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


4.0     Introduction

“The debate concerning high- versus low-intensity exercise continues, and although a significant amount of more recent research has focused on the aerobic benefits of high-intensity interval training (HIIT), experts cannot reach any single conclusion regarding its efficacy, because it all depends on the perspective from which they approach the argument.” (Porcari, Bryan & Comana, 2015, p.388).

In the third edition of their book, McArdle and Colleagues (2006, p.457) informed us that “Available evidence does not support superiority for either continuous or intermittent training to improve aerobic fitness. Both methods probably can be applied interchangeably.” This view is repeated in the fourth edition of their book (Katch, McArdle & Katch, 2011, p.435) “No one method has proved superior for either continuous or interval training to improve aerobic fitness. Both methods probably can be applied interchangeably.”

However, Katch and colleagues (2011, 435) do go on to state “Importantly, continuous LSD training gives the endurance athlete a more “task-specific” cardiovascular and metabolic overload that more closely mimics the duration and intensity of race conditions. Likewise, sprint and middle-distance athletes benefit from the intense metabolic demands and specific neuromuscular and fiber-type activation that interval training provides.”

In 1985, Robinson and colleagues suggested that ten days of either HIIT or MICT could improve cardiorespiratory fitness and glucose control and lead to reductions in TLR2 and TLR4 expression (markers of inflammation) in previously inactive, overweight/obese adults at elevated risk of developing type 2 diabetes. Robinson et al. (1985) also stated that MICT, which involved a longer duration of exercise, may be superior for reducing fasting glucose. In a study by Martins et al. (2016) involving 46 sedentary obese individuals (30 women) undertaking 12 weeks of isocaloric (having the same or a similar calorific value) programmes of HIIT, MICT or a short-duration HITT, their results indicated that the isocaloric training protocols exerted similar metabolic and cardiovascular improvements in sedentary obese individuals (no significant differences were observed between groups).

Thompson (2010) states “It is now recognised that a longer lasting fitness can be achieved if the original interval training is combined with sufficient aerobic endurance development to stabilise the improved cardio-respiratory response.”

Although there is now a larger body of research comparing HIIT with MICT, the methodology employed by researchers varies. For example, bikes versus running, varying duration of training, exercise intensities, exercise intervals and recovery intervals, and so on. This makes it somewhat problematic to accurately compare these studies. By the late 2010s, researchers are still reporting that none of the observed changes significantly differ between the training groups (Eskelinen et al., 2016; Heiskanen et al., 2016).

Kenney, Willmore and Costill (20012, p.337) outline the differences between high-intensity, low volume training and low-intensity, high volume training, stating:

“High-intensity, low-volume training can be tolerated only for brief periods. While this type of training does increase muscular strength in resistance training and total body speed and anaerobic capacity in high-intensity interval training, it provides little or no improvement in aerobic capacity. Conversely, low-intensity, high-volume training stresses the oxygen transport and oxidative metabolic systems, causing greater gains in aerobic capacity, but has little or no effect on muscular strength, anaerobic capacity, or total body speed.”

As well as training being person- and sport-specific, one must also note that a programme of regular exercise should include four elements (Garber et al., 2011):

  • Cardiorespiratory training (think aerobic and anaerobic, and power and endurance);
  • Resistance training (generally for strength);
  • Flexibility training; and
  • Neuromotor exercise training (i.e. functional training).

Porcari, Bryant and Comana (2015, p.391) also suggest “It may be particularly important to include multiple modalities of exercise (e.g., walking, cycling, and elliptical training) and even variations within a modality (e.g., steady-state exercise, interval training, and Fartlek training) to limit the risk for boredom, burnout or orthopedic injury from overuse as the volume of exercise builds.”

Keeping this in mind, this section of the article provides some of the perceived/actual advantages and disadvantages of interval training.

4.1     Advantages of Interval Training

Research suggests that interval training has a variety of benefits or advantages, and across multiple populations from athletes to the elderly. The following is a list of some of the perceived/actual advantages of interval training:

  • Individuals of mixed ability groups can work at the same time;
  • Generally, small numbers work at the same time;
  • Can be adapted for most sports; and
  • Can help to improve:
    • Cardiorespiratory fitness (aerobic and anaerobic).
    • Mental determination; and
    • Resistance to fatigue.
  • Interval training is a time-efficient way of obtaining the health benefits of exercise. It usually takes less time than a comparable MICT session.
  • “The advantage of the original interval training was that it brought about very rapid and significant improvements in performance.” (Thompson, 2010).
  • “HIIT builds ventilator power.” (Porcari, Bryan & Comana, 2015, p.388).
  • “Certainly, some research demonstrates how HIIT increases levels of free fatty acids (FFA) in the blood because of greater levels of circulating epinephrine, which is supposed to drive greater aerobic metabolism in the cells.” (Porcari, Bryan & Comana, 2015, p.388).
  • Research also suggests that high-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise (Bartlett et al., 2011). Despite a higher intensity and peak cardiorespiratory strain, participants generally prefer interval training versus moderate exercise (Astorino & Thum, 2016).
  • “Research has revealed that HIIT results in similar improvements in VO2max and mitochondrial density as bouts of lower-intensity exercise (LIE), but which form of exercise is more appropriate for a sprint athlete versus the overweight business executive simply seeking to improve health?” (Porcari, Bryan & Comana, 2015, p.388).
  • The shorter the distance of the run, the faster you can run the total distance of the workout.
  • One of the attractions of interval training is its measured, precise nature. Workouts can be tailored to an individual’s current ability level; similarly, they provide an accurate benchmark of one’s fitness, allowing achievable personal/competitive goals to be set.
  • Interval training’s repeatability facilitates comparisons to past and present performances.
  • Interval training possesses an almost infinite variety. By altering different segments of the workout, it is possible to come up with a new training session each time.
  • Interval training gives an individual a chance to challenge themselves. To put it bluntly, interval training is physically demanding, but there is something intriguing about that physical discomfort, about what it allows the individual to learn about themselves.
  • “Part of their popularity lies with the efficiency with which they can improve cardiovascular fitness and various other physiological parameters.” (Porcari, Bryant & Comana, 2015, p.92).
  • “Emerging research suggests HIIT may be a time-efficient strategy for improving the health of all populations if properly supervised.” (Porcari, Bryant & Comana, 2015, p.92).
  • “High intensity deep water training can improve aerobic power in elderly women.” (Quintana et al., 2006, p.117).
  • “[High-intensity] Deep water running with wet vest is a safe form of exercise for elderly with mobility limitations.” (Quintana et al., 2006, p.117).
  • “In conclusion, high intensity deep water running with vest improves submaximal work capacity, maximal aerobic power, and maximal ventilation with the effects transferable to land-based activities in elderly women.” (Quintana et al., 2006, p.123).
  • “A series of recent studies from McMaster University in Hamilton, Ontario (Canada), has clearly demonstrated that very high intensity, low-volume interval training can markedly increase aerobic capacity. Substantial increases in muscle oxidative capacity and endurance performance have been obtained in a training period as short as two weeks [Gibala & McGee, 2008]. These studies tend to seriously question the concept of specificity of training as discussed in chapter 11.” (Kenney, Wilmore & Costill, 2012, p.510).
  • “The “fat burning zone” at low intensities of exercise does NOT exist. The best approach is to think of energy expenditure as “a calorie is a calorie is a calorie,” rather than partitioning into carbohydrate and fat calories. To burn maximum calories in support of ongoing fat/weight loss, progress to a moderate-to-vigorous intensity/higher-volume exercise program and include interval training.” (Porcari, Bryant & Comana, 2015, p.111).
  • “Although there are not a lot of well-controlled studies, it appears that interval training with relatively brief (30-second) high-intensity elements is just as effective in terms of producing gains in anaerobic capacity as longer high-intensity bursts (where phosphagen depletion and lactate accumulation might be larger, and thus be expected to be more provocative of change).” (Porcari, Bryant & Comana, 2015, p.405).
  • Despite a higher intensity and peak cardiorespiratory strain, research suggests that participants prefer interval training versus moderate exercise.
  • In a group of nine adults with spinal cord injury (duration = 6.8 ± 6.2 year), peak oxygen uptake and heart rate were higher (p < 0.05) with HIIT (90% peak oxygen uptake and 99% peak heart rate) and sprint interval training (80% peak oxygen uptake and 96% peak heart rate) versus moderate intensity exercise (Astorino & Thum, 2016).
  • In a study of thirty-nine inactive, overweight/obese adults (32 women), biomarkers of cardiovascular risk and endothelial function were unchanged. However, HIIT and moderate intensity continuous training (MICT) produced different vascular adaptations in obese adults, with HIIT improving brachial artery flow-mediated dilation (FMD) and MICT increasing resting artery diameter and enhancing low flow-mediated constriction (L-FMC). HIIT required 27.5% less total exercise time and ∼25% less energy expenditure than MICT. (Sawyer et al., 1985).

Karlsen and colleagues (2017) argue that a growing body of evidence suggests that higher exercise intensities may be superior to moderate intensity for maximising health outcomes. In their review, Karlsen et al. (2017) discuss how aerobic high-intensity interval training (HIIT), in comparison to moderate continuous training, may maximise outcomes. They also provide practical advice for successful clinical and home-based HIIT.

4.2     Disadvantages of Interval Training

Research suggests that interval training has a variety of negatives or disadvantages. The following is a list of some of the perceived/actual disadvantages of interval training:

  • Can get boring (e.g. if continually using the same session).
  • Although interval training develops stamina, it can fail to condition the mind to racing hard over a continuous/prolonged racing period.
  • “The disadvantages were that it could be incredibly monotonous and the rapid improvements in fitness were matched by an almost as quick loss of fitness on cessation of training.” (Thompson, 2010).
  • According to Eskelinen and colleagues (2016), HIIT and MICT induce similar metabolic and functional changes in the heart, although myocardial vascular hyperaemic reactivity is impaired after HIIT. They further suggest this should be taken into account when prescribing very intense HIIT for previously untrained subjects.
    • Reactive hyperaemia or post- ischemic reactive hyperaemia is the increased blood flow to an organ or a tissue following a temporary blockage of an artery. The affected tissue can cause hyperaemia in one of two ways:
      • Through a response with release of vasodilatory substances such as adenosine, carbon dioxide, adenosine phosphate compounds, histamine, potassium ions and hydrogen ions.
      • Hyperaemia could be caused by a lack of oxygen which could affect opening of more pre-capillary sphincters in the affected tissue, thereby increasing blood flow.
  • “But research also demonstrates that the increased levels of blood lactate that follow HIIT begin to inhibit hormone-sensitive lipase activity, the enzyme responsible for mobilising fats. Furthermore, increased lactate levels act as a precursor for glycerol 3-phosphate, promoting the re-esterification of those FFAs into triglycerides within adipocytes (fat cells) if the FFAs are not taken into the muscle cells, but instead remain in the blood.” (Porcari, Bryan & Comana, 2015, p.388).
  • “The influx of this form of training with the mainstream and even special populations groups is raising concerns regarding overall safety and appropriateness.” (Porcari, Bryant & Comana, 2015, p.92).
  • “So, yes, intervals worked. But there was a catch. Runners often improved their times dramatically for a few weeks or months, but then tore Achilles tendons, or [p.90] one weird day woke up with bones that seemed to have aged forty years in the night and devastated by the conviction that running was infantile and meaningless. Such runners had gone (and the word is too feeble) stale.” (Moore, 2006, p.89-90).
  • “Runners who overdose on intervals can often need months before their systems are once more able to handle the lactic acid stress of repeatedly going in and out of oxygen debt. Then, six weeks later, they can be devastated all over again.” (Moore, 2006, p.89-90).
  • If the training overload is too much or improperly applied, for example the pace of the exercise intervals is too great, then maladaptation may occur. The first step toward maladaptation may be overreaching, a short-term decrease in performance capacity that is easily recovered from and generally lasts from a few days to two weeks. Overreaching may result from planned shock micro-cycles, as described in periodisation, or result inadvertently from too much stress and too little planned recovery (Fry et al., 1991; Fry & Kraemer, 1997; Kuipers, 1998).

4.3     Points to Consider When Planning an Interval Session

“It is now recognised that a longer lasting fitness can be achieved if the original interval training is combined with sufficient aerobic endurance development to stabilise the improved cardio-respiratory response.” (Thompson, 2010).

There are a number of practical points to consider when planning an interval training session (not an exhaustive list):

  • Determine Objectives:
    • The designer must consider specific parts of the body and the components of fitness on which soldiers/participants need to concentrate.
    • How does this interval session fit with the mission objectives of the soldiers or training programme for participants/athletes?
  • Select the Activities:
    • The interval designer should list all the exercises or activities that can help meet the objectives.
    • Which method of interval training is to be used and why?
    • Is the exerciser a runner, swimmer, cyclis, gym-based etc.
    • If gym-based, what exercises will you incorporate and why? (e.g. example, lower-body, upper-body or combination).
  • When do you require the interval session for;
  • What are the time/distance of the exercise intervals and why?
  • What are the time/distance of the work-to-rest or exercise-to-relief ratios and why?
  • Are you using an active or passive recovery and why?
  • Amount of time allocated;
  • Numbers of individuals to be trained;
  • What is the venue/area like? (e.g. straight track, 400m oval track, grass area, swimming pool (25m versus 50m), etc).
  • Number of repetitions and/or sets to be completed;
  • The group’s standard of fitness (e.g. trainee soldiers versus trained soldiers, untrained participants versus professional athletes or mixed-ability group);
  • Space and equipment available (e.g. indoors/outdoors, stopwatch or HR monitor).
  • Are there safety factors to consider?

4.4     Points to Consider When Conducting an Interval Session

There are a number of practical points to consider when conducting an interval training session (not an exhaustive list and delete as applicable):

  • The route is clearly marked and easy to follow (if not using a running track/swimming pool) or the exercises are easy to follow if gym-based.
  • If gym-based, the exercises are laid out in the correct order.
  • Avoid exercises that demand (too much) skill, especially in HIIT, as this will slow down the pace.
  • Teach exercises to particiapants on the first visit.
  • Gain benchmark indicators on first visit (e.g. heart rate).
  • Ensure participants have a ‘good’ baseline fitness (e.g. through PAR-Q).

4.5     Points to Consider After Interval Training

Once the interval training session has been completed the following points should be considered (not an exhaustive list):

  • Record times/distances for each participant.
  • Adjust each participant’s standards and targets as necessary (e.g. exercise intervals, recovery intervals, repetitions and sets).
  • Discuss the effect of DOMS and other running related injuries (especially applicable for untrained participants).

4.6     Safety Factors

“The influx of this form of training with the mainstream and even special populations groups is raising concerns regarding overall safety and appropriateness.” (Porcari, Bryant & Comana, 2015, p.92).

While injury is always possible in any vigorous physical activity, few interval workouts are really unsafe or dangerous. The keys to avoiding injury while gaining training benefits are using correct form, intensity and progression.

Further, participants with low fitness levels, such as trainee soldiers or new clients, should not do the higher-intensity forms of interval training highly fit soldiers/athletes can do. For example, it may be advisable for participants new to interval training to undertake a modified version first (e.g. low volume, moderate-intensity interval training) progressing to low volume, high-intensity and, perhaps later on, then to moderate-high volume, high intensity interval training.

It is not sensible to have recruits/new clients do multiple sets of exercise interval repetitions because they probably are not conditioned for them. On the other hand, a conditioned Royal Marine Commando company or elite athletes may use multiple sets of repetitons with good results.

The risk of injury to untrained participants will be increased due to their unconditioned status. For example, lower limb injuries due to the stress of fast running.

The key to doing gym-based interval training exercises safely is to use common sense (and interval training in general). Also, ballistic (that is, quick-moving) exercises that combine rotation and bending of the spine increase the risk of back injury and should be avoided, especially in HIIT workouts where form may deteriorate due to the pace of the workout. This is especially true if someone has had a previous injury to the back. If this type of action is performed, slow stretching exercises, not conditioning drills should be used.

HIIT-type workouts are beneficial when the participant is fit and conducts these workouts in a regular, progressive manner. However, a certain level of fitness is needed to do them safely. Therefore, participants should train progressively to build up to these workouts. Using such workouts for unconditioned participants increases the risk of injury and accident.

When a participant fails to maintain proper form/speed during interval training, they should slow down to regain proper form. Typically, inexperienced participants will perform the first one or two repetitions too qucikly. When this happens, it causes form to break down and affects the ability to maintain speed for the specified number of repetitions. Participants should be instructed to pay attention to their speed in order to maintain precision.

Particular areas for concern include:

  • Progression: This is the systematic increase in the intensity or duration of exercise activities. Proper progression allows the body to positively adapt to the stresses of training. When intensity or duration is increased too rapidly, the participant cannot adapt to the demands of training, and is unable to recover, leading to overtraining and possible injury. The variables should be gradually increased to produce the desired physiological effect:
    • Intensity (resistance and pace);
    • Exercise volume (number of sets and repetitions); and
    • Duration (time).
  • Adhering to the scheduled intensity and duration prevents the participant from progressing too fast. How fast the participant should progress also depends on how regularly they perform challenging activities and how much rest and recovery time they get.
  • Overtraining: This occurs when training involves excessive frequency, intensity and/or duration of training that may result in extreme fatigue, illness or injury. This may occur within a short period of time (days) or cumulatively (weeks/months) over the length of the training cycle and beyond. Overtraining often results from a lack of adequate recovery, rest or in some cases, a lack of nutrient intake. Thus, too much training, too little recovery, and/or poor nutrient intake (i.e. fuelling) may elicit both the physical and psychological symptoms associated with overtraining syndrome.
  • Overreaching: The term ‘overreaching’ refers to the earliest phase of overtraining. Overreaching consists of extreme muscle soreness that occurs as a result of excessive training with inadequate rest/recovery between hard training sessions. This process of overreaching occurs quickly after several consecutive days of hard training. Overreaching has both positive and negative results. When planned as part of the periodised training programme, for soldiers and athletes, overreaching is a planned component of their training for peak performance. Their higher fitness levels allows for a tolerance to this more intense training with proper rest/recovery and nutrient intake. Short-term overreaching followed by an appropriate tapering period can elicit significant strength and power gains. Muscle soreness and general fatigue are normal outcomes following a series of intense workouts; however, if these outcomes are never completely resolved and performance continues to decline, these may be the first indicators of overtraining syndrome. Coaches/trainers need to be able to recognise these symptoms, especially in the early stages of a training programme and need to adjust training and recovery for these participants.
  • Continued overreaching will lead to overtraining and elicit negative results. In many instances, participant that experience a degradation of performance (a loss of strength or speed) feel the need to train even harder. Contrary to their belief, pushing harder not only decreases the chance of improved performance, but increases the risk of injury. Recovery, rest, and proper nutrient intake will elicit more improvement than training harder. When the volume and intensity of exercise exceeds participants’ capacity to recover, they cease making progress and may even lose strength and endurance. Overtraining is a common problem in both resistance training and running activities. Improvements in strength and endurance occur only during the rest period following hard training. This process, referred to as supercompensation, takes 12 to 24 hours for the body to completely rebuild. If sufficient rest is not available, then complete recovery cannot occur. Overreaching as a training practice is not appropriate, nor is it recommended for beginners/trainee soldiers, especially for those who have low fitness levels, high foot time, and high training operational tempo. Overreaching may lead to overtraining syndrome and overuse injuries when hard training continues beyond a reasonable period of time.
  • Continued overreaching without adequate rest/recovery and nutrient intake leads to overtraining and eventually overuse injuries. The effects of overtraining syndrome may last weeks or months, inhibiting performance and possibly causing acute or chronic injuries that may limit or end training. Specific examples include rhabdomyolysis, pubic ramus stress fractures, compartment syndrome and femoral neck stress fractures.
  • Multiple training sessions per day have both positive and negative effects as they relate to performance improvement and injury control. Highly conditioned participants may respond well to an additional daily training session that challenges them differently than the one conducted earlier that same day. For example, speed training may be conducted during the morning session, with the use of endurance training machines (ETM) and agility exercises in the afternoon. Participants with lower fitness levels, such as those entering basic training or beginners, those recovering from injury, and soldiers returning from extended deployment, are better served with a second training session of lower intensity that targets specific needs for improvement, but does not lead to overtraining. Coaches/trainers should understand that “more is not better” and additional recovery time (rest) may elicit higher performance than the conduct of additional sessions.
  • Punitive Sessions: Soldiers commonly refer to these training sessions as ‘smoke sessions’ in the US and as ‘beastings’ in the UK. Many times in these types of sessions, the difficulty, intensity, and volume of exercise is too high and the purpose may be to punish soldiers by bringing them to the point of exhaustion. This type of training is a dangerous practice that inhibits building resiliency because performance is degraded, motivation is lowered, and risk of injury is high. Thus, training sessions for the sole purpose of smoking/beasting soldiers should have no place in a formal, structured training programme. Many times, in the past, these types of sessions have produced life-threatening conditions for soldiers such as heat fatalities, debilitating overuse injuries, and rhabdomyolysis and, in some cases, even led to permanent disability or death.

Continue reading Part 05