What happens in the body during interval training?

Your heart rate rises quickly, breathing becomes heavy, and the muscles begin to burn. After a short recovery period, the body feels partly ready again before the next work interval begins. Interval training alternates between periods of higher intensity and periods of lower intensity or rest.

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This structure makes it possible to accumulate more time at a high intensity than many people can sustain during continuous exercise. The heart, lungs, blood vessels, muscles, and nervous system are challenged simultaneously, but the physiological response depends on how hard and how long the intervals are, how much recovery you receive, and which activity you perform.

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Interval training is not one specific type of exercise

The term interval training mainly describes how the session is organised. Work and recovery periods alternate according to a planned structure, but the intensity and duration can vary considerably.

An interval session may include:

• short sprints lasting 10–30 seconds

• work periods lasting one to four minutes

• longer intervals lasting five to ten minutes

• active recovery with easy movement

• complete rest between efforts

• running, cycling, rowing, or swimming

• strength exercises arranged as circuit training

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Two sessions that are both described as interval training can therefore create very different demands. Ten 20-second sprints with long rest periods challenge the body differently from four four-minute running intervals with moderate recovery.

It is more precise to evaluate an interval session according to:

• the duration of each work period

• the intensity during the work period

• the duration of the recovery

• whether recovery is active or passive

• the number of repetitions

• total work duration

• the chosen activity

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Energy demand rises within seconds

When the work interval begins, the muscles rapidly require more energy. The immediate energy source for muscle contraction is adenosine triphosphate, abbreviated ATP.

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The muscles store only small amounts of ATP. It must therefore be continuously regenerated through several energy systems.

The three main systems are:

Energy system	Main role	Particularly important during
Phosphagen system	Provides very rapid energy from phosphocreatine	Short and explosive efforts
Anaerobic glycolysis	Breaks down glucose rapidly without direct oxygen use	Hard work lasting tens of seconds
Aerobic metabolism	Uses mainly carbohydrate and fat with oxygen	Longer work periods and recovery

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All three systems work simultaneously. The difference lies in how much each system contributes during different parts of the interval.

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During the first few seconds, stored ATP and phosphocreatine contribute substantially. If the effort continues, carbohydrate breakdown becomes increasingly important. At the same time, aerobic energy production rises as oxygen delivery increases.

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Oxygen uptake takes time to increase

When intensity suddenly rises, oxygen uptake cannot immediately reach the required level. The heart must beat faster, blood flow must be redistributed, and the muscle cells must increase aerobic energy production.

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At the beginning of the interval, a temporary gap therefore develops between energy demand and aerobic energy production. This is often described as an oxygen deficit.

The gap is covered partly through:

• phosphocreatine

• anaerobic breakdown of glucose

• oxygen already stored in the blood and muscle tissue

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During repeated intervals, oxygen uptake often begins from a higher level than during the first repetition. The body does not return fully to rest during the recovery period, and aerobic metabolism is activated more quickly when the next interval starts.

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This is one of the main reasons interval training can accumulate substantial time at a high oxygen uptake without each interval needing to be extremely long.

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The heart must deliver more blood to the muscles

Working muscles require more oxygen and nutrients, while carbon dioxide, heat, and other metabolic by-products must be transported away.

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Cardiac output describes how much blood the heart pumps each minute. It is determined by:

• heart rate

• the amount of blood pumped with each beat

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During interval exercise, both usually increase. The heart beats faster, while stroke volume rises until it reaches a level influenced by intensity, training status, body position, and the activity being performed.

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Blood flow is also redistributed. More blood is directed towards the active muscles, the heart, and the skin, while less blood is prioritised for systems that are less essential during the immediate task.

The result is:

• higher heart rate

• increased blood pressure during the work intervals

• greater blood flow to the muscles

• increased oxygen transport

• increased heat loss

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During recovery, the load decreases, but heart rate may not return to baseline. With short recoveries, the next interval therefore begins with the cardiovascular system already activated.

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Breathing increases for several reasons

Ventilation rises quickly when the interval begins. You breathe both deeper and faster to deliver more oxygen and remove more carbon dioxide.

Breathing regulation is influenced by:

• signals from the brain when movement begins

• nerve signals from muscles and joints

• increased carbon dioxide production

• changes in acidity

• body temperature

• stress hormones

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At moderate intensity, ventilation rises relatively steadily. At high intensity, breathing often increases disproportionately.

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This occurs partly because the body must remove additional carbon dioxide produced when bicarbonate helps buffer increasing acidity.

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Heavy breathing during intense intervals therefore does not mean only that the body lacks oxygen. It is also part of the regulation of carbon dioxide and acid-base balance.

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Lactate is not merely a waste product

During hard exercise, glucose breakdown increases. Pyruvate is formed during this process, and a larger proportion is converted to lactate when the rate of energy production becomes high.

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Lactate is often blamed for all fatigue and post-exercise soreness. This is an oversimplification.

Lactate can:

• be transported to other muscle fibres

• be used as an energy source

• be delivered to the heart

• be processed further in the liver

• help transfer carbon and energy between tissues

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A high lactate level indicates that glycolysis is operating rapidly, but lactate is not the main reason the muscle eventually loses force.

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Fatigue results from several simultaneous changes in the muscles and nervous system.

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Soreness the following day is also not caused by lactate remaining in the muscles. Lactate is removed or used relatively quickly after exercise, while delayed-onset muscle soreness is related to other processes.

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Why do the muscles begin to burn?

The burning sensation during hard intervals occurs when local muscular stress becomes high and the chemical environment within the muscle changes.

Several processes may contribute:

• accumulation of hydrogen ions

• changes in inorganic phosphate

• disruption of calcium handling

• changes in ion balance

• activation of pain-sensitive nerve endings

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These changes can reduce the muscle fibres’ ability to produce force and contribute to the sensation of burning and fatigue.

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Lactate is produced at the same time and is therefore often used as a marker of high glycolytic activity. This does not mean that lactate alone creates the burning sensation.

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The sensation usually decreases quickly when intensity is reduced. It is not in itself a sign that the muscle is being damaged.

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Recovery is an active part of the training

The recovery period between intervals is not simply waiting time. During recovery, the body attempts to restore the conditions required for the next work period.

Several processes occur:

• partial restoration of phosphocreatine

• reduction in heart rate and ventilation

• transport and use of lactate

• restoration of ion balance

• reduction in metabolic stress

• mental preparation for the next interval

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How complete the recovery becomes depends on the length of the recovery period.

Short recoveries:

• keep heart rate elevated

• maintain high oxygen uptake

• increase accumulated fatigue

• reduce performance during the next interval

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Longer recoveries:

• allow greater restoration of phosphocreatine

• make it possible to maintain higher speed or power

• reduce acute fatigue accumulation

• are more suitable for very explosive intervals

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There is therefore no universally correct recovery duration. The recovery must match the goal of the session.

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Active and passive recovery have different effects

During active recovery, you continue moving at a low intensity. During passive recovery, you stand, sit, or lie still.

Active recovery may:

• maintain blood flow

• support faster lactate use

• keep oxygen uptake elevated

• make the transition to the next interval less abrupt

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Passive recovery may:

• allow greater phosphocreatine restoration

• reduce energy expenditure between intervals

• make it easier to maintain maximal intensity

• be more suitable for sprint intervals

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Intervals aimed at a high oxygen uptake often use easy movement during recovery. Maximal sprint or explosive work more often benefits from more complete rest.

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Short and long intervals challenge the body differently

Short intervals allow a high speed or power output. Each repetition may be very intense, but the short duration limits how long the load is sustained.

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Longer intervals require a lower intensity, but provide more continuous time at a high oxygen uptake.

Interval type	Typical duration	Common primary demand
Sprint intervals	5–30 seconds	Explosiveness and anaerobic capacity
Short intervals	30–90 seconds	High glycolytic and aerobic stress
Medium-length intervals	2–5 minutes	High oxygen uptake
Long intervals	5–15 minutes	Endurance near threshold intensity

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The categories overlap. Intensity, recovery, and number of repetitions determine the actual physiological effect of the session.

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An interval session does not need to be maximal to be effective. Many sessions are best performed at an intensity that can be repeated with control throughout the whole workout.

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VO₂ max can improve

Maximal oxygen uptake, usually written as VO₂ max, describes the highest rate at which the body can take in and use oxygen during hard exercise.

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Interval training can improve VO₂ max through adaptations in both central and peripheral systems.

Central adaptations may include:

• increased stroke volume

• greater maximal cardiac output

• increased blood volume

• improved oxygen transport

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Peripheral adaptations may include:

• more capillaries around the muscle fibres

• larger and more effective mitochondria

• increased activity of aerobic enzymes

• improved oxygen extraction from the blood

• more efficient use of energy substrates

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Which adaptations dominate depends on training status and how the intervals are structured.

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Untrained people may improve quickly. Highly trained athletes usually require more precise control of intensity, total work duration, and recovery to continue progressing.

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The mitochondria adapt to the stress

Mitochondria are structures within muscle cells that produce much of the energy used during aerobic exercise.

Interval training can stimulate signalling pathways that increase:

• the number of mitochondria

• mitochondrial size

• activity of aerobic enzymes

• the ability to use oxygen

• the capacity to use carbohydrate and fat

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High intensity provides a strong signal because energy demand changes rapidly and the muscle cells experience substantial metabolic stress.

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This does not mean that interval training always produces better mitochondrial adaptation than easy endurance training.

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Low-intensity endurance exercise can be performed for much longer and may therefore create a large total training stimulus.

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The two forms of training complement each other. Interval training provides high intensity, while easy training makes it possible to accumulate more volume with lower stress.

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Blood vessels improve their regulation of blood flow

When the muscles work, blood vessels must widen to increase blood supply. Repeated interval training can improve vascular function and the ability to direct blood towards the tissues with the greatest demand.

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The endothelium lining the blood vessels responds to increased blood flow and shear stress. This can stimulate the production of nitric oxide, which helps the blood vessels dilate.

Over time, interval training may contribute to:

• improved endothelial function

• lower resting blood pressure in some people

• better regulation of blood flow

• improved oxygen delivery to the muscles

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The response is influenced by training status, age, disease, and total exercise volume.

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People with cardiovascular disease may benefit from appropriately adapted interval training, but intensity and supervision should be individually assessed.

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Insulin sensitivity can improve

Working muscles require glucose. Muscle contractions activate mechanisms that help transport glucose into the cells, partly independently of insulin.

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After exercise, muscle tissue may become more sensitive to insulin. This makes it easier to move glucose from the blood into the muscle cells.

Interval training may therefore contribute to:

• improved blood glucose regulation

• greater insulin sensitivity

• increased muscle glycogen storage

• lower post-meal blood glucose

• improved metabolic health

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The effect is particularly relevant for people with insulin resistance or type 2 diabetes.

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Training must still be adapted to medication, blood glucose response, complications, and physical capacity.

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People using insulin or medication that may cause hypoglycaemia should have a plan for monitoring, food intake, and possible dose adjustment in cooperation with healthcare professionals.

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Interval training also affects the nervous system

During high-intensity exercise, the brain and spinal cord must recruit motor units rapidly. The movement requires precise coordination while fatigue increases.

The nervous system must regulate:

• muscle activation

• movement technique

• force production

• breathing

• balance

• perceived exertion

• willingness to continue

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Neuromuscular quality is particularly important during sprint intervals. If recovery periods are too short, speed and technique decline quickly. The session then becomes more endurance-oriented and less specific to maximal sprint performance.

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During longer intervals, the challenge is more about maintaining a steady effort despite increasing discomfort.

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The brain evaluates how much effort is sustainable

Perceived exertion is created from information coming from both the body and the brain. Breathing, heart rate, muscle stress, temperature, pain, motivation, and expectations all contribute.

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If you know the work interval is almost finished, a high intensity may feel more manageable. The same speed may feel harder if you do not know how long the interval will last.

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Interval training may make high intensity easier to tolerate because the work is divided into clearly defined periods. Recovery provides both physiological and mental relief.

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Music, a training partner, and clear time targets may influence perceived exertion. These factors do not necessarily change the physiological load, but they may make the desired effort easier to maintain.

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Post-exercise oxygen consumption increases, but should not be exaggerated

After a hard interval session, oxygen uptake remains elevated for a period. This is called excess post-exercise oxygen consumption, abbreviated EPOC.

The additional energy is used partly to:

• restore energy stores

• normalise body temperature

• regulate hormones

• restore oxygen stores

• process metabolic by-products

• support recovery and repair

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Harder and longer sessions generally create a greater EPOC than short and easy sessions.

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The effect is often described as the afterburn effect, but the total amount of additional energy is usually more moderate than marketing claims suggest.

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Most of the energy expenditure still comes from the session itself and total daily activity.

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Interval training should therefore not be selected only to maximise calorie expenditure after exercise.

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Why can intervals cause nausea or dizziness?

Hard interval sessions can cause nausea, dizziness, or discomfort in some people.

Possible causes include:

• very high sympathetic activation

• reduced blood flow to the digestive system

• a large meal close to the session

• dehydration

• heat

• stopping suddenly after high intensity

• overbreathing

• unfamiliar exertion

• low energy intake

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Nausea does not mean that the session was more effective. If it happens frequently, intensity, meal timing, fluid intake, and recovery duration should be adjusted.

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A gradual cool-down may reduce dizziness. When intense exercise stops abruptly, the contribution of the muscle pump to venous return falls quickly. Easy walking or cycling can make the transition more controlled.

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How often should you perform interval training?

The appropriate frequency depends on training level, goals, session intensity, and the rest of the training week.

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For many recreational exercisers, one or two interval sessions per week may be sufficient. Beginners may start with one controlled session and use easier activity on the other days.

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More hard sessions are not necessarily better. Excessive high-intensity training may contribute to:

• persistent fatigue

• declining performance

• sleep problems

• heavy legs

• reduced motivation

• increased injury risk

• lower session quality

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Well-trained endurance athletes often perform most of their training at low intensity, even though interval sessions remain important for performance.

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Interval training is therefore a supplement to easier exercise rather than a replacement for all other endurance training.

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How to structure a controlled interval session

A simple interval session may include:

Warm-up

Ten to fifteen minutes with gradually increasing intensity. A few short accelerations may be added.

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Work intervals

Four work periods lasting three to four minutes at an intensity that feels hard but controlled.

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Recovery

Two to three minutes of easy activity between the intervals.

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Cool-down

Five to ten minutes with gradually decreasing intensity.

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The intensity should allow the final interval to be completed with roughly the same quality as the first. If speed falls sharply early in the session, the opening intensity was probably too high.

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A beginner may instead start with shorter intervals, such as:

• one minute of brisk walking or easy running

• one to two minutes of easy activity

• six to ten repetitions

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The activity can be adapted using a bicycle, elliptical trainer, rowing machine, swimming, or uphill walking if running causes pain or excessive mechanical stress.

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When high intensity should be adapted

Interval training can be adapted for most people, but maximal or near-maximal effort is not appropriate in every situation.

Extra caution is required with:

• known cardiovascular disease

• uncontrolled high blood pressure

• unstable angina

• severe respiratory disease

• recent illness or infection

• dizziness or fainting

• unexplained chest pain

• pregnancy with complications

• substantial musculoskeletal problems

• prolonged inactivity

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People with medical conditions do not necessarily need to avoid interval training. Adapted interval programmes are used in both cardiac and pulmonary rehabilitation.

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The intensity, exercise mode, and level of supervision must match the person’s health status.

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Stop the activity and seek urgent medical assessment if chest pain, fainting, severe breathing difficulty, sudden neurological symptoms, or a markedly irregular heart rhythm with significant discomfort occurs.

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Summary

Interval training alternates between periods of higher intensity and periods of lower activity or rest. During the work intervals, energy demand rises rapidly, and both anaerobic and aerobic systems contribute. Heart rate, ventilation, and blood flow increase, while the muscles use more glucose and oxygen. Recovery periods make it possible to restore part of the capacity and accumulate more high-intensity work. Over time, interval training can improve maximal oxygen uptake, mitochondrial function, vascular function, and insulin sensitivity. The effect depends on the duration, intensity, recovery periods, and total training load. Interval training should be used as part of a varied training programme and does not need to be maximal every time.

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Sources

• Buchheit, M. & Laursen, P. B. (2013). High-intensity interval training, solutions to the programming puzzle: Part I: Cardiopulmonary emphasis. Sports Medicine, 43(5), 313–338.

• MacInnis, M. J. & Gibala, M. J. (2017). Physiological adaptations to interval training and the role of exercise intensity. The Journal of Physiology, 595(9), 2915–2930.

• Gibala, M. J., Little, J. P., Macdonald, M. J. & Hawley, J. A. (2012). Physiological adaptations to low-volume, high-intensity interval training in health and disease. The Journal of Physiology, 590(5), 1077–1084.

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