Dr. Wells: Interval training a powerful way to improve fitness

Greg Wells PhD and Jessica Caterini BSc
6/3/2013 11:39:44 AM
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Once you have been training for a reasonable amount of time and have established your basic fitness level, interval training is a powerful way to improve both your overall fitness and specific running ability.  By understanding and applying the science of intervals, you will find that when the training plan on the wall calls for interval training, you are energized by the challenge instead of finding reasons to skip the workout.

Why Interval Training Works

Interval training is a form of exercise that involves varying the pace of your workout session anywhere from an easy jog right through to a maximal effort.  This combination engages both your aerobic energy system and type I muscles - which are used for endurance - and anaerobic energy systems and type II muscles which are used for power and speed.  The endurance parts of your system are highly efficient and produce very little waste, but they do not generate a significant amount of power.  The strength parts of your system produce the burst you need for sprinting or heavy lifting but are highly inefficient and produce a significant amount of waste in the form of lactic acid.  By engaging both systems at once you teach your body how to process metabolic waste while running, which improves your ability to recover after a hill or short sprint, and enhances your overall fitness. 

Intervals are shorter exercise blocks (usually from 10 seconds up to a few minutes long) that are separated by a rest period.  For example, 45 seconds at 50 per cent effort followed by 45 seconds at 65 to per cent effort, repeated a number of times. As your fitness improves, you can lengthen the amount of time you spend at the higher exertion level from 45 seconds up to 10 minutes or even longer.  You can also adjust the easy effort portion of the workout to make the training more difficult (less rest) or easier (more rest). The total duration of the interval-training set can be adjusted from just a few minutes when starting out up to 60 minutes for highly trained athletes. Make sure you work with a trainer or fitness professional to design the interval-training sessions and properly incorporate them into your overall fitness program.  And always make sure you warm up properly before doing your interval set.

You can use a heart rate monitor to measure your effort, or you can also use your breathing to know how you should be exerting yourself.  At easy running speeds, you should be able to carry on a conversation. Once you're past your first threshold where the anaerobic system starts to be activated, you should be able to hear yourself breathe.  As you increase your intensity beyond the threshold where the anaerobic system is contributing a significant amount of energy, you won't even be able to speak because you're breathing so hard. Many interval training sets demand that you work in the breathing too hard to speak zone. I often use this as a test to see if my athletes are working hard enough. If they can answer my questions after an interval I know they're not working hard enough!

Some runners use a track for their interval workouts so they can easily measure distances and others use more informal methods like picking up the pace between lamp posts along the side of the road.  You can also use your heart rate monitor to track your recovery between fast pieces. As your fitness improves, your heart rate will drop back to pre-interval levels faster.

What The Research Tells Us

Research has shown that interval sessions can improve overall endurance despite the fact that the total running distance may be quite short.  One study discovered that cyclists who engaged in 2.5 hours of interval training a week generated the same improvement in their capacity for endurance as riders who had done 10 hours of endurance-type training. (1)

Intervals are also beneficial because they increase calorie burn post-exercise.  This is a phenomenon called excess post-exercise oxygen consumption (EPOC) and is commonly known as "afterburn."  Afterburn occurs because the body needs to restore itself to the state it was in prior to the workout.  During afterburn, your body has to synthesize a substance called ATP, make repairs to your cells and fibres and process the lactic acid you created.  All of these processes improve your overall level of fitness.  Research has also found that afterburn may last up to a day after intense anaerobic strength training and that the body's metabolism is elevated for several hours after the athlete's heart returning to its resting rate. (2)

Improve Your Performance

To access the benefits of interval training, consider the following tips:

- Make sure you warm up properly.  Light activity like walking or jogging, dynamic activation exercises, and some short duration speed work such as a few sprints to activate your anaerobic system should all be done before you start your interval training session.
- 1-2 interval training sessions per week is often enough to stimulate adaptation in your muscles
- Maintaining a heart rate of 50-55 per cent of maximum during recovery will teach your body to efficiently remove waste products between intervals because you are moving oxygen through the muscles but not producing lactic acid.
- Keep the interval session distance shorter than your typical running workout.
- Concentrate on maintaining proper running form when you increase your speed.
- Concentrate on smooth transitions from slow to fast running and also when you're slowing down.
- Consider training with other people who will help you maintain your pace and increase your motivation.
- Always include an active recovery/cool down phase to help your muscles clear out waste products and to speed recovery. I recommend at least 10-15 minutes of light exercise after your interval training session.

Nutrition and Psychology

In terms of nutrition and fueling, it is essential that you support your interval training with a proper recovery process.  Cooling down properly, taking in proper amounts of carbohydrates and proteins after the workout to fuel regeneration, consuming appropriate fluids to rehydrate and getting plenty of sleep to ensure muscle repair are all critical.  We will have another video and article on the details of post-exercise recovery and sleep.

Interval training requires a different kind of mindset than normal training.  You need to be relaxed and calm so that you can breathe freely and run smoothly despite the intensity that comes with a near all-out effort.  Sustaining your effort during intense portion of your interval sessions is critical if you are going to optimize the benefits to your cardio-vascular and muscular systems.


(1) Gibala MJ, Little JP, Essen MV, Wilkin GP, Burgomaster KA, Raha S, Tarnopolsky MA. (2006). Short term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance. J Physiol 575(3): 901-911.

(2) Shuenke MD, Mikat RP, McBride JM (2002).  Effect of an acure period of resistance exercise on excess post-exercise oxygen consumption: implications for body mass management. Eur J Appl Physiol 86(5). 411-417.

Greg Wells Ph.D. (, @drgregwells) is an Assistant Professor at the University of Toronto in the Faculties of Medicine and Kinesiology. He was the sport science analyst for the Olympic Broadcast Consortium during the 2010 & 2012 Games, and is the author of Superbodies: Peak Performance Secrets from the World's Best Athletes. Jessica Caterini is a member of the Human Physiology Research Unit in the Faculty of Kinesiology at the University of Toronto.

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Dr. Greg Wells Biography

Greg Wells, Ph.D. is a scientist and physiologist who specializes in health and performance in extreme conditions. Most recently, Dr. Wells was the on-camera sport science and sport medicine analyst for the CTV Broadcast Consortium during the 2010 Vancouver Olympics and Canada's 2012 Olympic broadcast for London 2012. Dr. Wells is currently an Assistant Professor at the Faculty of Kinesiology & Physical Education at the University of Toronto where he directs the Human Physiology Research Unit. Previously, Dr. Wells served as the Director of Sport Science at the Canadian Sport Centre where has had the opportunity to work with dozens of athletes who have won medals at Commonwealth Games, World Championships and the Olympic Games. Dr. Wells also believes that to truly understand extreme conditions you should experience them yourself. To this end he continues to build on his experiences as a former international level competitive swimmer, as a marathon runner having twice completed the world's toughest marathon 600 miles north of the arctic circle, and participated in the 11,000 km Tour D'Afrique bike race - the longest bike race in the world.

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