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Table 5 (below) was constructed by reviewing the results of articles that examined the maximum oxygen uptake, muscle mass and insulin-resistance reduction reponse to aerobic, resistance and interval training. Although fewer interval sprinting studies have been completed, interval sprinting has resulted in as big or bigger positive changes in maximum oxygen uptake, muscle mass and insulin-resistance than both aerobic and resistance exercise.

The typical amount of reduction of insulin resistance in study participants in randomised controlled studies lasting greater than 2 weeks was between 20% and 50%. We now think interval sprinting is especially suitable for individuals who suffer from type 2 diabetes and metabolic syndrome.

Table 5. A summary of the maximum oxygen uptake, muscle mass and insulin-resistance reduction repsonse to aerobic, resistance and interval training in randomised control trials lasting at least 12 weeks.

Interval sprinting and special populations

There are now over 50 articles published in scientific literature examining different aspect of interval training. Surprisingly, a number of these articles have examined the effects of interval training on the health of special populations, including heart disease, intermittent claudication and diabetic patients, depressed individuals, older people, postmenopausal women and post-pregnant women. Interval training protocols in these studies have varied but most research groups have used longer interval exercise at a less than all-out intensity for 2 minutes, followed by low-intensity exercise or rest for 4 minutes. Thus, although cycling for a 2-minute bout cannot be classed as sprinting, the exercise was still performed at high intensity and followed by a rest. Although more research needs to be done in this area, the preliminary results are promising.

Heart disease patients

The heart is the organ that pumps blood to all body tissues; if it stops pumping, death quickly follows. There are a number of heart diseases but 2 major ones are coronary artery disease and chronic heart disease. Coronary artery disease occurs when the arteries that provide blood to the heart don’t work properly; if they become blocked, blood cannot get to the heart. Blocked arteries in the heart can be caused by smoking, high cholesterol, hypertension, type 2 diabetes and inherited genes. Chronic heart disease is a condition where the heart does not pump normally and is usually caused by a weak cardiac muscle or faulty heart valves. Heart disease is the major cause of death in most Westernised countries.

A number of studies have investigated the effect of interval training on patients possessing coronary heart disease and chronic heart failure. A series of studies carried out on coronary artery bypass patients by one research team in the 1990s showed that, compared to control groups, their physical performance significantly improved after a program of interval training.
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Another team examined the effect of interval exercise on stent function following heart surgery. Results showed that high-intensity interval exercise improved stent function, increased aerobic fitness and reduced inflammation.
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A different study compared the effects of aerobic interval training and moderate continuous aerobic training on aerobic fitness and quality of life after coronary artery bypass grafting.
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Four weeks of interval and continuous aerobic exercise showed a significant increase in aerobic fitness of all participants; however, 6 months later, the interval group had greater aerobic fitness than the continuous exercise group. Interval training protocols in these studies involved cycling at less than all-out intensity for 2 minutes, immediately followed by rest for 4 minutes.

With regard to chronic heart disease patients, one study found that 16 weeks of high-intensity interval training enhanced functional capacity and quality of life.
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Another study compared the effect of moderate aerobic and high-intensity exercise on cardiovascular function in heart failure patients: aerobic fitness was increased more with aerobic interval training and was associated with greater improvements in left ventricular function.
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A different study also showed that high-intensity interval exercise was better than moderate continuous aerobic exercise for increasing aerobic fitness in coronary artery patients.
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Collectively, research examining interval exercise and coronary artery disease and chronic heart failure has shown that interval training increases aerobic fitness in far less time than conventional moderate aerobic exercise. Quality of life was also consistently improved, as were a number of indicators of heart function. An overview of this area has been provided by Ernst.
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Chronic obstructive pulmonary disease patients

Chronic obstructive pulmonary disease affects breathing and is characterised by chronic bronchitis or emphysema, which results in narrowed airways in the lungs. Chronic obstructive pulmonary disease is typically caused by smoking, which inflames the lungs. In the US it is the third leading cause of death and it has been calculated to cost over US$42 billion in increased health care and lost productivity. Estimates suggest that chronic obstructive pulmonary disease will become the fourth leading worldwide cause of death by 2030.

Patients with this disease usually have trouble performing aerobic exercise and typically have an overall poor quality of life. When performing continuous aerobic exercise, chronic obstructive pulmonary disease patients typically experience breathing discomfort and have to stop for a rest. As exercising and resting is the basis of interval training, it appears that this form of exercise is suitable for chronic obstructive pulmonary disease patients. A 12-week study compared interval with aerobic exercise and found that patients with chronic obstructive pulmonary disease significantly improved their exercise tolerance after interval exercise.
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Continuous aerobic exercise also improved exercise tolerance but involved twice as much exercise time. After both interval and aerobic exercise, quality of life was significantly improved. These results have been replicated in a number of studies. In another study, researchers showed that both interval exercise and continuous moderate aerobic exercise resulted in positive changes to muscle function, however interval training caused fewer training problems.
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Interval training protocols in these studies typically involved a less than all-out intensity cycling for 30 seconds, immediately followed by a rest for 30 seconds.

Overall, research examining interval exercise and chronic obstructive pulmonary disease has shown that interval sprinting increases aerobic fitness in less time than moderate aerobic exercise. Quality of life was also consistently improved. Importantly, interval training caused fewer training problems, such as shortness of breath and breathing discomfort. Because interval training allows chronic obstructive pulmonary disease patients to tolerate harder intensity exercise for longer periods of time with less breathing and leg discomfort, it appears to be superior to other training modalities.
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Metabolic syndrome and diabetic patients

Metabolic syndrome is a condition distinguished by having a lot of belly fat, high blood pressure, insulin resistance and bad blood-lipid profiles. It is a precursor to type 2 diabetes and is typically an outcome of an unhealthy diet and being sedentary. Incidence of type 2 diabetes has substantially increased during the last 50 years in a similar fashion to rates of obesity: in 2010 there were about 285 million people possessing metabolic syndrome compared to about 30 million in 1985. Worryingly, the World Health Organization has estimated that by 2025, 50% of the world’s type 2 diabetics will be people of Asian descent.

Long-term complications caused by type 2 diabetes are heart disease, stroke, retinopathy, kidney disease and nerve degeneration. Type 1 diabetes is a form of diabetes that results from the inability of beta cells in the pancreas to produce insulin. The degradation of the beta cells is usually brought about by our immune systems. In Western countries, type 1 diabetes comprises about 10% of the total diabetic population. Most people who develop type 1 diabetes are usually of average weight and healthy in comparison with those who develop type 2 diabetes.

Aerobic exercise has been shown to be beneficial for reducing symptoms of metabolic syndrome and type 2 diabetes. The effects of interval training, however, have been less examined but initial results are promising. For instance, one study placed 32 metabolic syndrome patients on a 16-week program of interval training and found that many health risk factors were reversed.
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Another examined the effects of resistance and interval exercise training on skeletal muscle function in people with type 2 diabetes.
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They found that 10 weeks of resistance and interval training in unfit type 2 diabetic patients resulted in improvements in muscle function and blood pressure. Interval training protocols in these studies involved less than all-out intensity cycling for 2 minutes, immediately followed by resting for 4 minutes. A further study investigated the effects of low-volume interval sprinting – 10 cycle bouts of 60 seconds with 60-second rest periods for 20 minutes – on glucose regulation and skeletal muscle metabolic capacity.
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Results showed that low-volume interval sprinting rapidly improved glucose control and induced skeletal muscle adaptations that were beneficial for the health of patients with type 2 diabetes.

The effects of a single 10-second sprint on glucose levels of exercising type 1 diabetics have also been investigated. It is well established that moderate intensity aerobic exercise increases the risk of hypoglycemia (low blood-sugar levels) after exercise in those with type 1 diabetes. Therefore, the study investigated whether a short, 10-second cycle sprint would prevent the rapid fall in blood sugar levels typically associated with moderate aerobic exercise in individuals possessing type 1 diabetes.
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Their results indicated that for individuals possessing type 1 diabetes who participated in moderate aerobic exercise, a 10-second maximum sprint at the end of aerobic exercise prevented a fall in blood-sugar levels. Another study assessed whether 30 minutes of interval sprinting resulted in less lowering of blood-sugar levels compared to 30 minutes of continuous aerobic exercise.
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The decline in blood sugar levels was less with interval sprinting compared with aerobic exercise in people with type 1 diabetes.

Collectively, research examining interval exercise and metabolic diseases such as type 2 diabetes has shown that interval training consistently increases insulin sensitivity and reverses a number of risk factors. An overview of this area has been provided by Kessler and colleagues.
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Depressed individuals

Depression is a long-term mood disorder and is usually defined as having continuous unhappiness and reduced enjoyment of everyday life for greater than 2 weeks. It has been estimated that about 1 in 7 people in Westernised countries are clinically depressed. People who experience depression typically have a range of health problems, such as cardiovascular disease, headaches, back pain, anxiety attacks and poor-quality sleep. Depressed individuals exhibit chronic increased levels of cortisol, which causes increased belly fat accumulation.

In 2008, investigators from Amsterdam found that depressed people had twice the risk of gaining belly fat over a 5-year period compared to people without depression. These authors suggested that storing fat in the belly puts depressed people at much greater risk for cardiovascular disease and diabetes. It has been shown that depressed individuals have much greater incidence of heart disease and diabetes.

Interestingly, these investigators found no association between depression and obesity. This finding suggests that, despite being of normal weight, depressed people had elevated levels of belly fat. Another study examining women found similar results. The study assessed depression levels and belly fat of middle-aged African-American and Caucasian women and found a strong relationship between depression and belly fat; no association between depression levels and subcutaneous fat was found. Although it is not clear how depression causes an increase in belly fat, it is possible that depression triggers the accumulation of belly fat by increasing the production of cortisol and inflammatory compounds.

Can depression be reduced by exercise? Yes. Research has shown that all kinds of moderately vigorous exercise tend to alleviate clinical depression. For depression reduction, moderately vigorous exercise such as fast walking, jogging and strength training compared to easy exercise produced the best results. Depression has been shown to change in weeks, however longer exercise programs seem to produce greater reductions in depression levels. Whether or not the positive effect of exercise on depression is related to decreased belly fat is unknown. That longer, moderately vigorous exercise tends to alleviate clinical depression more may indicate that these kinds of exercise programs result in a greater reduction of belly fat.