Interval training has been used by athletes for years to build fitness. Interval training combines short, high-intensity bursts of speed, with slow, recovery phases, repeated during one exercise session. An early form of interval training, fartlek (a Swedish term meaning “speed play”) was casual and unstructured. A runner would simply increase and decrease pace at will.
Today, athletes use more structured interval training workouts and high-intensity interval training (HIIT) to build speed and endurance. This variation of interval training and speed work can be a simple or sophisticated routine, but the basics are still the same as the original fartlek training.
Interval training is built upon alternating short, high-intensity bursts of speed with slower, recovery phases throughout a single workout. Interval workouts can be highly sophisticated and structured training that is designed for an athlete based on their sport, event, and current level of conditioning.
An interval training workout may even be designed based upon the results of anaerobic threshold testing (AT) that includes measuring the blood lactate of an athlete during intense exercise. But less formal interval training is still beneficial for average people who aren’t competitive athletes.
How Interval Training Works
Interval training works both the aerobic and the anaerobic system. During the high-intensity efforts, the anaerobic system uses the energy stored in the muscles (glycogen) for short bursts of activity. Anaerobic metabolism works without oxygen, but the by-product is lactic acid.
As lactic acid builds, the athlete enters oxygen debt, and it is during the recovery phase that the heart and lungs work together to “payback” this oxygen debt and break down the lactic acid. It is in this phase that the aerobic system is using oxygen to convert stored carbohydrates into energy.
It’s thought that by performing high-intensity intervals that produce lactic acid during practice, the body adapts and burns lactic acid more efficiently during exercise. This means athletes can exercise at a higher intensity for a longer period of time before fatigue or pain slows them down.
Benefits of Interval Training
Interval training adheres to the principle of adaptation. Interval training leads to many physiological changes including an increase in cardiovascular efficiency (the ability to deliver oxygen to the working muscles) as well as increased tolerance to the build-up of lactic acid. These changes result in improved performance, greater speed, and endurance.1
More benefits include:
- Avoids injuries associated with repetitive overuse, which are common in endurance athletes
- Benefits people with medical conditions such as COPD and metabolic syndrome2
- Burns more calories*
- Includes cross-training in an exercise routine
- Increases training intensity without overtraining or burnout
*According to the American College of Sports Medicine, more calories are burned in short, high-intensity exercise. as opposed to long, slow endurance exercise. Studies show it can be at least as effective as moderate-intensity continuous exercise in losing body fat.3
Precautions and Safety Tips
Keep in mind that interval training is extremely demanding on the heart, lungs, and muscles, and it’s important to have an OK from your physician before you start interval training. You should also have a solid base of overall aerobic fitness before performing high-intensity training of any kind.
- Assess your current conditioning and set training goals that are within your ability.
- Keep a steady, but challenging pace throughout the interval.
- Start slowly. For example: walk 2 minutes/run 2 minutes. In general, longer intervals provide better results.
- Train on a smooth, flat surface to ensure even effort.
- Warm up before starting intervals.
How to Build Interval Training Workouts
Designing the right interval training routine can be sophisticated or casual. Elite athletes may go to sports performance lab to have blood lactate and exercise metabolism testing performed to determine the best interval training routine. On the other end of the spectrum, you can use the casual “speed play” interval training (fartlek) without timing.
You can vary your work and recovery intervals based on your goals. Four variables you can manipulate when designing your interval training program include:
- Duration (distance or time) of work interval
- Duration of rest or recovery interval
- Intensity (speed) of work interval
- Number of repetitions of each interval
- Longer recovery intervals: A longer recovery interval teamed with a shorter work interval allows you to go all-out on the work interval. For example, a 30-second sprint teamed with a 1-minute recovery.
- Longer work intervals: You can shorten the rest and lengthen the work interval as you advance. This burns more calories and builds endurance.
- Mixed work intervals: You can vary the length and intensity of the work intervals in your workout, with some being at the highest effort and others being at a moderately high effort, or making the work intervals of different lengths within the same workout.
- Untimed intervals: As with fartlek, you simply pay attention to how you feel and set your intensity and duration accordingly.
Build the number of repetitions over time. To improve, increase intensity or duration, but not both at the same time. Make any changes slowly over a period of time. Beginners should start with short intervals (under 30 seconds), fewer repeats, and more rest. Elite athletes can up the intensity, time, and frequency of training. Few athletes benefit from performing intervals more than two times per week.
Aerobic Interval Training (AIT)
With aerobic interval workouts, you alternate between moderate- to high-intensity exercise work intervals with a recovery interval. Your work interval is below 85% of your maximum heart rate. Aim for a recovery effort that brings your heart rate down to 100 to 110 bpm during the rest interval.
You can use any cardio activity, such as running, walking, cycling, elliptical trainer, etc. The workout can be as short as 10 minutes (after a warm-up of at least 5 minutes) or can be as long as 60 minutes for those who are advanced.
Beginners may use shorter work intervals and longer recovery intervals. As fitness improves, the work and recovery intervals can be adjusted so the work intervals are longer (as much as 10 minutes) and the recovery intervals shorter (such as 2 minutes)
Here is a typical AIT workout:
- Warm-up for 5 to 10 minutes at a comfortable level of exertion.
- Pick up your speed or exertion to your recovery level for 2 to 3 minutes.
- Increase speed or difficulty for 1 to 2 minutes to bring you up into a higher heart rate zone, but not exceeding 85% of your maximum heart rate.
- Return to your recovery pace or exertion for 2 to 5 minutes.
- Repeat work and recovery intervals as needed for your chosen workout length.
You can do aerobic interval workouts two or more times per week.
Anaerobic or High-Intensity Interval Training (HIIT)
In anaerobic interval workouts, your work interval is at an all-out effort, bringing your heart rate to 85% to 100% of your maximum heart rate. You can use any cardio activity, such as running or cycling, that can bring your heart rate up to the anaerobic zone. These workouts are usually shorter because they are so intense, often just 20 minutes after a warm-up.
The rest interval is usually twice as long as the work interval, such as 30 seconds of sprinting followed by 1 minute of recovery. The warm-up should be longer than with less intense intervals, in the range of 10 to 15 minutes. Due to the intensity, allow 24 to 48 hours of recovery between HIIT workouts.
An example of an anaerobic interval workout:
- Warm-up for 5 minutes at an easy to moderate effort, then come up to your recovery interval effort for 5 minutes.
- Work interval 30 seconds: Sprint all-out, as fast as you can.
- Recovery interval 1 minute: Return to your recovery interval effort.
- Repeat work and recovery intervals three to seven times.
- End with a cool down for 5 minutes at an easy effort.