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Growth hormone also induces fat burning by coaxing fat cells to release fat. Growth hormone has been shown to increase during high-intensity exercise. With regard to interval sprinting, one study investigated the growth-hormone response to 30-second treadmill sprinting.
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Male and female athletes displayed a marked growth-hormone response, and there was a greater growth-hormone response for sprint exercise compared to endurance-trained athletes, suggesting that regular interval sprint training increases growth-hormone levels. In this study, growth-hormone concentration was still 10 times higher than baseline levels after 60 minutes of recovery.

Figure 10. The norepinephrine 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.

How to do interval cycle sprinting

The 8-second sprint/12-second recovery pedalling cadence is recommended. It is best to wear shorts but a tracksuit bottom can be worn when sprinting as long as it does not get tangled up with the pedals. Trainers are suitable for footwear, and when cycling inside, it is a good idea to have a towel handy to remove excess sweat. In average temperature conditions (20–24° C), 20 minutes of interval sprinting typically results in a sweat loss of around 0.3kg. You can measure your sweat loss during exercise by simply measuring your nude weight before and immediately after an interval sprinting session: if you weigh 70kg nude before sprinting and 69.7kg nude after the session then you would have lost 0.3kg of sweat. Sweat loss is usually significantly greater when exercising in hot, humid conditions. Information concerning choosing and setting up the bike, selecting a suitable pedal rate and resistance, and determining your optimal heart rate and rating of perceived exertion are included below.

Figure 11. The set-up for performing interval sprinting on a stationary bike.

Choosing a stationary bike

First, choose a bike of sufficient quality to withstand sprinting at high pedal rates. Of the range of stationary bikes available, the most suitable ones for interval sprinting are those that allow you to set a pedal resistance that is independent of power output. Some of the newer electronic bikes do not allow you to do this, so when you sprint against a pedal resistance of 1.0kg, for example, instead of the resistance staying the same in the recovery phase, the bike will increase the resistance to produce the same amount of work generated during the sprinting phase. What should be easy pedalling during the recovery phase becomes hard exercise, and having a hard recovery phase will prevent the body from oxidising the waste products produced during sprinting. It will also make the workout excessively demanding, tiring you out more quickly.

How to set up the bike

First, adjust the saddle height so when you’re sitting on the bike you have about a 5% bend at the knee. Adjust the handle bars so your hands rest on top of them. Make sure that the pedals have foot grips. The correct set-up for sprinting is shown in Figure 11.

Choosing the pedal rate and resistance

We know that the production of catecholamines during bike interval sprinting is brought about by moving the legs quickly: cycling fast is more important than cycling hard. Your pedal rate will be influenced by a range of factors, such as your fitness, age, health, leg muscle mass and height. It is best to start with a comfortable pedal rate and then increase it if your rating of perceived exertion is less than 12 and your heart rate is low for your age.

The pedal rate is more important than the load. This means a woman with no health issues could start on a 0.5kg resistance and sprint at 100 revolutions per minute. Fitter or stronger women may require a heavier load, say 1.0kg, and sprint at more than 100 revolutions a minute, while a smaller, older female who is unfit might need to start at a pedal resistance of 0.5kg and a pedal rate of 90 revolutions per minute. A man with no health issues should start at 1.0kg resistance and a pedal rate of 100+ a minute, while fitter, stronger men should be able to sprint at a pedal rate of 115 revolutions a minute.

Stationary bikes have a number of different ways of varying pedal resistance. Increasing pedal resistance is similar to cycling up a hill: the steeper the hill, the greater the pedal resistance. Stationary bikes typically control pedal resistance by either tightening a band around the wheel or by slowing the wheel with magnets. Most bikes will quantify pedal resistance in kilograms or pounds. On the Monark bike made in Sweden, for example, tightening the strap on the wheel from 0.5kg to 1.0kg when pedalling at 100 revolutions per minute will double the power output, which is how much power you are generating when cycling and is typically measured in watts. Watts are calculated by multiplying the pedal rate by the pedal resistance. Thus, cycling at 100 revolutions per minute at a resistance of 1.0kg is equivalent to a power output of 100 watts. If all this sounds too technical, don’t worry. In their first session, most people should try to pedal at a rate of 100 revolutions per minute with a resistance of 0.5kg.

You will have to increase the load and pedal rate as you enhance your fitness with training. For the first week of training, if you’re unfit, you should try 10 minutes of interval sprinting and then see how you feel at the end of the session and when you wake up the next morning. If everything is fine, then you should increase the exercise time to 15 minutes during week 2. By week 3, most people should be able to complete the full 20 minutes of interval sprinting. After 2–3 weeks, you will probably find that your sprint pedal rate is too low. If your rating of perceived exertion is low (12 or less) then you should increase your pedal rate. Try an increase of 5 revolutions per minute, for example, from 90 to 95, and see how your rating of perceived exertion and heart-rate change.

Heart rate and rating of perceived exertion during interval sprinting

Monitoring your heart rate is important, although, working out your optimal interval sprinting heart rate is tricky. Typically, exercise physiologists determine exercise heart rates firstly by calculating an individual’s maximum exercise heart rate which they do by continually increasing the power output on a bike until the exercising person reaches exhaustion and their heart rate will not increase any further. Maximum heart rates typically, but not always, reduce with age; a person in their twenties may have a maximum heart rate of 200 beats per minute, whereas a person aged 60 years may have one of 160 beats per minute. For interval sprinting, the optimal heart rate is typically around 75–80% of a person’s true maximum heart rate: for a young person an exercise heart rate averaging 150 beats per minute would be appropriate, while for a 70-year-old person an exercise heart rate averaging 113 beats per minute may be optimal.

Unfortunately, most people do not know their maximum heart rate. It can be estimated by using the Karvonen formula, which simply calculates the required heart rate by assuming maximum heart rate can be estimated by subtracting a person’s age from 220. Thus, a person aged 20 would have an estimated maximum heart rate of 200 beats per minute: 220 – 20 = 200. A person aged 60 would have one of 160 beats per minute: 220 – 60 = 160. However, young people can have a maximum heart rate of 160 beats per minute and older people, especially if they are fit, of over 200 beats per minute.

If you do not know your true maximum heart rate, the best strategy is to use a trial-by-error approach. For example, an unfit but healthy 20-year-old male could start interval sprinting by using a pedal rate of 100 revolutions per minute at a resistance of 0.5kg. If his true maximum heart rate was 200 beats per minute, then his exercise heart rate should be around 150 beats per minute. As discussed below, the rating of perceived exertion for a heart rate of 150 when interval sprinting is around 14 – somewhat hard to hard. If the rating of perceived exertion was 17 – very hard – then it is likely that the true maximum heart rate of this individual is lower than 200 beats per minute, therefore a heart rate of 150 beats per minute when exercising would likely be too much. Optimal target heart rates are around 150–160 beats per minute for most people in their 20s, 130–140 beats per minute for most people in their 40s, and 120–130 beats per minute for most people in their 60s.

Heart rate will continue to increase during a 20-minute session of interval sprinting. Thus, at the start of exercising you do not want your heart rate to be too high, for example, 160 beats per minute, as it will end up around 170 beats per minute. Heart rate is also affected by heat, which means exercising in warm, humid conditions will result in much higher heart rates, as the heart has to work harder to cool you by shunting blood to your skin.

It is also important to measure your heart rate at the end of the 4-minute cool-down period. If your heart rate got up to 160 beats per minute it should get down below 100 beats per minute by the end of the 4-minute cool-down. As you get fitter, your cool-down heart rate should decrease more quickly.

How to do your first session of interval sprinting

Before starting a high-intensity interval training program you should be medically screened. This is especially important if you are older, have any risk factors, have any diseases or are on medication. Being screened means the possible positive or negative effects of interval training on you are being assessed by a physician. If you live in Australia, an accredited exercise physiologist will also be able to give you more information and advice for this kind of exercise.

Make sure you have decided on a suitable pedal rate – 90–130 revolutions per minute for most untrained people – and pedal resistance – between 0.5kg and 1.5kg. Perform a 4-minute warm-up using continuous cycling at 60–90 revolutions per minute with a resistance of 0.5kg. This should result in a rating of perceived exertion of around 11. If your rating of perceived exertion is higher – say, 13 – then you should decrease the resistance and pedal rate. If you are not using LifeSprint music (see below) then you need to time the warm-up, sprinting, recovery and cool-down phases. You can download apps for iPhones and Android devices that allow you to create your own interval sprinting program. The LifeSprint music will prompt you to get ready for the sprint through a 3, 2, 1 countdown, and the sprint component of the music is fast whereas the recovery is slow. Therefore, you have to learn to sprint during the 8-second fast music and slow your pedalling rate during the 12 seconds of slow music. If you are timing yourself, increase your pedal rate 3 seconds before the actual sprint.

When sprinting, the pedal rate displayed on your bike will always lag behind the real pedal rate, although some bikes do not display or record pedal rate. By the end of the 8-second sprint, the bike should have caught up and be displaying the real pedal rate. It’s important that you learn the feeling of your targeted pedal rate so it can be performed throughout the 8-second sprint, without relying on the bike to tell you what your rate is.

When sprinting, pushing and pulling with your legs is very important. When you push down on the pedal, you activate the quadriceps muscle, located on your upper thigh, and when you pull the pedal up with your legs, you activate the hamstrings, located underneath your thigh. It is much easier to cycle at high pedal rates if you use this push/pull technique. If you only use a push action, which is typical for beginners, then it makes the sprinting feel much harder. Using the push/pull technique involves more leg muscle mass and should produce more fat burning. It is advisable to push back slightly on the handlebars with your arms to stabilise your pelvis to prevent rocking and bouncing. This avoids possible soreness after interval sprinting exercise.

What does interval sprinting feel like?

For most people, the first couple of sprints of a session feel quite hard, but when they have established a rhythm it gets easier. Most people will not start breathing heavily until after about 5 minutes of sprinting; this is also the time they typically start sweating. We think this is because this form of exercise starts to deplete the short-term energy supplies in the exercising muscles after about 5 minutes, meaning the body has to switch to sugar stores inside the muscle to provide the fuel for sprinting. Towards the end of the sprinting session, we think fat stored inside the muscle is also used, and it continues to be used during the recovery period. Breathing is likely to become heavier.