improving the ability of muscles to use fats during exercise, preserving intramuscular glycogen
- Enhancing the speed at which muscles recover from high intensity exercise
There is convincing evidence that aerobic exercise can exert psychological benefits. In addition to cardiovascular and respiratory effects of exercise, psychological and emotional benefits of exercise such as well being, treatment of insomnia, reduction of stress stress have been well documented.
Abnormal Responses to Aerobic Exercise
Individuals with suspected cardiovascular disease or any other type of disease that may produce an abnormal response to exercise should be appropriately screened and tested before the initiation of an exercise program. However, abnormal responses may occur in individuals without known or diagnosed disease and thus routine monitoring of exercise response is important and can be used to evaluate the appropriateness of the exercise prescription and as an indication that further diagnostic testing may be indicated. In general, responses that are inconsistent , with the normal response guidelines described previously are considered abnormal responses. From the parameters described, HR and BP are most commonly assessed during exercise. The failure of HR to rise in proportion to exercise intensity, a failure of SBP to rise or a decrease in SBP 2:20 mm Hg during exercise, and an increase in DBP 2:15 mm Hg would all be examples of abnormal responses to aerobic exercise.