The Art and Science of Interval Training for Sports and Fitness
By Roy Stevenson

Interval training is an extremely potent training technique that will improve your endurance racing performances. It’s also widely avoided by recreational endurance athletes because many consider it too complex or too hard. This should not be the case, and endurance athletes, runners, cyclists and swimmers of all levels can utilize this mode of training to significantly improve their fitness and competition performances.

Benefits of Interval Training (IT)

Improves competition - IT simulates the stress of race pace and conditions athletes for competition.
Improves Neuromuscular Coordination - Your nervous system gets used to running at a faster pace.
Less lactic acid accumulation at a given pace - IT trains the athlete to race faster and accumulate less lactic acid at a given pace.
Thermoregulatory system is not as stressed - Body heat does not accumulate as rapidly as during continuous endurance sports.
Runners of all abilities can use it - Joseph and Kimberlie Nitti, in their book Interval Training for Fitness write, “Interval training can benefit almost any health person, from beginning exercisers to world-class endurance athletes”.

Unfortunately, a lack of knowledge and understanding of interval training principles wastes many endurance athletes’ workouts--and worse, causes injury or sickness if overdone.

Defined as “repeated bouts of high intensity activity, each followed by a limited rest period”, interval training involves running, swimming or cycling a short distance repeatedly, at a speed that is always greater than could be sustained continuously for the full session.

In other words you swim, run or cycle shorter bursts faster than you would race, with much slower recovery intervals between these fast bursts. This is how it gets the name “intervals”. With slower recovery intervals we eventually adapt to sustaining the higher workload for a longer period. And by manipulating the length and speed of the recovery interval we create the desired training effect. e.g. short recovery intervals create an oxygen debt, enabling us to tolerate more lactic acid, so we are better prepared for races.

Interval Training Terminology

Interval: The recovery distance between fast bursts.

Repeat (aka repetition): The fast burst part of interval training.

Energy System: The fuel supply systems that the body uses during various types of (intensity) of exercise.

Anaerobic: Fast, high intensity exercise where you cannot supply enough oxygen to the working muscles, and consequently have to stop or slow down.

Aerobic: Steady state exercise where you take in enough oxygen to supply the muscles’ demands. This would be running, swimming or cycling at a pace you can maintain for a long time.

Finding the Right Energy System for Your Interval Sessions

Three energy systems can be stressed with interval training. The first is the Adenosine Triphosphate--Phosphocreatine (ATP-PC) System. If you train for very short-term fast energy release in activities lasting less than 10 seconds, you’re in this zone. These are high power activities like the shot put, 100m sprints, 25-meter swim, etc.

The second system is the lactic acid system, primarily using glucose as its energy source. Its’ other name is anaerobic glycolysis. Track events lasting 1-3 minutes, such as the 800 meters, and the last lap of the 1500 meters use this energy pathway. Some activities, such as the 400 meters, use both the ATP-PC and lactic acid systems because the time periods for both overlap.

The third energy system is the aerobic system, for events lasting longer than 3 minutes. It uses oxygen as the catalyst for energy release and burns fats and carbohydrates.

Summary of the Three Energy Systems

Unless you’re a sprinter or field-eventer, the ATP-PC system is not worth isolating in your training. The lactic acid system becomes more important because middle distance runners and sprint swimmers stress it when they compete. But the main system recreational and competitive endurance athletes should stress in their training is the aerobic pathway.

Key Guidelines to Using Intervals Successfully

It’s important to exercise the right energy system your competitive racing distance stresses. (More about this follows). Your fast bursts need to be the correct distance for your main racing event.

It’s critical that you recover completely between interval workouts so your energy reserves are replenished and your muscle tissue has time to recover and rebuild.

Don’t do too many fast interval bursts in each workout. (Guidelines follow).

Experiment with adjusting your recovery interval distance to get the desired training effect for your racing distance. You should be able to complete your workout feeling like it was a challenge, but attainable.

Interval workouts should not become the focal point of your weekly training program--avoid becoming over competitive with yourself and obsessing over your interval times.

Establish your limitations with all of the above by trial and error.

The Devil’s in the Details

Figuring out the details of your interval training workouts can be tricky; you’ll need to use the acronym DIRT to help, where. . .

D = Distance of each fast burst
I = Interval, or length of recovery (jog, walk, swim or cycle) between fast bursts
R = Repetitions. How many fast bursts we do in one session
T = Time for each fast burst

This is an especially appropriate acronym--you’ll feel like dirt if you miscalculate any of these factors.

Most endurance athletes err by using incorrect distances for their fast bursts and/or recovery distance and wonder why their times aren’t improving (or even getting slower). 
Now lets go back to the acronym DIRT to establish our workouts. We’ll use distance running to illustrate the physiology of interval training. And for general fitness buffs, the “Table for Suggested Workouts for Beginners” can be used for good result.

D = Distance: The Length of Your Interval Bursts

The fast bursts in an interval workout need to be long enough to dip into the aerobic system--that is, they should be 3-10 minutes long.Because of the precise nature of the distances and times you’ll be running, interval training is best done on the 400-meter track. How far should you be running in your fast bursts? Distances that stress the aerobic system includes 800m (2 laps), 1200m (3 laps), 1600m (4 laps), and even 2000 meters (5 laps).

However, be warned: many runners waste their time doing 100, 200 and 400 meter bursts until they’re blue in the face, and wonder why their times don’t improve. They’re simply not exercising the right energy system for their racing distance.

However, you will still get benefits from doing occasional shorter interval bursts--you’ll improve your neuromuscular coordination of at high speed, making you more efficient, and thus go faster in your races. The disadvantage of these shorter faster intervals is that as intensity increases, so do your chances of injury, because of the higher impacts.

I = Interval: What to do in the Recovery Intervals

Walking or jogging, or a combination, is recommended in the interval between fast bursts. Your first goal is to adapt to the interval workouts by attaining the maximum number of repetitions over these distances. Then, for continued improvement, speed up the fast burst, or decrease the recovery interval between them.

Decreasing the length of the recovery interval between fast bursts achieves great results. This is because your energy sources (ATP and glucose) are not permitted to completely resynthesize, so you draw on the emergency back up system, the lactic acid system. Your body eventually adapts by generating smaller amounts of lactic acid at the same pace, enabling you to cruise at a “higher wattage”, or a much faster pace with less lactic acid building up.

R = Repetitions: How Many Fast Bursts should You Do in an Interval Workout?

The cumulative distance of the fast bursts in your interval workouts should add up to 1.5 to 2 miles for beginners. Running a cumulative total of fast bursts exceeding 3 miles in an interval workout significantly increases your chances of injury, while incurring diminishing benefits.

For example, a session of 8 times 400 meters should be the absolute maximum number prescribed--and that would certainly not be recommended for your first interval workout. You might start with 4 repeats of 400 meters, adding two to that workout next time you do it. Once you reach 8 or 10 x 400 meter repeats comfortably, for example, you can pick up your pace by about 2-3 seconds for following sessions.

T = Time: How to Estimate the Speed of Your Fast Interval Bursts

The longer the fast bursts, the slower they need to be because of our limited ability to supply oxygen to the working muscles and to disperse fatiguing by products (such as lactic acid) as they build up. If you do mile repeats for example, they should be done 10-25 seconds faster than your average 10K race pace, or slightly faster than your average 5K race pace.

For the speed of other track interval workouts, see the table below.

Putting It All Together:
An Example: Sub 40-Minute 10K

Let’s look at a runner trying to break 40 minutes for a 10K race. He or she needs to average under 6:27 mile pace, so 1600 meter (one mile) repeats should be done at around 6:10 pace. Start with 1 of these and build to 2, then 3.

For the same goal, running 800 meter repeats should be slightly faster than the 1600 meter pace, as the distance is shorter--say around 2: 50-3:00 per repeat. Start with 2 of these repeats, and build to 4. Repeats over 400 metres (one lap of the track) should be at about 85-90 seconds each--start with 4 x 400 metre repeats, gradually building to 8.

Table for Suggested Interval Workouts for Beginners

Because of the high risk of illness or injury it’s critical for you to adapt to your interval workouts, rather than let them flatten you. Allow at least 48 hours between these high intensity workouts, and if you’re over 30 years old, one interval workout a week is sufficient. Your muscles, tendons, ligaments and connective tissues need much longer to recover past this age, as they lose their elasticity and resilience.

If you’re under 30, two interval workouts a week are possible. You’ll find out very quickly if you’re not recovering properly because you still won’t be able to walk without pain two days later.

Moving Up to a Higher Level of Interval Training

If you are running faster than your previous times for 2-3 consecutive workouts, it’s time to ramp up your pace. When you can comfortably perform the workout, it’s time to increase the pace or decrease the recovery interval. Your heart rate (taken for one minute) immediately after finishing the last repetition should drop over the weeks. This is also a sign your cardiovascular system is ready to handle more.

Other Advice on surviving (and thriving on) your Interval Workouts

We are all different, and no two athletes will respond to an interval workout in the same way. Thus, avoid competing with others in your workout to reduce your risk of injury. I’ve seen countless runners get injured from thrashing themselves in interval workouts against faster teammates.

Warm-Up

A good warm-up is critical before interval workouts. A solid 15-25 minute jog with stretching and some fast “stride-outs” of up to 50 meters should be included in the warm-up. With these “stride-outs”, you pick your tempo up to a medium-paced sprint to prepare you for the fast interval bursts.

Likewise for good recovery, a 15-minute jog and stretching to cool down afterwards will help disperse metabolic waste products that have built up, reducing your muscle soreness the next day.

Recovering from Interval Training Sessions

Your interval workout should not be so exhausting you cannot recover for the next day’s training, which should be a slow recovery jog on a soft surface. Most important of all is that you have an adequate aerobic conditioning base before you attempt interval training. I’d suggest 8-10 weeks of steady state, aerobic running to prepare you for the rigors of interval speed work.

Interval Training Variations

Another type of interval running you can do is fartlek (a Swedish word meaning “speedplay”). Here you run hard through a park or woods for a certain period of time (for example 5 minutes), then jog to recover, then another fast burst, and so on. This type of training provides a break from the regimentation of track intervals.

Finally, avoid mixing different distances in the same interval session on the track. For example, 4 x 400 metres, then 6 x 200 metres. This training obviously requires a shift in pace to be effective, greatly increasing your probability of injury from muscle strains, hamstring tears, etc. During this workout your body and legs have just adjusted to one pace, then are expected to adjust to another distance. It confuses your neuromuscular system, inhibiting the establishment of the motor learning pattern of running at one tempo.

Interval Workouts for Swimmers and Cyclists?

Interval training is an integral part of swimmers’ and cyclists’ preparation for racing. For D.I.R.T. guidelines for these sports, find a good local coach to help you establish these workouts.

Good luck with this fast training, and remember when in doubt, slow down the pace, or consult with your coach or a runner who is experienced in using interval training.