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Increasing Aerobic Fitness & Interval Training

Interval training is a popular topic, but it's nothing new. Records of it being used to improve athletic performance go as far back as 1912, when Finnish Olympic champion runner Hannes Kolehmainen used it for training his 10-kilometer pace.

Countless studies, such as this 2008 study from American Journal of Physiology, have since examined the specific effects of interval training on markers of aerobic fitness and revealed this type of training to be comparable, and in many cases superior, to traditional steady-state cardiovascular exercise.

Defining Aerobic Fitness

Aerobic fitness is another term for cardiovascular fitness. It's a measure of the ease and efficiency with which your heart pumps blood and oxygen to working muscles and how efficiently those muscles are able to use the oxygen.

Aerobic fitness is difficult to quantify. It depends on your age, gender, exercise habits, genetics and other physiological factors. It also depends on your goals. Someone who wants to run their first 5k is going to measure cardiovascular fitness differently from a competitive cyclist.

How hard your heart works during activity and at rest is a good marker of how aerobically fit you are, no matter your goals. You can feel it while you're working out by how hard your heart beats and how fatigued you feel, and you can measure it at rest by taking your pulse. On average, the resting heart rate of a healthy adult is about 70 beats per minute, while a well-conditioned heart beats more slowly at rest — 40 to 50 beats, or fewer, per minute.

The Case for Interval Training

As with any other muscle in your body, you can improve aerobic capacity by training your heart to work more efficiently. Any type of dynamic large muscle cardiovascular activity — running, biking, skating — will improve aerobic fitness when done at the right intensity for long enough.

Traditional steady-state or continuous workouts, such as running or cycling for more than 30 minutes at a moderate pace, are effective and certainly can be part of your training regimen. However, research shows that shorter, more intense interval workouts can garner the same — or better results — in a lot less time.

In a study published in The Journal of Physiology in 2008, participants who performed four to six repeats of a 30-second all-out sprint followed by 4.5 minutes three days per week improved skeletal muscle oxidative capacity , which is a measure of a muscle's capacity to use oxygen, comparably to participants who performed 40 to 60 minutes of lower-intensity steady-state exercise. The major difference was training volume, which averaged 1.5 hours weekly for interval training versus 4.5 hours for continuous training.

In many cases, interval training has proved to be more effective than steady-state. The 2008 American Journal of Physiology looked at the effects of intervals versus continuous training in sedentary subjects. Over an eight-week period, V02 max — a measurement of the heart's efficiency under maximum exertion — improved by 15 percent in the interval training group versus a 9 percent improvement in the continuous training group. In addition, only the interval training group saw improvements in both maximal cardiac output and skeletal muscle oxidative capacity. Researchers concluded the improvements were due to the fluctuations in workload and oxygen uptake involved in interval training.

Read more: 5 Ways to Supercharge Your HIIT Routine

Using Interval Training to Improve Aerobic Fitness

Interval training is an effective method for improving aerobic fitness for both beginner exercisers and athletes -- and everyone in between -- but the way it's implemented will differ depending on fitness level.

The intensity of your intervals is relative. If you're just starting out a walking program, your intervals of high-intensity might be simply speed walking. If you're a jogger who wants to work up to running, your intervals would be a running pace, and recovery would be an easy jog or a fast walk. A runner looking to increase speed would alternate sprint intervals and recover at a jogging pace.

Whatever your goal, the aim is to continue to increase the intensity of your intervals. This results in overload — continually increasing the amount of stress placed on the heart and lungs to elicit the physiological adaptations that result in increased aerobic fitness.

Workouts are typically based upon a ratio of work to rest. For example, a 1:1 ratio of 30 seconds of intensity followed by 30 seconds of rest, or a 1:2 ratio of 30 seconds of intensity followed by 1-minute of recovery.

For general fitness purposes, either of these are effective. For specific goals, workouts can be modified accordingly. For example, a power athlete, such as a sprinter, would typically perform shorter periods of high effort, while a distance runner would perform longer high-effort intervals.

In terms of intensity, the high-intensity interval typically ranges from 80 percent to 100 percent of maximum effort — but, again, it's specific to training goals. For the average exerciser looking to improve aerobic capacity, the general advice is to simply work as hard as you can during the high-effort intervals.

Interval training workouts should be kept short — about 20 minutes, plus a warm up and cool down is typical. Do as many repeats as you can in that time.

Frequency and Cross-Training

Interval training is highly effective, but because of its intense nature and the stress it places on the body, it shouldn't be done in every workout. One to three interval training sessions per week is adequate. On the other days, do longer less-intense cardio workouts or rest. Adequate recovery is key to improving your fitness without injury.

Read more: 5 Myths About HIIT

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About the Author

Jody Braverman is a professional writer and editor based in Atlanta, GA. She received a Bachelor of Arts in English from the University of Maryland, and she is a certified personal trainer, fitness nutrition specialist, and yoga teacher.

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