This article on interval training is divided into five parts for easier reading:


2.0     Introduction

“Interval Training is always Repetition Training – not all Repetition Training is Interval Training.” (Thompson, 2010).

This section of the article looks at the methodology behind interval training. Areas discussed in this section include:

  • What is continuous training?
  • What is repetition training?
  • What is interval training?
  • The training variables.
  • Determining your ‘correct’ interval training pace.
  • The recovery interval.
  • Why interval training?
  • Rules of Effective Interval Training.
  • Seeing What Happens to the Amount of Oxygen You Consume.
  • Ventilatory threshold.
  • Frequency of interval training.

Figure 1 provides an outline of the various methods of training discussed in this article.

Figure 1: Methods of training

Two types of aerobic training include the continuous and intermittent methods. Both the resistance and functional methods of training have developed interval training models.

Some training regimens, such as Fartlek Training (Part 03, Section 3.3), combine both continuous and interval approaches.

2.1     What is Continuous Training?

“Continuous training involves continuous activity without rest intervals. This can vary from long, slow distance (LSD) training to high-intensity endurance training.” (Kenney, Wilmore & Costill, 2012, p.222).

The continuous method, more commonly known as continuous training, is any type of physical activity/exercise without rest intervals – it is often contrasted with the intermittent method and is generally viewed as a traditional method of training.

It is also known as (not an exhaustive list) continuous exercise, continuous moderate exercise (CME), continuous cardiovascular exercise, steady state run, long steady/slow distance (LSD), and (traditional) endurance training.

Continuous training requires sustained, steady state aerobic exercise and, because of its submaximal nature, may continue for a considerable time in relative comfort – distance rather than speed is the main objective of continuous training. This makes LSD training ideal for people beginning an exercise programme or wanting to reduce excess body fat.

“LSD training became extremely popular in the 1960s. [and was] introduced in the 1920s by Dr. Ernst Van Auken, a German physician and coach…” (Kenney, Wilmore & Costill, 2012, p.222).

Continuous training can be performed at one of three intensities:

  • Low-intensity Continuous Training (LICT), approximately 60% to 70%-75% maximum heart rate (MHR);
  • Moderate-intensity Continuous Training (MICT), approximately 70%-75% to 85% MHR; and
  • High-intensity Continuous Training (HICT), approximately 85% to 90% MHR.

Sometimes referred to as low-, moderate- or high-intensity aerobic continuous exercise (LICE, MICE and HICE respectively).

Elite endurance athletes overload the cardiovascular and energy transfer systems using continuous exercise training at nearly the same intensity as competition. This specifically activates the slow-twitch muscle fibres in sustained exercise. A champion middle-distance runner may run 5 miles continuously in 25 minutes during workouts at a heart rate of 180 beats per minute; this pace does not exhaust the athlete but still nearly duplicates race conditions. By finishing each exercise session with several all-out sprints stopped 30 to 40 seconds before exhaustion, the athlete also trains the short-term anaerobic system (glycolysis) that contributes to race performance, particularly at the finish. A marathon runner trains as a slightly slower pace than a middle-distance athlete to simulate the intensity, distance and energy requirements of actual competition.

Exercise methodologies noted as suitable for continuous training include (not an exhaustive list):

  • Indoor and outdoor cycling;
  • Walking, jogging and running
  • Indoor and outdoor rowing;
  • Stair climbing and simulated climbing;
  • Nordic skiing;
  • Elliptical training;
  • Aerobic riding, aerobic dancing and bench step aerobics;
  • Hiking;
  • In-line skating;
  • Rope skipping; and
  • Swimming, and water aerobics.

2.2     What is Repetition Training?

“Let’s first take a quick look at traditional ‘Repetition Training’, since all interval training is a specific type of repetition training. You are aware that coaches and athletes frequently use repetition training by breaking training distances down into parts, with the parts being repeated, hence ‘Repetition Training’. A typical, simple repetition session might be 15 repetitions of 400m, which would be referred to as a ‘400m rep session’ or doing ‘400m reps’.” (Thompson, 2010).

With classic repetition training the recovery period, unlike other forms of interval training, is long enough for the athlete to get their ‘breath back’. As such, a longer recovery period is required (e.g. 4-6 minutes) unlike newer forms of interval training where the recovery period maybe reduced to 2-4 minutes or even less than 60 seconds.

2.3     What is Interval Training?

“Researchers, coaches and athletes have a variety of concepts of interval training, the only point of agreement being that interval training involves alternating bouts of exercise and recovery.” (Daniels & Scardina, 1984, p.327).

With this in mind, just as there are a variety of alternative names for interval training, there are a variety of definitions, including:

  • “High-intensity interval training (HIT), which involves repeated 30–300-s bouts of aerobic exercise at an intensity ranging from 85 to 100% of VO 2max interspersed by recovery periods of equal or shorter duration.” (Daniels and Scardina 1984).
  • “One can exercise at an intensity that normally proves exhausting within 3 to 5 minutes using preestablished spacing of exercise-to-rest intervals. This approach forms the basis of the interval training program. From a practical perspective, the exerciser applies various work-to-rest intervals using “supermaximum” effort to overload the specific systems of energy transfer. (Katch, McArdle & Katch, 2011, p.200).
  • “Interval training consists of repeated bouts of high- to moderate-intensity exercise interspersed with periods of rest or reduced-intensity exercise.” (Kenney, Wilmore & Costill, 2012, p.220).
  • “Interval exercise refers to the basic pattern of alternating periods of more intense effort with period[s] of less intense effort, or complete rest, within a single training session.” (Gibala, 2016).
  • “‘Interval Training’ is simply and very specifically any repetition training where the training effect takes place during the recovery intervals between the faster paced runs.” (Thompson, 2010).
  • “Interval training involves repeated short to long bouts of rather high intensity exercise (equal or superior to maximal lactate steady-state velocity) interspersed with recovery periods (light exercise or rest).” (Billat, 2001, p.13).
  • “In general, muscular activity can be sustained beyond 10 min at a level demanding about 75% to 80% VO2max, called the lactate or ventilatory threshold. Interval training allows short periods of muscular contraction for 1 to 2 min at 100% to 110% VO2max.” (Tipton, 2014, p.260).

“The idea that interval training can be identified by a specific intensity, duration, or number of exercise bouts or by the amount or type of recovery between bouts of exercise is not valid. Rather, interval training has come to mean any type of intermittent training which, with manipulation of the number, intensity and duration of work bouts and amounts of recovery, is used to produce a particular type of stress on the body.” (Daniels & Scardina, 1984, p.327).

Saltin et al. (1976, p.23-51) suggest that – using an interval training of 3 minutes at 100% of the minimal velocity associated with the maximal oxygen consumption (vVO2max) determined in an incremental test interspersed with 3 minutes at 50% vVO2max – interval training has the following characteristics:

  • The intensity is defined as the average power output; for the interval training described above, the average intensity is equal to (100 + 50)/2 = 75% vVO2max [about 75% of maximal oxygen uptake (VO2max)];
  • The time-ratio for the high and low exercise duration; for the interval training described above, the time ratio 3/3 = 1;
  • The amplitude is the ratio of the difference between the intensity of the different periods (heavy or recovery run) with the average velocity; for the interval training described above, since the average velocity is 75% vVO2max, the amplitude is: 100-75/75 = 33%;
  • The duration and the distances run at high and low velocities.

In summary, periods of intense activity interspersed with moderate to low energy expenditure characterise many sport and life activities.

Interval training simulates this variation in energy transfer intensity through specific spacing of work and recovery periods. With this approach, an individual trains at an inordinately high exercise intensity with minimal fatigue that would normally prove exhausting if done continuously. Rest-to-exercise intervals vary from a few seconds to several minutes depending on the energy system(s) overloaded.

There are a number of factors or variables which help to formulate the interval training session as outlined below.

2.4     The Training Variables

There is general agreement that rather specific guidelines are available which determine the amount and intensity of work and the amount of rest necessary to produce specific results, and it is suggested that types of training be identified based on the specific characteristics of each particular type of training rather than placing all types of intermittent training in an all-inclusive category called ‘interval training’. In other words, the training should be specific to the athlete and their sport.

With this in mind, when undertaking interval training, many would suggest there are four variables to consider, easily remembered by the mnemonic DIRT:

  • Distance of each speed interval:
    • Refers to the distance to be covered (or time taken) during each work period.
  • Interval of recovery between speed intervals:
    • Aka the ‘recovery period’.
    • It is during this recovery period, especially the first 10 to 15 seconds, that most of the training effect occurs.
    • Besides the duration of the recovery period, the activity figures into the equation (e.g. rest, walking, jogging or running).
    • One of the original principles of interval training, that is still generally accepted, is that the next speed interval should not begin until the individual’s pulse has lowered to 120 beats per minute.
    • Work-to-Recovery Ratio (WR Ratio) (Section 2.6):
      • 2:1 WR Ratio: Recovery times are half as long as the interval work time.
      • 1:1 WR Ratio: Recovery times are equal to the interval work time.
      • 1:2 WR Ratio: Recovery times are twice as long as the interval work time.
    • WR Ratio is an important characteristic that differentiates ‘old’ interval training and its younger high-intensity interval training relative.
  • Repetitions of speed intervals:
    • The number of fast sprints/runs to be performed.
    • In longer training sessions, repetitions can be broken down into sets, with a longer recovery interval than between individual repetitions.
  • Time of each repetition:
    • How fast each repetition should be run.
    • Time can be constant or variable depending on the goal of the training session.

McArdle, Katch and Katch (2006, p.457) utilise slightly different phrasing, but mean the same thing:

  • Intensity of exercise interval;
  • Duration of exercise interval;
  • Duration of recovery interval; and
  • Repetitions of exercise-recovery interval.

Kenney, Wilmore and Costill (2012, p.221) note six “primary variables” to consider:

  • Rate of the exercise interval: Determine the intensity of the exercise interval either by establishing a specific duration for a set distance or by using a fixed percentage of the person’s maximal heart rate (MHR). Intensity can be described as:
    • Supra-maximal (or supermaximum), maximal and submaximal; or
    • High-intensity, moderate-intensity or low-intensity.
  • Distance of the exercise interval: The distance of the exercise interval is determined by the requirements of the event, sport, or activity. Individuals who run or sprint short distances, such as track sprinters, basketball players, and football players, will utilise short exercise intervals of 30 to 200 metres (33-219 yards), although a 200 metre sprinter will frequently run over distances of 300 to 400 metres (328-437 yards). A 1,500 metre runner may run exercise intervals as short as 200 metres to increase speed; but most their training would be at distances of 400 to 1,500 metres (437-1,640 yards), or even longer distances, to increase endurance and decrease fatigue or exhaustion during a race.
  • Number of repetitions and sets during each training session: The number of repetitions and sets should be determined by the needs of the sport, event or activity. Generally, the shorter and more intense the exercise interval, the greater should be the number of repetitions and sets. As the exercise interval is lengthened in both distance and duration, the number of repetitions and sets should be correspondingly reduced.
  • Duration of the rest or active recovery interval: The duration of the recovery interval (whether active or passive) will depend on how rapidly the individual recovers from the exercise interval. The extent of recovery is best determined by the reduction of the individual’s heart rate to a predetermined level during the recovery interval. For younger individuals (30 years of age or younger), heart rate is generally allowed to drop to between 130 and 150 bpm before the next exercise interval begins. For those over 30 years, since MHR decreases ~1 bpm per year, one subtracts the difference between the individual’s age and 30 years from both 130 and 150. So, for a 45-year-old, one would subtract 15 bpm to obtain the individual’s recovery range of 115 to 135 bpm. The recovery interval between sets can be established in a similar manner, but generally the heart rate should be below 120 beats/min.
  • Type of activity during the active recovery interval: The type of activity performed during the recovery interval for land-based training can vary from complete rest (passive recovery) to slow walking or rapid walking to jogging/slower running (active recovery). In the pool, slow swimming using alternative strokes or the primary stroke is appropriate. In some cases, usually in the pool, total rest can be used. Generally, the more intense the exercise interval, the lighter or less intense the activity performed in the recovery interval. As the individual becomes better conditioned, they will be able to increase the intensity of the exercise interval or decrease the duration of the recovery interval, or both.
  • Frequency of training per week: The frequency of training will depend largely on the purpose of the interval training. A world-class sprinter or middle-distance runner typically works out five to seven days a week, although not every workout will include interval training. Swimmers use interval training almost exclusively. Team sport athletes can benefit from two to four days of interval training per week when interval training is used only as a supplement to a general conditioning programme. The coach or athlete who is interested in the specific details of how to organise and administer an interval training programme should refer to the classic text by Fox and Mathews (1974). These authors have provided many excellent examples of how interval training can be used for various types of sports. For beginners and those with medical ailments, one session per week may be appropriate.

Billat (2001a, p.15) presents ‘variables’ through a classification system “…of the different types of interval training according to the specific velocities of a race, the time limit at these velocities and ‘physiological velocities’: the velocity at maximal oxygen uptake (VO2max), the critical velocity (i.e. the asymptote of the velocity-time limit relationship), and the velocity at maximal lactate steady state.”

For most people, running continuously at a 4 minute mile pace will exhaust them within a minute due to rapid lactate accumulation. However, running at this speed for only 15 seconds followed by a 30 second recovery interval enables a person to accomplish 4 minutes of running at this near record pace. Of course, this does not equate to a 4 minute mile, but during 4 minutes of running, the person covers a one mile distance even though the combined exercise and recovery intervals require approximately 11 minutes 30 seconds.

Further, the variables are interrelated, and changing one will affect the others and even the training effect/output. Understanding how these variables and the training output interact will enable a person to modify their workout to their advantage.

As a practical example, a person could complete eight repetitions of 400 metres in 90 seconds with a 2 minute recovery interval (8 x 400 @ 90; 2). To increase the difficulty of this interval training session a person could make four changes to the above variables (although it is wise to only alter ONE of them at a time):

  • The number of repetitions could be raised to 10 or 12;
  • The distance could be increased to 600 or 800 metres;
  • The pace could be dropped to 85 or 80 seconds a lap; or
  • The recovery time could be reduced to 1:45 or 1:30.

In contrast, if the original interval training session was considered too challenging, changing any of the variables the other way would make it easier to complete.

As a final thought, the shorter the distance a person runs, the faster they can run the total distance of the workout. For example, 5 x 1,000 metre exercise intervals with recovery intervals can be ran faster than 5,000 metres continuously; 10 x 500 metre exercise intervals with recovery intervals can be ran faster than 5 x 1,000 metres; and 20 x 250 metre exercise intervals with recovery intervals can be ran faster than 10 x 500 metres.

“Exercise duration is probably the most appropriate variable to manipulate initially, building the exercise session by 10%, or 5 to 10 minutes every week or two over the first 4 to 6 weeks. Thereafter, and once adherence is developed, progressions can be implemented by increasing exercise frequency and then exercise intensity, but the progressions should always remain consistent with the individual’s goals.” (Porcari, Bryant & Comana, 2015, p.391).

With so many possible combinations of these variables, there is nearly unlimited potential to vary workouts and ‘never get bored’. Although runners tend to pay more attention to the pace and distance of each running period, the benefit from interval training sessions occurs from a combination of running and recovery. The recovery intervals are very important to the design and effectiveness of the workouts. That combination of running and recovery is what makes interval runs different from continuous runs.

2.5     Determining Your Correct Interval Training Pace

“Brief, all-out exercise interspersed with recovery represents a specific application of interval training for anaerobic conditioning.” (Katch, McArdle & Katch, 2011, p.432).

There are a number of different ways to determine an individual’s ‘correct’ interval training pace, with three examples using distance, VO2max and rate of perceived exertion given below. Further, an individual may perform a single-paced or multi-paced session.

Katch, McArdle and Katch (2011, p.435-436), referencing Fox and Matthews (1974), suggest the following distance method is appropriate for determining a person’s correct interval training pace for running and swimming:

  • Add 1.5 to 5 seconds to the person’s ‘best time’ for training distances between 60 and 220 yards (55 to 200 metres) for running and 15 and 55 yards (14 to 50 metres) for swimming.
    • If a person covers 60 yards from a running start in 8 seconds, the exercise duration for each repeat equals 8 + 1.5 or 9.5 seconds.
    • Add 3 seconds to the best running time for interval training distances of 110 yards (100 metres).
    • Add 5 seconds to a distance of 220 yards (200 metres).
    • This particular application of interval training most effectively trains the intramuscular high-energy phosphate component of the anaerobic energy system.
  • For training distances of 440 yards (400 metres) running or 110 yards (100 metres) swimming, determine the exercise rate by subtracting 1 to 4 seconds from the average 440 yard portion of a mile run or 110 yard portion of a 440 yard swim.
    • If a person runs a 7 minute mile (averaging 105 seconds per 440 yards), the interval time for each 440 yard repeat ranges between 104 seconds (105 – 1) and 101 seconds (105 – 4).
  • For run training intervals beyond 440 yards (and swim intervals beyond 110 yards) add 3 to 4 seconds to the average 440 yard portion of a mile run or 110 yard portion of a 440 yard swim.
    • In running an 880 yard (800 metre) interval, the 7 minute miler runs each interval in about 216 seconds ([105 + 3] x 2 = 216).

Another method of determining a person’s correct interval training pace is through VO2max pace – the maximum volume of oxygen your muscles consume per minute (or how efficiently your body uses oxygen). The Cooper VO2 Max Test is an aid to determining a person’s VO2max.

  • For recreational runners, generally, VO2max pace is:
    • Between mile and 2-mile (3k) race pace;
    • About 20 to 25 seconds per mile faster than 5k race pace;
    • About 40 to 45 seconds faster per mile than 10k race pace; and
    • 95 to 100 percent of maximum heart rate.
  • For competitive and highly trained runners, generally, VO2max pace is:
    • Equal or very close to 2-mile (3k) race pace;
    • About 10 to 15 seconds per mile faster than 5k race pace;
    • About 25 to 30 seconds per mile faster than 10k race pace; and
    • 95 to 100 percent of maximum heart rate.

To improve VO2max, running faster than VO2max pace is not any better than running at VO2max pace; doing so adds more fatigue, causing the current or next training session to suffer.

Rate of perceived exertion (RPE) is for those who do not use heart rate monitors, or other technology, and wish to rate their training effort by ‘how it feels’. Further information can be found here.

Regardless of the method an individual uses to determine their correct interval training pace, they should not run the first exercise interval of the training session so fast that they cannot match that pace during the rest of the session. Finally, the pace desired will be affected (amongst other factors) by the individual’s sport/event and motivation level.

2.6     The Recovery Period

“Anaerobic and aerobic power training programs are designed to train the three metabolic energy systems: ATP-PCr system, anaerobic glycolytic system, and oxidative system.” (Kenney, Wilmore & Costill, 2012, p.223).

The recovery interval (also known as recovery period or relief interval) occurs either (Katch, McArdle & Katch, 2011):

  • Passively, known as rest-relief or passive recovery; or
  • Actively, known as exercise-relief or active recovery.

The recovery interval duration represents a multiple of the exercise interval, and is known as the work-to-rest ratio (WR Ratio) or exercise-to-relief interval (Katch, McArdle & Katch, 2011):

  • A 1:3 WR Ratio overloads the immediate energy system (ATP-PCr).
    • For a person who runs 10 second intervals, the recovery interval equals 30 seconds.
  • For training the short-term glycolytic energy system, the recovery interval doubles (WR Ratio of 1:2), i.e. a 2 minute recovery interval follows a 1 minute exercise interval.
    • These specified ratios allow sufficient restoration of high-energy phosphates and lactate removal so subsequent exercise proceeds with undue fatigue.
  • For training the long-term aerobic energy system, the WR Ratio usually equals 1:1 or 1:1.5.
    • For example, during a 60- to 90-second exercise interval oxygen uptake increases rapidly to a high level.
    • Although some lactate accumulates during this relatively intense exercise, the duration remains brief enough to prevent exhaustion.
    • A 1 to 2 minute recovery interval permits exercise to begin again before oxygen uptake returns to its pre-exercise level.
    • Consecutive repeat exercise-relief intervals ensures that cardiovascular response and aerobic metabolism eventually maintain near-maximal levels throughout the exercise intervals and recovery intervals.
    • Performing continuously at this exercise intensity exhausts the person within several minutes, and training would cease.
  • 2:1 WR Ratio: For a person who runs 20 second intervals, the recovery interval equals 10 seconds. HITT-type sessions general utilise a 2:1 WR Ratio.

“The onset of fatigue, or the inability to continue exercise at a given intensity, depends on fitness level and training status, exercise intensity, and environmental conditions.” (Porcari, Bryant & Comana, 2015, p.78).

During the recovery interval, an individual’s heart rate declines at a proportionally greater rate than the return of blood to the heart (In other words, a lot of blood is coming back to the heart), but the heart rate drops quickly because the individual has stopped running fast. The slower heart rate allows more blood to enter the left ventricle and results in a brief increase in stroke volume (the amount of blood the heart pumps with each beat). The increase in stroke volume overloads the heart, making it stronger and enabling the skeletal muscles to be cleared of waste products quickly because of the increased blood flow to the muscles. Because stroke volume peaks during the recovery interval and because interval training sessions have many recovery intervals, stroke volume peaks many times, providing a stimulus for improving maximum stroke volume and therefore the capacity of the oxygen transport system.

2.7     Why Interval Training?

“Research has shown that athletes can perform a considerably greater volume of exercise by breaking the total exercise period into shorter, more intense bouts, with rest or active recovery intervals inserted between the intense bouts.” (Kenney, Wilmore & Costill, 2012, p.220).

A sound rationale forms the basis for interval training. In the example of a continuous run by an average person at a 4 minute mile pace (Section 2.4, above), the predominant energy for exercise comes from the short-term anaerobic energy pathway with rapid lactate accumulation. The individual becomes exhausted within 60 to 90 seconds. In contrast, running at this speed for 15 second intervals or less places significant demands on the immediate energy (ATP-PCr) system with little lactate accumulation. Recovery becomes predominantly ‘alactic’ in nature and occurs rapidly (The anaerobic alactic energy system provides massive bursts of energy in very short periods of time. Generally speaking, the anaerobic alactic energy system can only be dominant for, at most, 20 seconds before the anaerobic lactic and aerobic energy systems take over.). The subsequent exercise interval can begin after only a brief recovery interval. Repetitively linking specific exercise intervals and recovery intervals eventually places considerable demand on aerobic energy metabolism.

“In interval training, as with other forms of physiologic conditioning, exercise intensity must overload the specific energy system(s) desired for improvement through sport-specific muscle activation.” (Katch, McArdle & Katch, 2011, p.435).

One of the attractions of interval training is its measured, precise nature. Training sessions can be tailored to an individual’s current level of ability/fitness and can also provide an accurate benchmark of their fitness, enabling achievable (and competitive) goals to be set. Interval training’s repeatability facilitates comparisons of past and present performance.

Manipulation of the training variables (Section 2.4) enables a great variety of sessions, with the opportunity to create a new session every time you train!

Interval training sessions have a nil/low choreography level and there is no fancy footwork patterns or complicated arm movements (subject to the requirements of the sport/activity). Interval training provides an excellent cardiovascular workout and, although interval training features a high-intensity level, this can be adjusted to the individual’s fitness level, meaning each individual can work at a level of intensity suited to them.

The best part about interval training is that everyone can benefit. The only real restrictions are:

  • The fitness level of the participants;
  • The contraindications of the participants (if applicable);
  • The space available; and
  • The imagination of the instructor/trainer/coach (see Part 03).

2.8     Rules of Effective Interval Training

These ‘rules’ are aimed at the recreational exerciser rather than professional/elite athletes, although the general ideas can be applied to both.

  • Rule One:
    • Go to each interval training session with a goal, and a plan.
    • Do not just say “I need to get fast and I am going to work-out on the track.”
    • Different workouts have different training effects; 20 x 200 and 4 x 1,000 have little in common besides the total distance being the same.
    • A marathon runner and a 1,500 metre runner will perform different types of workouts; the former might do repeat miles to increase stamina, the latter, sets of 200s to hone a finishing kick.
    • Know what your seasonal racing goals are, what phase of your training you are in, and what you need to work on at the current time.
    • With this knowledge, and the understanding of the basic principles of interval training, you can sensibly follow the next rule:
  • Rule Two:
    • Be flexible in your workouts, but within reason.
    • An elite athlete will not go to the track to increase their midrace performance, witness another athlete working on their sprint finish, and suddenly swap. However, some individuals are prone to swap and change during a session. Some consider that speed-work is speed-work, and it does not matter what they do, it has just got to make them faster.
    • Beware of falling prey to group mentality (aka groupthink), for example doing what everyone else is doing because you may feel embarrassed.
    • Refer back to Rule One, and see if you can adapt or modify the workout to fit your training goals.
    • If not, you are probably better doing a workout on your own, even if it is psychologically tougher than training within a group.
  • Rule Three:
    • Knowing this principle allows you to tailor a workout to your own needs and ability.
    • Say a group or training partner is doing the above workout (Rule One), but it is just a bit beyond your current fitness level, or you are tired from the weekend’s long run or race.
    • To bring it within your reach, you could slightly slow the pace, for example finishing your laps in 95 or 100 seconds or stop after reaching the 150 metre mark.
    • Alternatively, you could do only six repetitions, which would also lessen your total workload (if the workout proved even tougher than you thought, you could make two or even three of these reductions).
    • Theoretically, you could also lengthen your recovery interval, but in a group setting that may be impractical.
    • What change(s) you make will depend on your goal(s) for the given training session.
    • To learn to run six minute miles in a race, keep the pace the same and reduce the distance and/or reps this time, trying to increase them in the future when your fitness improves.
    • On the other hand, if you are working on being able to run farther at a higher pace, slowing down but completing the total distance may be a better option.
  • Rule Four:
    • Be flexible and honest when assessing your own training for the day (and over the season) and realise that, like continuous/endurance training, speed-work’s benefits do not appear immediately but accrue gradually through a steady accumulation of workouts.
    • Intervals are supposed to be tough, to provide the training stimulus for physiological callousing and improvement, and it is sometimes difficult to differentiate between physical fatigue and mental fatigue.
    • However, if your body is really not up to the day’s workout, make it easier, using these rules and the training principles above, or even have the courage and common sense to skip it entirely, knowing that in workouts as well as warfare, discretion is the better part of valour (aka pick the battles you are going to win).

2.9     Seeing What Happens to the Amount of Oxygen You Consume

Interval training sessions are very demanding, but they are an excellent method for improving an individual’s cardiovascular conditioning. Figure 2 illustrates what occurs during an interval training session.

Figure 2: The increase in oxygen consumption (VO2) during an interval training session

  • During the first exercise interval, the volume of oxygen you consume (VO2) initially rises rapidly and begins to plateau toward the end of the exercise interval.
  • During the recovery interval, VO2 decreases rapidly, at first, because the individual has stopped running hard and, therefore, quickly decreased demand for oxygen.
  • If the recovery interval is short and active (equal to or less than the time spent running, a 1:≤1 WR Ratio), VO2 will not decrease all the way back down to what it was when the session began.
  • That is exactly what you want to happen because the next exercise interval then begins with your VO2 elevated.
  • VO2 then rises again during the subsequent exercise interval, to a point higher than during the first exercise interval.

If planned right, VO2 reaches VO2max after a couple of exercise intervals, which is the goal of the workout. These workouts are difficult because your muscles are consuming oxygen as fast as they can and also relying on some anaerobic metabolism (to produce energy without oxygen) so you can hold the fast pace. Of course this is just an illustration, a beginner exerciser, for example, will generally start and progress at a lower intensity, although the principle of a rising and falling VO2max applies.

2.10     Ventilatory Threshold

Exercise intensity is recognised as the most important variable factor of exercise programming to optimise maximal oxygen uptake. Echoing this is a slow, but inexorable trend, to move away from ‘antiquated’ models of percentage of maximal heart rate (% MHR) and percentage heart rate reserve (% HRR) to the concept of ventilatory threshold (VT) (ACE, 2011).

Research demonstrates that HIIT builds ventilatory power (the speed and magnitude with which individuals move air into and out of the lungs). In contrast, low-intensity exercise builds ventilatory endurance (the capacity of the endurance muscles to sustain work and resist fatigue). Porcari, Bryan and Comana (2015, p.388) suggest that “Both are critical to ventilation and the delivery of adequate levels of oxygen to the muscle cells.”

The American Council on Exercise Integrated Fitness Training (ACE IFT™) Model was one of the first cardio training models to adopt the VT concept, stating that research demonstrates how certain markers reflect significant events occurring within the energy pathways and with fuel utilisation during exercise. ACE (2011) describes the two VT categories:

  • “VT1 represents the transition in one’s primary fuel from fats to carbohydrates, representing the onset of our loss in aerobic efficiency (caloric quality) and a noticeable increase in blood lactate levels.
  • VT2 represents the point where carbohydrates contribute exclusively to energy production via the aerobic and fast glycolytic pathways, and where we begin to overwhelm our capacity to tolerate lactate spillover into the blood (scientifically defined as OBLA, although most refer to it as lactate threshold).”

Onset of blood lactate accumulation or OBLA is the point during exercise at which lactic acid builds up in the blood and fatigue sets in. The point when an individual “hits a wall,” so to speak. It is also known as the lactate threshold or anaerobic threshold.

ACE (2011) suggests that while many cardio programmes focus on maximising caloric quantity (i.e. burn more calories per unit of time), they should also consider building caloric quantity (i.e. greater fat utilisation per unit of time and faster recoveries) – combining both “implies training your body to continue to burn fat into higher intensities of exercise.” (ACE, 2011).

For an example of an ACE 2-zone training model to improve VT1 look here.

2.11     Frequency of Interval Training

The exact frequency with which an individual performs interval training will be person- and sport-specific. For example some track athletes may undertake one or two interval training sessions each day of training, whilst recreational sprinters may only perform one or two per week.

Professional athletes will follow a specific, tailored and seasonal training programme which will most likely incorporate interval training as an integral component. Training intensity will fluctuate depending on factors such as upcoming races, altitude, season (winter versus summer) and so on.

Most recreational exercisers will most likely wish to limit their interval training sessions to between one and three per week. However, they may increase the frequency for an upcoming race or event.

When discussing obesity and weight management Porcari, Bryant and Comana (2015, p.676) state “Include some cardiorespiratory workouts that are of higher intensity for a shorter period. This may best be realized with high-intensity, continuous training or with interval training. To avoid physiological and orthopedic stress and injury, complete only one higher-intensity workout per week.” (Porcari, Bryant & Comana, 2015, p.676).

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