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The effect of interval sprinting on depression has not been examined, however, given the large reductions in belly fat occurring after interval sprinting and the connection between belly fat and depression, its potential appears to be significant. Interval sprinting may be suitable for depressed patients because it can be fun, can be performed in a group, is time-efficient and has been shown to reduce belly fat.

Intermittent claudication patients

Intermittent claudication is skeletal muscle pain experienced as aching, cramping and numbness, which usually occurs in the calf muscles when walking and is only relieved by resting. Intermittent claudication is due to peripheral artery disease brought about by blockages of the arteries of the leg. People who smoke, have high blood pressure or possess type 2 diabetes have a greater incidence of intermittent claudication. Men over 50 have the highest incidence of intermittent claudication, and it affects around 5% of people in Western populations.

A number of studies have shown that regular aerobic exercise can improve intermittent claudication symptoms. Patients typically walk for 5 minutes, rest for 5 minutes, and then repeat this pattern for 20–30 minutes. However, one study found that high-intensity training at 80% of maximum oxygen uptake was more effective than an identical volume of low-intensity training for improving aerobic fitness in patients with intermittent claudication.
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The high-intensity training patients carried out 2 minutes of speed walking on a treadmill, followed by 3-minute sit-down resting periods. This was performed 8 times. Another study conducted a high-intensity rehabilitation program with intermittent claudication patients that lasted 12 weeks. Patients walked on a treadmill for 6 minutes at a speed that brought about ischaemia and maximum claudication pain. When patients reached this level, they stopped walking and rested for 3 minutes. Patients performed this protocol 6 times per session. Results showed that those patients who successfully completed the 6 sessions showed the greatest decrease in claudication symptoms. The authors concluded that, as no adverse events were experienced, patients with intermittent claudication can safely tolerate high-intensity exercise programs.
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Obese and overweight adults and children

Obesity is a condition in which body and belly fat accumulation negatively affects health. It increases an individual’s chances of heart disease, type 2 diabetes, sleep apnoea and certain cancer types. Obese individuals also have a reduced life expectancy. People are classified as obese when their BMI exceeds 30. Obesity is typically caused by genetic influences, excessive food energy intake and being sedentary, although some people can become obese because of endocrine disorders and certain medications. There has been a dramatic increase in overweight and obesity over the last 50 years in both developed and developing countries.

Does participation in aerobic exercise significantly reduce body fat of obese and overweight individuals? The answer is no; regular aerobic exercise only results in a minor loss in total body fat. It results in a greater loss of belly fat if individuals are prepared to exercise for at least 1 hour, 5 times per week. The good news is that participation in interval sprinting does significantly impact on body fat if individuals are prepared to exercise for 1 hour per week. We have shown that interval sprinting results in a significant reduction of total body fat and belly fat of overweight – where BMI equals 28 – women and men.
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Other researchers studied the effects of a 12-week, high-intensity exercise program on obese older men and women and found a significant reduction in belly fat, while subjects in a moderate-intensity group showed no decrease in belly fat.
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Similarly, another study found that interval training significantly reduced belly fat in older men and women.
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Importantly, they found that reductions in belly fat were strongly related to reductions in insulin resistance – the greater the reduction in belly fat, the greater the improvement in insulin sensitivity.

With regard to childhood obesity, one study compared the effects of high-intensity exercise and a multi-treatment strategy on a number of cardiovascular risk factors in obese adolescents.
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One group performed aerobic interval training twice per week for 12 weeks: 4 minutes of hard uphill running on a treadmill followed by 4 minutes of rest. Another group undertook a multidisciplinary approach over 12 months, including dietary changes, moderate exercise and psychological advice twice a month. The results indicated that high-intensity exercise brought about a greater reduction in cardiovascular risk factors than the multi-treatment strategy.

Overall, research examining the effects of high-intensity exercise and interval sprinting exercise on the obese and overweight has shown that this kind of exercise decreases body fat to a greater extent than continuous aerobic exercise. Belly or visceral fat has been shown to be reduced by interval sprint training in far less time than by aerobic exercise. Because interval training, especially on the stationary bike, is easily performed by obese adults and children, and because it results in more subcutaneous and belly fat loss, it appears to be superior to other kinds of training.
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Postmenopausal women

Premenopausal women are typically younger than 46 and tend to have less belly fat than men, as they store their fat in their legs, hips and on the back of their arms. This differing pattern of fat storage for women and men is mainly an outcome of the hormone oestrogen. As women go through menopause in their late 40s and early 50s, however, oestrogen production stops or slows down, resulting in increased belly fat accumulation. Scientists have suggested that the reduction in oestrogen is also accompanied by an increase in the stress hormone cortisol which, as mentioned previously, helps increase belly fat. Thus, women older than 46 tend to have greater increases in belly fat than males. Women in their late 40s can find their waistlines increasing even if they don’t gain much weight, as increasing belly fat forces the abdominal wall outwards.

Importantly, diet is relatively ineffective at reducing the belly fat stores of postmenopausal women. In one study, postmenopausal women only lost belly fat when exercise, in the form of walking, was added to a diet. Interval training, however, has been shown to result in greater belly fat reductions in postmenopausal women. The previously mentioned study of middle-aged men and women showed a 48% decrease in belly fat, measured by MRI, with an 18% decrease in subcutaneous fat after steady-state aerobic exercise 2 days per week and interval training 1 day a week for 8 weeks.
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Another study carried out on 32 middle-aged metabolic syndrome men and women over 16 weeks involved interval training 3 times per week. Aerobic fitness of participants was enhanced by 26%, whereas body weight was decreased by 2.3kg.
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Overall, research examining the effect of interval exercise on postmenopausal women’s health has shown that interval training or sprinting increases aerobic fitness in less time than moderate aerobic exercise. Body fat also decreases to a greater extent after interval training interventions compared to continuous aerobic exercise. Belly or visceral fat has also been shown to be reduced by interval training in far less time than aerobic exercise.

Pregnancy and interval sprinting

Many women increase their food intake excessively when pregnant, and while there are general guidelines to help women understand how much weight gain is appropriate during pregnancy, there are no specific guides regarding the amount of fat that should include. Weight gain during pregnancy is influenced by the baby’s weight and the mother’s increase in blood volume and body fat. Some fat stores are increased during pregnancy to enhance breast feeding but excessive fat gain increases a number of health risks for mother and baby. Unfortunately, a significant amount of women increase their belly fat stores during pregnancy. Although many women think they have to ‘eat for 2’ when pregnant, only a small number of extra calories are needed. While pregnant women should not be encouraged to diet to lose weight, as this may harm the health of the growing baby, a healthy diet and regular physically activity are important for the long-term health of both mother and baby.

But what about after pregnancy? Given that many women will retain their increased belly fat stores after giving birth, and that interval sprinting has been shown to decrease belly fat in non-pregnant women, it follows that interval sprinting may be the optimal exercise for reducing these unwanted belly fat stores. Research studies, however, are needed to confirm this relationship.

With regard to conception, no studies have investigated the effect of interval training on conception rates. Some research has been done with aerobic exercise and it seems that really hard exercise, like marathon training, is detrimental, while moderate exercise – 3 45-minute sessions of aerobic exercise per week for example – is beneficial for conception. So where would interval sprinting fit in? Interval sprinting is performed at a harder intensity but is much shorter than a 45-minute bout of aerobic exercise. In our studies with women aged between 18 and 30, we did not have any reports of menstrual irregularity after training 3 times per week for 12 or 15 weeks. With regard to belly fat and conception, it has been shown that those women possessing elevated belly fat stores have a reduced rate of conception. As interval sprinting has been shown to reduce belly fat in women, it is feasible that interval sprinting may enhance conception rates, but research into this area is required.

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In the research described above, all subjects or patients were medically screened before participating in high-intensity interval training. It is important that you seek medical advice on the possible positive or negative effects of interval training on your health before beginning an interval sprinting regime. The potential interaction between any medication you might be taking and interval training should also be estimated. For example, if you have heart disease and are taking some form of beta blocker, you will not be able to elevate your heart rate to the recommended level during exercise.

An accredited exercise physiologist (EP) qualified to give advice on exercise and beneficial life changes, such as diet, stress-management and sleep quality enhancement, together with a physician who is supportive of lifestyle change strategies is the ideal team to help as you embark on an interval training program. They will be able to advise you on what form of exercise to do, how long each session should last and how many times per week you should exercise and at what intensity. You can find an exercise physiologist near you by visiting www.essa.org.au.

If you are just interested in improving your fitness and health, then most types of continuous, steady-state exercise will be effective. If, however, you want to lose belly fat and improve your insulin sensitivity, as well as improve your aerobic and anaerobic fitness and increase muscle mass, then interval sprinting is the best option. See Chapter 3 for more information on incorporating the interval sprinting program in your life.

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The points highlighted above outline the way we should be exercising if we want to lose belly fat. It’s important to remember the following:

• Aerobic exercise can result in a decrease in belly fat but people have to exercise at a moderately hard intensity for at least 5 hours per week.
  
• Resistance exercise does not seem to decrease belly fat in most people.
  
• Interval sprinting has resulted in an 17% decrease in belly fat when males exercised for 1 hour per week for 12 weeks.
  
• We’re not sure how interval exercise works on belly fat, but it’s likely that increased fat burning during and after exercise and a possible decrease in appetite are the main factors.
  
• Interval training has been used successfully in a number of special populations, such as heart disease and type 2 diabetes patients.
  

Now you’re familiar with how belly fat affects your health and the positive effects interval sprinting can have on the amount of belly fat you carry, let’s outline a 8-second sprint/12-second recovery interval sprinting program, the benefits such a program offers, equipment you’ll need and how to begin training.

What happens to heart rate and hormones during interval sprinting?

Heart rate

There are a number of acute responses to interval sprinting, but 3 that are important for belly fat loss are heart rate, blood lactate and fat-burning hormones. Your heart-rate response depends on what type of interval sprinting protocol you undertake, but typically it is significantly elevated during interval sprinting exercise and declines slightly during the recovery between sprints. For example, peak heart rates during the hard Wingate test typically exceed 170 beats per minute during a 30-second all-out cycle sprint, with the average heart rate across the 30-second sprint being 150 beats per minute. Studies have found a smaller heart-rate response for an interval sprinting protocol consisting of 10 6-second sprints interspersed with a 30-second recovery.
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Heart rate increased to 142 beats per minute after the first sprint and then increased to 173 beats per minute following sprint 10. Heart-rate response to the 8-second/12-second program typically averages around 150 beats per minute after 5 minutes of interval sprinting, which then increases to 160 beats per minute for young adults after 15 minutes of exercise.
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As can be seen in Figure 5 (below), the heart rates of young adults gradually increase during the 20 minutes of 8-second/12-second interval sprinting. In the 8-second/12-second program, there is typically a small heart rate decrease of around 3–5 beats per minute during each recovery period.

So we know that heart-rate response to interval sprinting varies depending on your age. In young adults, heart rate typically averages around 150 beats per minute after 5 minutes of interval sprinting, which then increases to 160 beats per minute after 20 minutes of exercise. For middle-aged adults, optimal heart-rate response is generally lower, around 140 beats per minute after 5 minutes of interval sprinting, increasing to 150 beats per minute after 20 minutes of exercise.

Figure 5. The heart-rate response of young adults to 1 session of interval sprinting consisting of an 8-second sprint and 12 seconds of easy pedalling for 20 minutes.

Measuring heart rate

Heart rate during exercise and recovery can be measured manually or by a heart-rate monitor. The best place to measure heart rate during exercise or exercise recovery using the manual or palpation method is at the radial artery, located in the wrist. The fingers should be used to locate a pulse rather than the thumb.

Figure 6. Locating the radial pulse at the base of the wrist.

To locate the radial pulse on your wrist, position the index and middle fingers on the opposite wrist, approximately 1.5cm on the inside of the wrist, below the index finger (see Figure 6, above). When the pulse is found, count the number of beats for 1 minute. The per-minute rate can also be calculated by counting for 10 seconds and multiplying by 6.

A heart-rate monitor is much easier and gives a much more accurate reading of your heart rate than using the finger palpitation method. There are numerous heart rate smartphone apps available for iPhones and Android devices that can measure heart rate. Using a heart-rate monitor, as pictured in Figure 7 (above), is also useful when you want to record your heart rates so you can examine them later. Such monitors also allow you to download your records to a computer.

Figure 7. Assessing heart rate by using a heart-rate monitor.

Rating of perceived exertion

Exercise physiologists measure how hard people feel they are working during exercise by using a rating of perceived exertion scale (see Appendix D). When exercising, people are asked to answer the question ‘How hard are you working?’ by indicating their perceived effort level on a scale that ranges from 6 to 20.
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When performing lower-intensity interval sprinting such as the 8-second sprint/12-second recovery program, the rating of perceived exertion is usually around 12 and increases to just over 15 by the end of the session. This can be seen in Figure 8 (above).

When performing aerobic exercise, the rating is typically equivalent to about one-tenth of the exercise heart rate. Thus, a rating of 16 would typically accompany a heart rate of 160 beats per minute for young adults in their 20s. For aerobic exercise, if you increase the intensity of exercise as you get fitter, your rating of perceived exertion typically stays the same. We have shown that this is not the case with interval sprinting.
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With interval sprinting exercise, the rating is typically equivalent to about one-eleventh of the exercising heart rate, so a rating of 14 would typically accompany a heart rate of 150 beats per minute for young adults. As people get fitter with repeated interval sprinting, they will typically increase pedal rate and resistance. After interval training, however, instead of staying the same, the rating of perceived exertion tends to go up. This is probably because the greater force developed by skeletal muscles needed to cope with higher pedal resistance loads on the bike causes higher exertional perceptions. Thus, as force production increases, the greater amount of signalling received by the brain’s sensory centres could be the reason why the rating of perceived exertion goes up after interval sprinting training.

Figure 8. The rating of perceived exertion response of young adults to 1 session of interval sprinting consisting of an 8-second sprint and 12 seconds of easy pedalling for 20 minutes.

Blood lactate levels

Lactate is a chemical that accumulates in the blood when an individual does anaerobic exercise, which is performed at a higher intensity than aerobic exercise. Anaerobic exercise uses energy systems that do not require oxygen, relying mainly on glycolysis to release energy into the muscles, and is typically brought into action when performing brief, high-intensity bouts of exercise continuously, as with interval sprinting. Heavy use of the glycolytic pathways during this kind of exercise results in enhanced acidity in the exercising muscles – lactate – which then spills into the blood.

Exercise physiologists measure blood lactate levels to gauge how much energy is being derived from the glycolytic pathways. Blood lactate levels are low while at rest but can significantly increase during anaerobic exercise. For example, blood lactate levels during the high-intensity Wingate test protocols are typically 6–10 times higher than those at rest. Lactate levels gradually increase during longer, lower-intensity interval sprinting protocols such as the 8-second/12-second for 20 minutes protocol and are usually between 2 and 4 times greater than those at rest after 5 minutes of interval sprinting for both trained cyclists and un-trained women who took part in our study (see Figure 9). Lactate levels were about 5 times greater than those at rest after 15 minutes of interval sprinting. Thus, lactate levels immediately increase at the start of interval sprinting and continue to slowly increase throughout the exercise session. Despite increasing lactate levels during sprinting exercise, it appears that fat transport is also increased. For example, a 20-minute session of 8-second/12-second sprinting resulted in elevated levels of glycerol, reflecting increased release of fatty acids into the blood.
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Blood lactate levels gradually increase during interval sprinting such as the 8-second/12-second for 20 minutes protocol and are usually 3–4 times greater than resting values at the end of a 20-minute session.

Figure 9. Blood lactate response of young adults to 1 session of interval sprinting consisting of an 8-second sprint and 12 seconds of easy pedalling for 20 minutes.

Hormones

Hormones that have been found to increase during interval sprinting include catecholamines and growth hormone, both chemicals integral to fat release and burning. Catecholamine response is typically elevated after Wingate sprints for both men and women. Catecholamine response to less-intensive interval sprinting protocols have also been shown to be elevated. For example, norepinephrine response to long (24-second sprint/36-second recovery) and short (6-second sprint/ 9-second recovery) bout intermittent treadmill exercise was significantly elevated post exercise. Our study also found significantly elevated epinephrine and norepinephrine levels after 20 minutes of interval sprinting cycle exercise – 8-second/12-second and 24-second/36-second programs – in trained and untrained young women (see Figure 10).
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Another study examined the catecholamine response of 12 males who performed 10 6-second cycle sprints with a 30-second recovery between sprints. Compared to baseline, plasma epinephrine increased 6.3 times, whereas norepinephrine increased 14.5 times at the end of sprinting.
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This large catecholamine response to interval sprinting is in contrast to moderate, steady-state aerobic exercise which typically brings about small increases in epinephrine and norepinephrine. The interval sprinting catecholamine response is an important finding, as catecholamines are the major drivers of fat release from both belly and muscle depots.