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Interval training

Interval training is a structured form of exercise that alternates short bursts of intense physical activity with periods of lower-intensity recovery or rest, designed to enhance cardiovascular fitness, endurance, and overall performance.^[1] This method allows individuals to achieve significant physiological adaptations in less time compared to continuous moderate exercise, making it versatile for athletes, fitness enthusiasts, and clinical populations.^[2] The origins of interval training trace back to the early 20th century, with systematic development in the 1930s through the Swedish fartlek method—meaning "speed play"—pioneered by coach Gösta Holmér, which emphasized unstructured variations in pace during runs.^[3] In parallel, German coach Woldemar Gerschler and physiologist Hans Reindell formalized the interval method at the University of Freiburg, using controlled repetitions of high-effort sprints followed by heart-rate-monitored recovery to build aerobic capacity in athletes.^[4] By the 1950s, sprint interval training emerged for elite Olympic competitors, pushing efforts to near-maximal heart rates, while the 1996 Tabata protocol—developed for Japanese speedskaters—popularized high-intensity variants reaching 170% of VO2 max (maximal oxygen uptake) during work phases.^[2]^[5] Interval training encompasses a spectrum of intensities, from moderate aerobic intervals to high-intensity interval training (HIIT), where efforts exceed 80% of maximum heart rate for 15 seconds to several minutes, interspersed with recovery.^[6] Key benefits include improved aerobic capacity through enhanced mitochondrial function and oxygen utilization, reduced body fat, and increased muscle endurance, often yielding results comparable to longer steady-state workouts in half the time.^[7] For health applications, it lowers blood pressure, improves insulin sensitivity, and supports management of chronic conditions like cardiovascular disease and type 2 diabetes, though supervision is recommended for beginners or those with health risks.^[8] Its adaptability to various activities—such as running, cycling, or swimming—has driven its widespread adoption in sports science and public health guidelines.^[9] As of 2025, high-intensity interval training ranks as the sixth top fitness trend worldwide according to the American College of Sports Medicine.^[10]

Fundamentals

Definition and Principles

Interval training is a structured exercise protocol that involves repeated bouts of relatively intense activity alternated with periods of rest or low-intensity recovery. This method allows individuals to perform higher intensities than would be sustainable in continuous exercise, enabling greater overall training volume and stimulus for physiological improvements. The key principles of interval training revolve around manipulating work-to-rest ratios, intensity levels, interval durations, and session structure to optimize training outcomes. Work-to-rest ratios typically range from 1:1 (equal time working and recovering) to 1:3 (three times more recovery than work), depending on goals such as building endurance or power; for instance, a 1:1 ratio might involve 2 minutes of high effort followed by 2 minutes of recovery. Intensity is commonly prescribed as a percentage of maximum heart rate (e.g., 80-95% of HRmax), rating of perceived exertion (RPE) on the Borg scale (e.g., 15-18 out of 20), or relative to VO2 max (e.g., 90-100% VO2max for work intervals). Intervals generally last 30 seconds to 5 minutes to balance fatigue accumulation and recovery, while a full session includes a 5-10 minute warm-up at low intensity to prepare the body, the main interval sets, and a 5-10 minute cool-down to facilitate recovery and reduce injury risk.^[11]^[12]^[13] At its core, the physiological rationale for interval training lies in alternating intensities to simultaneously stress both aerobic and anaerobic energy systems, promoting enhancements in endurance capacity and power output without excessive fatigue. By accumulating time at high intensities during work phases while allowing partial recovery, the approach taxes oxidative metabolism for aerobic gains and glycolytic pathways for anaerobic improvements. A simple illustrative format is 4x400-meter runs at a challenging pace (e.g., 80-90% effort) alternated with 200-meter jog recoveries, demonstrating a balanced 1:1 ratio to build running economy. High-intensity interval training represents a common modern variation emphasizing shorter, more vigorous work bouts.^[14]^[15]

Historical Development

Interval training traces its roots to the early 20th century, when European coaches began developing structured methods to enhance athletic performance in runners, drawing from observations of occupational endurance activities. In the 1930s, Swedish coach Gösta Holmér introduced fartlek, a unstructured form of speed play that served as an early precursor to more systematic interval approaches by alternating intensities during runs in natural settings. Concurrently, German coach Woldemar Gerschler and cardiologist Hans Reindell pioneered the Freiburg interval method at the University of Freiburg, emphasizing short, high-intensity runs (100–400 meters) to elevate heart rate to 180 beats per minute, followed by recovery periods until it dropped to 120 beats per minute, aimed at strengthening the cardiovascular system in distance runners.^[3] In the 1950s, Swedish physiologist Per-Olof Åstrand conducted influential studies demonstrating the physiological benefits of interval training, particularly its effects on maximal oxygen uptake (VO₂ max), through controlled experiments on athletes that highlighted improvements in aerobic capacity with repeated high-intensity bouts. This era also saw Czech runner Emil Zátopek popularize interval training through his rigorous regimens, including extensive sessions of 400-meter repeats, which contributed to his dominance at the 1952 Olympics where he won gold in the 5,000m, 10,000m, and marathon events. By the 1960s, American coach Bill Bowerman at the University of Oregon integrated interval methods with continuous training, adapting European techniques for U.S. track athletes and fostering a balanced approach that influenced collegiate and elite distance running programs.^[3]^[16] During the 1970s and 1980s, amid the aerobic exercise boom popularized by jogging and group classes, interval training expanded beyond elite athletics into general fitness programs, incorporating varied intensities to make cardiovascular workouts more engaging and accessible to recreational participants. The method experienced a significant resurgence in the 1990s and 2000s, driven by Japanese researcher Izumi Tabata's 1996 study on high-intensity intermittent training, which compared short bursts of near-maximal effort (20 seconds work, 10 seconds rest) to moderate continuous exercise and showed superior gains in both anaerobic capacity and VO₂ max, spurring widespread adoption in sports science and public health applications.^[3]^[17]

Types and Variations

High-Intensity Interval Training

High-intensity interval training (HIIT) is a subtype of interval training that involves repeated bouts of short-duration, high-intensity exercise alternated with brief recovery periods of lower intensity or rest. The high-intensity phases are typically performed at 80-100% of maximum heart rate or VO2max, while recovery intervals occur at 40-50% of maximum heart rate, resulting in total session durations of 20-30 minutes. This structure maximizes cardiovascular and metabolic demands within a condensed timeframe, distinguishing HIIT from longer, moderate-intensity continuous training.^[18]^[19]^[20] Several established protocols exemplify HIIT's structured approach. The Tabata protocol, developed by Japanese researcher Izumi Tabata, consists of eight rounds of 20 seconds of near-maximal effort (approximately 170% VO2max) followed by 10 seconds of rest, totaling about 4 minutes of intense work. The Gibala method, pioneered by exercise physiologist Martin Gibala, features 8-12 cycles of 60 seconds at 95% VO2max interspersed with 75 seconds of active recovery. Another common regimen is the 4x4 interval protocol, which includes four 4-minute efforts at 90-95% of maximum heart rate separated by 3-minute active recovery periods. These protocols can be adapted across various activities to suit different fitness levels.^[17]^[21]^[22] HIIT sessions are versatile and can utilize bodyweight exercises like burpees, high knees, or squat jumps, or incorporate equipment such as stationary bicycles, treadmills, ellipticals, or rowing machines commonly found in gym environments. Unlike general interval training, which may involve moderate intensities around lactate threshold, HIIT emphasizes supramaximal efforts in protocols like Tabata to heighten metabolic stress, enabling greater time efficiency and dual enhancements in aerobic capacity and anaerobic power. This focus on extreme intensities makes HIIT particularly effective for busy individuals seeking substantial physiological benefits in minimal time.^[20]^[5]

Fartlek and Other Aerobic Variations

Fartlek training, a Swedish term translating to "speed play," was developed in the late 1930s by coach Gösta Holmér as a method to revitalize the Swedish cross-country running team amid competition from Finnish athletes.^[23] Unlike rigidly timed intervals, fartlek involves unstructured variations in pace during a continuous run, where athletes intuitively accelerate and decelerate based on terrain, effort, or landmarks, typically lasting 30 to 60 minutes.^[24] This approach emphasizes fluidity, allowing runners to blend aerobic endurance with spontaneous surges without predefined recoveries, fostering a playful yet challenging session.^[25] Other aerobic variations build on similar principles of sustained effort with periodic intensity shifts, tailored for endurance development. Pyramid intervals feature progressively increasing and then decreasing durations or distances, such as starting with 400 meters at a moderate pace, building to 1,600 meters, and descending back, followed by short jog recoveries to maintain aerobic focus.^[26] Tempo intervals, in contrast, consist of longer sustained efforts near the lactate threshold—often 20 to 40 minutes total, broken into segments like 3 x 10 minutes—with active recoveries to enhance aerobic capacity without venturing into anaerobic territory.^[27] These methods provide structured flexibility, differing from high-intensity interval training by prioritizing steady-state aerobic work over maximal bursts. Primarily applied in distance running and cycling, these variations suit endurance athletes seeking to improve sustained performance over varied terrains.^[28] Distance runners incorporate fartlek on trails to simulate race undulations, while cyclists use pyramid or tempo sessions on roads to boost threshold power during long rides.^[29] The emphasis on continuous flow helps develop aerobic efficiency for events like marathons or gran fondos. Key advantages include enhanced mental toughness through adaptive pacing decisions and improved race-pace versatility, as athletes learn to respond to internal cues rather than clocks.^[25] This intuitive training also promotes mind-body awareness, reducing monotony and sustaining motivation in aerobic-focused regimens.^[24]

Anaerobic and Sprint-Based Variations

Anaerobic and sprint-based variations of interval training emphasize short, maximal-intensity efforts that primarily rely on the anaerobic energy systems, fostering improvements in power output, speed endurance, and repeated sprint ability. These protocols typically involve explosive bursts lasting 10-30 seconds, performed at near-maximal velocity, with recoveries of 1-4 minutes to allow partial replenishment of phosphocreatine stores and clearance of metabolites like lactate. Such training is particularly effective for enhancing alactic power (efforts under 10 seconds) and lactic tolerance (efforts up to 30 seconds), targeting fast-twitch muscle fibers that contribute to rapid force production and fatigue resistance.^[30]^[31]^[32] One common format is walk-back sprinting, where athletes perform a series of maximal sprints over distances of 100-400 meters, followed by a walking recovery back to the starting point, which serves as active recovery to maintain light circulation without excessive fatigue accumulation. This method builds speed endurance by simulating progressive demands on the anaerobic systems while incorporating 1-2 minutes of low-intensity recovery per repetition, often structured in 6-10 sets to accumulate volume without compromising sprint quality. Research on similar repeated sprint protocols demonstrates that walking back enhances recovery efficiency compared to passive rest, preserving subsequent sprint performance in track athletes.^[33] Cluster sets represent another anaerobic variation, consisting of multiple short sprints (e.g., 5-10 seconds each) grouped with brief micro-rests of 10-20 seconds between intra-set efforts, followed by longer inter-set recoveries of 2-4 minutes. This structure allows for higher total sprint volume while minimizing velocity decrement and metabolic fatigue, making it suitable for developing repeated sprint capacity in team sports. Studies show cluster sets maintain greater power output across repetitions compared to traditional continuous sets, particularly benefiting fast-twitch fiber recruitment and anaerobic power in athletes.^[34]^[35] Hill repeats provide a resistance-enhanced sprint-based approach, involving uphill sprints of 10-30 seconds at maximal effort, with downhill walking as recovery to return to the start, typically in 6-10 repetitions. The incline increases force demands on the lower body, amplifying activation of fast-twitch fibers and alactic/lactic pathways without the need for additional equipment. This variation is widely used in track and field as well as team sports like soccer to improve explosive acceleration and anaerobic capacity, with evidence indicating significant gains in sprint performance following short-term protocols.^[36]^[37]

Physiological Mechanisms

Energy Systems Involved

Interval training primarily activates the body's three metabolic energy systems—the phosphagen (ATP-PC) system, the glycolytic system, and the oxidative system—based on the intensity and duration of the work and recovery phases, allowing for targeted stressing of anaerobic and aerobic pathways simultaneously.^[38] The phosphagen system dominates during very short, high-power intervals lasting 0–10 seconds, providing rapid ATP resynthesis without oxygen through the hydrolysis of stored ATP and the subsequent breakdown of phosphocreatine (PCr). This process involves the reactions ATP → ADP + P_i + energy and PCr + ADP → Cr + ATP, enabling explosive efforts such as sprint starts but depleting quickly within 5–10 seconds of maximal activity.^[38]^[39] For moderate-duration intervals of 10 seconds to 2 minutes, the glycolytic system becomes predominant, relying on anaerobic breakdown of glucose or glycogen to produce ATP, with pyruvate reduced to lactate as a byproduct under high-intensity conditions. The net reaction is glucose → 2 lactate + 2 ATP, yielding energy faster than aerobic processes but leading to lactate accumulation and associated acidosis that limits sustained effort beyond approximately 2 minutes.^[38]^[40] The oxidative system engages primarily during recovery periods and longer intervals exceeding 2 minutes, utilizing oxygen to oxidize carbohydrates, fats, and sometimes proteins for ATP production via the electron transport chain, contributing a smaller but increasing share of energy as exercise duration extends. This aerobic pathway supports replenishment of PCr stores and clearance of lactate during rest intervals in training sessions.^[38]^[41] These systems interact dynamically during interval training, with the phosphagen system initiating high-intensity bouts, rapidly transitioning to glycolysis for sustained power, and the oxidative system contributing throughout—especially in recovery—to buffer fatigue and restore anaerobic capacity; for instance, in a 30-second maximal sprint, contributions are approximately 23% from phosphagen, 49% from glycolysis, and 28% from oxidative metabolism.^[38]^[42]

Adaptations to Training

Interval training induces a range of chronic physiological adaptations that enhance the body's capacity for aerobic and anaerobic performance over time, primarily through repeated exposure to high-intensity efforts interspersed with recovery periods. These changes occur at the cardiovascular, muscular, neural, and hormonal levels, often surpassing those from equivalent volumes of moderate continuous training.^[43] Cardiovascular adaptations include increased stroke volume, which improves cardiac output and oxygen delivery during exercise, as observed in studies comparing high-intensity interval training (HIIT) to moderate-intensity continuous training (MICT). Resting heart rate typically decreases, reflecting enhanced parasympathetic tone and cardiovascular efficiency. VO2 max, a key measure of aerobic capacity, improves by 5-15% after 4-12 weeks of interval training, with meta-analyses showing average gains of approximately 4-5 ml/kg/min; for instance, the seminal 1996 Tabata study reported a 14% increase (7 ml/kg/min) in trained athletes after six weeks of high-intensity intermittent exercise.^[43]^[17] Muscular adaptations involve greater mitochondrial density in skeletal muscle fibers, boosting oxidative energy production by 25-35% after just 6-7 sessions of HIIT or sprint interval training (SIT). Capillary proliferation enhances blood flow and nutrient delivery to muscles, while the lactate threshold rises, allowing sustained higher intensities before fatigue onset; these changes are comparable or superior to those from MICT in work-matched protocols.^[43]^[44] Neural adaptations feature improved motor unit recruitment, particularly of fast-twitch fibers, leading to more efficient muscle activation and force production during intermittent efforts. Hormonally, interval training elevates growth hormone and catecholamine (epinephrine and norepinephrine) responses, which promote muscle remodeling, fat metabolism, and metabolic signaling, contributing to overall training-induced improvements.^[36]^[43]

Benefits and Effectiveness

Performance Enhancements

Interval training has been shown to enhance performance in endurance sports by improving race times and delaying the onset of fatigue. For instance, in distance runners, a 6-week program of sprint interval training resulted in significant reductions in 3000 m run times, with a large effect size of 1.53, corresponding to approximately 3-5% faster performances compared to traditional continuous training.^[14] Similarly, high-intensity interval training (HIIT) protocols have led to about 5.9% improvements in 3000 m endurance performance in well-trained athletes.^[45] In power and speed domains, particularly for team sports, interval training boosts sprint velocity and repeated sprint ability. An 8-week sprint interval training intervention in collegiate tennis players enhanced repeated sprint capacity, reducing total sprint times by approximately 4.84 seconds over multiple efforts and improving the best single sprint performance more than HIIT.^[46] These gains translate to better acceleration and maintenance of high speeds during intermittent play, as seen in sports like soccer and rugby where repeated-sprint training increases overall sprint proficiency.^[47] For anaerobic capacity, interval training elevates peak power output and facilitates quicker recovery between high-intensity efforts, notably in cycling. Short-duration, high-intensity intervals (e.g., 12 × 30 seconds at 175% peak power) over 3 weeks improved 40 km time-trial performance by 2.4%, while longer intervals near race pace (8 × 4 minutes at 85% peak power) yielded a 2.8% enhancement, both outperforming other protocols.^[48] Meta-analyses confirm the comparative superiority of interval training over steady-state continuous training for key performance metrics. HIIT produces a small but beneficial edge in VO₂max gains (1.2 mL·kg⁻¹·min⁻¹; 95% CI: 0.3-2.1) relative to continuous endurance training, alongside greater improvements in endurance performance measures such as time-to-exhaustion.^[49]^[50] These adaptations, including elevated VO₂max, underpin the observed performance uplifts across domains.^[45]

Health and Fitness Outcomes

Interval training, particularly high-intensity interval training (HIIT), offers substantial metabolic benefits that extend to general health improvement. It enhances fat oxidation capacity, with studies demonstrating up to a 36% increase in whole-body fatty acid utilization after short-term protocols involving repeated sprints.^[51] HIIT also improves insulin sensitivity by 23% to 58% in various populations, including those with type 2 diabetes, through mechanisms like increased muscle glycogen depletion and mitochondrial adaptations.^[51] Additionally, HIIT elevates excess post-exercise oxygen consumption (EPOC), leading to prolonged calorie expenditure post-workout—often greater than that from moderate-intensity continuous training (MICT)—which supports metabolic efficiency.^[52] In terms of cardiovascular health, interval training contributes to reduced blood pressure and a lower risk of metabolic syndrome, as evidenced by research from the 2010s and later. For instance, HIIT protocols have shown reductions in systolic blood pressure of approximately 4-9 mmHg compared to controls in individuals with hypertension or prehypertension after 8-16 weeks, with some evidence of superiority over MICT in certain populations.^[53] Meta-analyses from the decade confirm that HIIT improves cardiometabolic risk factors, including lower fasting glucose and waist circumference in those with metabolic syndrome, thereby mitigating overall cardiovascular disease risk.^[54] For weight management, interval training promotes greater calorie burn efficiency due to its high energy demands and elevated EPOC, facilitating fat loss in time-limited routines. A 12-week HIIT program resulted in significant reductions in body weight and fat mass among obese adults, comparable to or exceeding MICT outcomes.^[55] Its shorter duration enhances adherence for busy individuals, with compliance rates averaging 89.4% across supervised sessions, supporting sustained weight control efforts.^[56] These outcomes are particularly relevant for specific populations, such as sedentary adults and older individuals. In sedentary groups, HIIT improves glycemic control and insulin sensitivity after just 2 weeks, offering an accessible entry to fitness without requiring high baseline activity levels.^[57] For older adults and those with diabetes, 12-week HIIT interventions enhance glycemic regulation, reducing HbA1c levels and improving mitochondrial function to better manage blood sugar.^[58] Such benefits underscore interval training's role in promoting health across diverse, non-athletic demographics.

Practical Implementation

Designing Interval Workouts

Designing effective interval training programs begins with a thorough assessment of the individual's current fitness level, specific goals, and available resources. Fitness level can be evaluated through standardized tests such as the Cooper 12-minute run for aerobic capacity or simple self-assessments like the ability to perform bodyweight exercises without fatigue.^[59] Goals should be clearly defined, such as building endurance through longer aerobic intervals or developing power via shorter, high-intensity bursts, while considering equipment like treadmills, bikes, or bodyweight options to ensure accessibility.^[60] This initial evaluation aligns the program with the principle of individuality, tailoring workouts to the person's starting point and objectives to maximize adherence and results.^[61] Once assessed, structuring the workout involves selecting the appropriate interval type based on the goal—for endurance, opt for moderate-intensity intervals lasting 2-5 minutes; for power, use short, explosive efforts of 10-30 seconds. Work-to-rest ratios should be established to match fitness levels, with beginners typically starting at a 1:2 ratio (e.g., 1 minute of work followed by 2 minutes of recovery) to allow adequate replenishment of energy stores without excessive fatigue.^[62] Progression occurs weekly by gradually increasing volume (e.g., adding intervals) or intensity (e.g., shortening recovery or raising effort level), ensuring the program adheres to the specificity principle by targeting the desired energy systems.^[59] Progression models provide a systematic framework for advancing the program over time. In a linear model, training volume increases steadily—such as adding one repetition per session—while maintaining consistent intensity, which is suitable for novices building a base.^[63] Periodized models, by contrast, cycle through phases of varying intensities and volumes over weeks or months, such as a high-volume/low-intensity block followed by low-volume/high-intensity, to prevent plateaus and optimize adaptations.^[64] These approaches follow established guidelines from organizations like the American College of Sports Medicine, which recommend progressive overload to drive improvements without overtraining.^[65] Monitoring ensures the workout remains effective and appropriately challenging, using tools like the Borg Rating of Perceived Exertion (RPE) scale, which ranges from 6 (no exertion) to 20 (maximal exertion), with high-intensity intervals targeting 15-17 for most trainees. Heart rate zones, calculated as percentages of maximum heart rate (e.g., 80-90% for vigorous efforts), provide objective feedback via wearables to confirm intensity alignment.^[12] A sample beginner workout might include a 5-minute warm-up, followed by 3 intervals of 1 minute at high effort (RPE 15-17 or 80-90% max HR) alternated with 2 minutes of active recovery (easy jogging or walking), and a cool-down, totaling 20-25 minutes to build confidence without overwhelm.^[66]

Applications Across Sports and Activities

Interval training is widely adapted in team sports to replicate the stop-start nature of competition, enhancing players' ability to perform repeated high-intensity efforts. In soccer, shuttle runs are a staple, often structured using protocols derived from the 30-15 Intermittent Fitness Test, such as 15-second running efforts at velocities based on the test (e.g., 95% of the final velocity attained) followed by 15-second passive recoveries to mimic the intermittent demands of match play and improve aerobic and anaerobic fitness.^[67] This individualized approach has been shown to elicit physiological adaptations comparable to traditional training while being more specific to game scenarios.^[67] Supramaximal intermittent shuttle-run training has also demonstrated effectiveness in enhancing aerobic and anaerobic measures in soccer players.^[68] In basketball, suicide drills—progressive sprints from the baseline to the free-throw line, half-court, and opposite baseline with immediate returns—serve as high-intensity intervals that build court-specific agility, speed, and recovery capacity under fatigue.^[69] Research indicates these drills improve change-of-direction speed, a critical skill in basketball, when incorporated into conditioning programs.^[70] For individual sports, interval training emphasizes discipline-specific mechanics to optimize performance in sustained or explosive efforts. In swimming, athletes execute sets of intense swims, such as 4x50-meter all-out freestyle efforts with 4-minute recovery intervals that include light swimming or flip-turn transitions, to develop VO2 max and stroke efficiency.^[71] These protocols allow for targeted improvements in speed and endurance while facilitating active recovery to clear lactate. In cycling, hill intervals involve 4-5 minute climbs at 90-100% of maximum effort, followed by 3-6 minutes of easy descent or spinning recovery, which enhance climbing power and overall aerobic threshold.^[72] Such sessions, often on 6-8% gradients, have been validated for increasing time-to-exhaustion in performance testing.^[73] In fitness contexts beyond competitive sports, interval training provides versatile, time-efficient options for general conditioning. Gym-based HIIT circuits typically rotate through multi-joint exercises like kettlebell swings, battle ropes, and bodyweight squats in 30-60 second bursts with 10-30 second transitions, forming a full-body workout that boosts metabolic rate and muscular endurance.^[74] These circuits, adhering to ratios like 1:1 work-to-rest, are scalable for various fitness levels and supported by evidence for improving cardiorespiratory fitness in recreational populations.^[75] For home use, bodyweight interval sessions—such as alternating 20-40 second rounds of push-ups, mountain climbers, and air squats with equal rest—offer accessible conditioning that requires no equipment and can be completed in 20 minutes to enhance overall strength and fat oxidation.^[76] Interval training is particularly beneficial for special populations when modified with reduced durations, lower intensities, and extended recoveries to ensure safety and progression. For overweight or obese individuals, low-volume protocols like 10-20 second sprints interspersed with 1-2 minute walks have demonstrated reductions in body fat percentage and improvements in VO2 max over 12 weeks, with adherence rates comparable to moderate continuous exercise.^[77] Beginners can start with shorter efforts, such as 20-second high-knee marches followed by 40-second rests, gradually building tolerance as per guidelines that prioritize enjoyment and sustainability.^[2] In rehabilitation settings, such as post-cardiac event recovery, modified intervals (e.g., 1-minute efforts at 60-80% max heart rate with 2-minute recoveries) improve exercise capacity and vascular function without exceeding safe thresholds, as outlined in clinical protocols.^[78] These adaptations align with recommendations for clinical populations, emphasizing medical clearance and progressive overload.^[79]

Risks and Considerations

Potential Drawbacks and Injuries

Interval training, particularly high-intensity variants, is associated with a heightened risk of overuse injuries due to the repetitive high-impact nature of the exercises involved. Common examples include shin splints (medial tibial stress syndrome), which arise from excessive stress on the tibia and surrounding muscles during rapid acceleration and deceleration phases, and muscle strains in the lower extremities from sudden bursts of effort.^[80]^[81] Novices face an elevated risk for these injuries owing to inadequate conditioning and improper form, which amplifies biomechanical stress on joints and soft tissues during intense intervals.^[80] Physiologically, interval training can induce excessive fatigue and elevate cortisol levels, contributing to overtraining syndrome characterized by persistent exhaustion, hormonal imbalances, and diminished recovery capacity. Acute sessions of high-intensity interval training have been shown to significantly increase post-exercise cortisol concentrations, potentially leading to chronic stress responses if training volume is not managed.^[82]^[83] In rare cases, particularly extreme high-intensity sessions, interval training can lead to rhabdomyolysis, a serious condition involving the rapid breakdown of muscle tissue that releases harmful substances into the bloodstream, potentially causing kidney damage or failure. This risk is higher in untrained individuals, those with inadequate hydration, or when multiple muscle groups are intensely worked without proper progression.^[84] Cardiovascular strain represents another concern, with intense efforts potentially triggering arrhythmias in untrained individuals whose hearts are unaccustomed to sudden demands. This risk is particularly relevant for sedentary beginners, where abrupt high-intensity bouts may overload the cardiovascular system, increasing susceptibility to irregular heart rhythms during recovery phases.^[85] Evidence from emergency department data indicates that injuries linked to high-intensity interval training rose by 144% between 2012-2016, coinciding with surging popularity, with notable increases in lower extremity strains (127%), upper extremity strains (124%), and knee/ankle sprains (125%).^[81]

Guidelines for Safe Practice

Before beginning any interval training session, individuals should perform a mandatory warm-up consisting of 10 to 15 minutes of dynamic movements, such as light jogging or arm circles, to increase blood flow, elevate core temperature, and prepare muscles and joints for high-intensity efforts.^[86] Additionally, selecting appropriate footwear with adequate cushioning and support, along with suitable terrain that minimizes impact—such as even surfaces on tracks or treadmills—helps reduce the risk of strains and other musculoskeletal issues.^[20] To ensure safe progression, beginners should limit interval training to 2 to 3 sessions per week, gradually increasing intensity or duration only after establishing a base of moderate exercise, while incorporating at least one full rest day between sessions to allow for muscle repair.^[87] Post-workout recovery can be optimized by consuming carbohydrates within 30 to 60 minutes after exercise, such as through a meal including fruit or whole grains, to replenish glycogen stores and support energy restoration.^[88] During sessions, participants must monitor for warning signs including dizziness, excessive shortness of breath, or sharp pain, stopping immediately if any occur and seeking medical advice if symptoms persist.^[89] Those with pre-existing conditions, such as cardiovascular issues, should consult a healthcare professional before starting to tailor the program and ensure safety.^[90] Best practices include maintaining proper hydration by drinking 16 to 24 ounces of water 2 hours before training, sipping 7 to 10 ounces every 10 to 20 minutes during the session, and rehydrating with 16 to 24 ounces per pound of body weight lost afterward, particularly in warm environments.^[91] Integrating adequate sleep—aiming for 7 to 9 hours per night—enhances overall recovery by promoting hormonal balance and tissue repair.^[88] Furthermore, high-intensity interval sessions should not exceed 3 to 4 per week to prevent overtraining and allow sufficient adaptation time.^[87]

References

[1]
Interval training: More workout in less time (and you can do it)
Apr 20, 2018 · Interval training uses short bursts of strenuous activity to ramp up your heart rate and boost your fitness.
[2]
HIIT (High Intensity Interval Training) - The Nutrition Source
Interval training was first introduced in the 1950s as a higher intensity form called sprint interval training, which reached 100% maximum heart rate and was ...
[3]
History and perspectives on interval training in sport, health, and ...
Interval training was developed for athletes in the early 20th century. It was systemized in Sweden as Fartlek, and in Germany as die interval Method, in the ...
[4]
How Interval Training Started | Revo2lutionRunning.com
Mar 24, 2016 · Interval training originated in the 1930s by coach Waldemar Gerschler and physiologist Hans Reindell of Germany's Freiburg University to develop fitness in ...
[5]
High-Intensity Interval Training: For Fitness, for Health or Both?
Aug 16, 2019 · The basic structure of HIIT is generally understood – short periods of high intensity exercise alternating with usually longer but still brief ...Missing: definition | Show results with:definition
[6]
Evidence-Based Effects of High-Intensity Interval Training on ...
Research has indicated that high-intensity interval training induces numerous physiological adaptations that improve exercise capacity.
[7]
Exploring the benefits of high-intensity interval training - Mayo Clinic
Mar 25, 2016 · High-intensity interval training (HIIT) is emerging as an effective and more time-efficient alternative to traditional endurance-based exercise programs.Missing: definition | Show results with:definition
[8]
Interval training: A shorter, more enjoyable workout? - Harvard Health
Jun 1, 2024 · A 20-minute HIIT session will burn more calories, strengthen your heart more, and help your body use oxygen more efficiently.
[9]
Exercise Standards for Testing and Training | Circulation
Rather than using heart rate alone to clinically determine intensity of exercise, the 6 to 20 Borg scale of perceived exertion is useful (Table 4). Special ...
[10]
Rate of Perceived Exertion (RPE) Scale - Cleveland Clinic
The RPE scale is used to measure how hard your body works during exercise. It runs from 0 – 10, using numbers to rate how easy or difficult you find an ...Missing: VO2 | Show results with:VO2
[11]
Why and How to Do Aerobic Training, Including High-intensity ...
Always begin any aerobic training session with a warm-up period. This is a period of at least 5 minutes of lower-intensity, active exercise. The warm-up should ...Aerobic Training Methods · Intensity Of Exercise · Preventing Injury
[12]
Effects of 6-week sprint interval training compared to traditional ...
Feb 5, 2025 · This training method is designed to enhance both aerobic and anaerobic fitness by pushing the body to work at maximum effort during the intense ...
[13]
How to Run Faster: 6 Tips to Improve Your Running Speed - Still I Run
Jan 17, 2025 · Example: 4x400m repeats with 90 seconds of recovery jogs in between. 400m is one full loop on a race track. Beginner variation: Try 6x200m ...
[14]
Interval training for performance: a scientific and empirical practice ...
Interval training was first described by Reindell and Roskamm and was popularised in the 1950s by the Olympic champion, Emil Zatopek. Since then middle- and ...
[15]
Effects of moderate-intensity endurance and high-intensity ... - PubMed
Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Med Sci Sports Exerc. 1996 Oct;28(10):1327-30 ...Missing: URL | Show results with:URL
[16]
How to 'HIIT' your exercise targets - HonorHealth
This involves exercise at high-exertional levels, from 80 to 95% of maximal heart rate for short periods, followed by a recovery period that is usually 40 to 50 ...Missing: definition 80-100%
[17]
HIIT: What It Is, Health Benefits, and Getting Started
Meanwhile, the recovery periods are typically done at 40 to 50 percent of your maximal heart rate and may vary in duration. You'll continue alternating ...
[18]
High-Intensity Interval Training - ACSM's Health & Fitness Journal
High-intensity interval training (HIIT) consists of alternating short periods of intense exercise with recovery periods of passive or mild-intensity movement.
[19]
Physiological adaptations to low-volume, high-intensity interval ...
The protocol is still time efficient in that only 10 min of exercise is performed over a 20 min training session. Importantly, this practical, time-efficient ...
[20]
Aerobic high-intensity intervals improve VO2max more ... - PubMed
High-intensity aerobic interval training resulted in significantly increased VO2max compared with long slow distance and lactate-threshold training intensities.
[21]
Tabata training: one of the most energetically effective high-intensity ...
Apr 19, 2019 · One of the most effective training methods, ie, 'Tabata training,' is reviewed herein from the viewpoint of the energetics of exercise.
[22]
Tabata training: one of the most energetically effective high-intensity ...
Apr 19, 2019 · The interval training method known as Fartlek training was invented by the Swedish coach Gösta Holmér in the 1930s [2]. Dr. Woldemar ...
[23]
Fartlek Run 101: Your Guide to Fartlek Workouts - TrainingPeaks
A fartlek run is a type of running workout that involves running continuously at varying paces. It challenges your body to adapt to various speeds.What Is A Fartlek Run? · Fartlek Improves Your Mental... · Three Sample Fartlek...<|control11|><|separator|>
[24]
What Is a Fartlek Run and How Can It Help You Get Faster?
Dec 8, 2020 · Fartleks vary your pace and time to improve mind-body awareness, mental strength, and stamina.
[25]
https://www.runnersworld.com/training/a34824872/fartlek-run/
[26]
[Ultimate Guide] Zone 3 Training: Sweet Spot or Grey Zone? - INSCYD
Zone 3 training is also known as tempo training or the tempo zone. Especially runners use tempo intervals in their terminology. Fans of zone 3 training often ...
[27]
Best Cycling Workouts for Building Aerobic Endurance - CTS
Mar 7, 2025 · To build greater cycling aerobic endurance, two of the best cycling workouts to use are Aerobic Tempo and Sweet Spot Tempo intervals.
[28]
https://trainright.com/best-cycling-workout-for-building-aerobic-endurance/
[29]
The Role of High-Intensity Interval Training (HIIT) in Neuromuscular ...
... fast-twitch fibers, enhancing explosive power. Hybrid fibers ... Sprint Interval Training (SIT) effectively enhances power, speed, and anaerobic capacity ...
[30]
Energy metabolism characteristics of sprinters in speed endurance ...
Oct 1, 2025 · Sprint Interval Training (SIT) typically involves maximal intensity efforts lasting 20–30 s, with recovery periods of 2–4 min. This protocol ...
[31]
Impact of on-field repeated sprint training on aerobic fitness and ...
Repeated sprint training (RST) improves both aerobic and anaerobic capacities; however, its effects on players in different positional roles remain ...
[32]
Effects of Repeated Sprint Training With Progressive Elastic ...
Each sprint lasted 4 seconds followed by 26 seconds of semiactive recovery (walking back to the starting line). The subjects had 3 minutes of passive ...
[33]
Effects of Cluster vs. Traditional Sets Complex Training on Physical ...
Incorporating cluster sets in COX training is more effective than traditional sets for maximizing physical performance attributes and peak power output, as well ...
[34]
Cluster sets lead to better performance maintenance and minimize ...
Dec 8, 2024 · Cluster sets lead to better performance maintenance and minimize training-induced fatigue than traditional sets. Jos Antonio P ez-Maldonado ...
[35]
Neuromuscular Adaptations to Short-Term High-Intensity Interval ...
High-intensity interval training contained 30-second all-out sprints on a hill with a gradient of 9.5%. Six sprints in total were performed in a session and 4- ...
[36]
The effects of uphill vs. level-grade high-intensity interval training on ...
With regard to running performance, the results indicate that both uphill and level-grade interval training can induce significant improvements in a run-to- ...
[37]
Interaction among Skeletal Muscle Metabolic Energy Systems ...
The purpose of this paper is to provide a contemporary explanation of the muscle metabolic response to different exercise intensities and durations.
[38]
The contribution of energy systems during 30-second lower body ...
May 24, 2024 · In both groups, ATP-PCr system contribution was greater in IST compared with SMT, whereas the glycolytic and oxidative systems had a higher ...
[39]
Effects of high-intensity interval training on aerobic and anaerobic ...
May 8, 2025 · ... fast-twitch muscle fibers ... sprint interval training in taekwondo athletes: effects on cardiorespiratory fitness and anaerobic power.
[40]
Performance and energy systems contributions during upper-body ...
During upper-body sprint intervals, glycolytic contribution was higher initially, but decreased. ATP-PCr contribution increased in later bouts, while oxidative ...
[41]
Effects of sprint distance and repetition number on energy system ...
This study aims to compare the effect of sprint distance and repetition number on performance, physiological responses, and energy systems contributions.
[42]
Physiological adaptations to interval training and the role of exercise ...
May 1, 2017 · This brief review considers the role of exercise intensity in mediating physiological adaptations to training, with a focus on the capacity for aerobic energy ...
[43]
Physiological and performance adaptations to high-intensity interval ...
Short-term HIIT is a potent stimulus to induce physiological remodeling similar to traditional endurance training despite a markedly lower total exercise ...
[44]
Aerobic high-intensity intervals are superior to improve V̇O 2max ...
Oct 31, 2022 · HIIT improves V̇O 2max more than SIT. Given the importance of V̇O 2max for most endurance performance scenarios, HIIT should typically be the chosen interval ...INTRODUCTION · METHODS · RESULTS · DISCUSSION
[45]
Effects of sprint interval training compared to high intensity interval ...
This study compared the effects of sprint interval training (SIT) versus high-intensity interval training (HIIT) on repeated sprint capacity and ...
[46]
Integrating Repeated Sprints in Football Codes - HIIT Science
Repeated-sprint training (RST) is a unique and attractive HIIT format for preparing football code athletes, such as soccer, rugby union, rugby 7s and rugby ...
[47]
Effects of different interval-training programs on cycling time-trial ...
Conclusions: Interval training with work bouts close to race-pace enhance 1-h endurance performance; work bouts at much higher intensity also appear to improve ...Missing: capacity | Show results with:capacity
[48]
(PDF) Effectiveness of High-Intensity Interval Training (HIT) and ...
Aug 5, 2025 · Results The meta-analysed effect of endurance training on VO2max was a possibly large beneficial effect (4.9 mL•kg-1•min-1; 95% confidence ...
[49]
Meta‐analysis of high‐intensity interval training and alternative ...
Oct 5, 2025 · While HIIT and SSD are similarly effective in enhancing VO2max, HIIT provides greater improvements in endurance performance measures.
[50]
High-Intensity Intermittent Exercise and Fat Loss - PMC - NIH
Nov 24, 2010 · Regular HIIE has been shown to significantly increase both aerobic and anaerobic fitness. HIIE also significantly lowers insulin resistance and ...
[51]
Acute interval running induces greater excess post-exercise oxygen ...
Apr 22, 2024 · We conclude that HIIT results in greater total EE and EPOC, as well as higher percentage of energy from lipid during EPOC than EE matched MICT in male college ...Exercise Protocols · Results · Epoc After Hiit And Mict
[52]
Effects of high-intensity aerobic interval training vs. moderate ...
May 7, 2010 · Higher levels of physical activity and cardiorespiratory fitness have been shown to reduce the risk of HPT in healthy normotensive persons.
[53]
High intensity interval training exercise-induced physiological ...
Nov 10, 2020 · HIIT improves certain clinical aspects in people with MetS (BG, SBP, DBP and WC) compared to people with MetS who do not perform physical exercise.
[54]
Effectiveness of high-intensity interval training for weight loss in ...
Jul 20, 2021 · This study aimed to determine whether a 12-week HIIT was more effective than MICT for weight loss in obese adults.
[55]
Rates of compliance and adherence to high-intensity interval training
Nov 21, 2023 · Meta-analyses were conducted to discern differences in compliance and adherence between HIIT vs. MICT. Sensitivity analyses, publication bias, ...
[56]
Effectiveness and Safety of High-Intensity Interval Training in ... - NIH
Indeed, a single bout of exercise increases insulin sensitivity for up to 48 hours into recovery, and this is accompanied by improved glycemic control in ...
[57]
High-Intensity Interval Training Improves Glycemic Control, Cellular ...
Jul 17, 2023 · A 12-week HIIT program significantly improves diabetes by reducing insulin resistance; regulating mitochondrial biogenesis; and decreasing oxidative stress ...
[58]
https://www.mdpi.com/1648-9144/59/7/1320
[59]
https://www.nsca.com/contentassets/de9aebfe7a7340b69217b99bb13862a7/basics_of_strength_and_conditioning_manual.pdf
[60]
https://www.nsca.com/contentassets/8323553f698a466a98220b21d9eb9a65/foundationsoffitnessprogramming_201508.pdf
[61]
[PDF] Training for Cardiovascular Fitness
Interval training to improve aerobic power should involve low or high-intensity intervals of at least 60 to 90 seconds duration, with 1 to 2 minutes recovery in ...
[62]
Periodization Training Simplified: A Strategic Guide | NASM Blog
An example of linear periodization is adding weight (load) to a given exercise each training session until that 4- or 8-week block is completed. An example of ...
[63]
CURRENT CONCEPTS IN PERIODIZATION OF STRENGTH AND ...
Secondly, the linear model helps ensure that each training parameter (strength, power, speed) is addressed in step‐wise progression. Advancement to other ...
[64]
[PDF] Progression Models in Resistance Training for Healthy Adults
This model has compared favorably with linear periodized and nonperiodized multiple-set models (14) and has been shown to produce superior strength increases ...
[65]
HIIT Workouts for Beginners (3 Running & Interval Training Routines)
Feb 2, 2024 · HIIT involves short bursts of high-intensity exercise followed by rest. A beginner example is a 3-minute walk, then 60 seconds of power walking ...Missing: sample | Show results with:sample
[66]
Adaptations to Optimized Interval Training in Soccer Players
30-15 Intermittent Fitness Test. The test comprised 30 s shuttle runs with 15 s passive recovery between efforts. Participants commenced the test with an ...
[67]
The Effects of a Supramaximal Intermittent Training Program on ...
Oct 11, 2023 · This study investigated the effectiveness of supplementing regular preseason soccer training with a supramaximal intermittent shuttle-run training (ISRT) model.
[68]
Effect of Active vs. Passive Recovery on Repeat Suicide Run Time
Aug 9, 2025 · This study was conducted to evaluate the difference between active and passive recovery methods during successive suicide runs.
[69]
Change of Direction Speed Tests in Basketball Players - NIH
This review aimed to summarize the CODS test varieties for basketball players on publications until 2019 and identify recent trends regarding what types of ...
[70]
Interval training for maximum endurance (VO2max) development
This study attempted to determine which training set would best stimulate maximum endurance development.
[71]
5 Different Types of Hill Work You Should Be Doing Now
Hill workouts can be manipulated in many ways through five variables: grade, intensity, volume, length, and time.Short Steep Hills (<12 Sec... · Short Steep Hills (>12 Sec... · Moderate Hills (12 To 30...<|separator|>
[72]
Hill intervals: improve your climbing | Cycling Weekly
Feb 1, 2013 · Hill intervals are also ideal for increasing your aerobic capacity. Find a hill of around six-eight per cent gradient and make sure you are warmed up ...
[73]
HIIT Workouts: Programming, Exercises, and Benefits - NASM Blog
When creating circuits for 30-20-10 workouts, for the best results, allow for 10-15 seconds to transition between exercises. Key things to remember about high ...
[74]
HIGH-INTENSITY CIRCUIT TRAINING USING BODY WEIGHT
The American College of Sports Medicine (ACSM) recommends 8 to 12 repetitions of a resistance training exercise for each major muscle group at an intensity of ...
[75]
https://journals.lww.com/acsm-healthfitness/fulltext/2013/05000/high_intensity_circuit_training_using_body_weight_.5.aspx
[76]
Low-volume high-intensity interval training improves ...
Nov 7, 2020 · Low-volume HIIT may induce significant improvements in cardiometabolic health, especially VO 2max, WAI and well-being in obese individuals after only 12 weeks.
[77]
Benefits and Risks of High-Intensity Interval Training in Patients With ...
Apr 15, 2019 · Despite the enthusiasm surrounding HIIT, its main advantage over MICT appears to be short-term exercise performance outcomes and indices of vascular function.
[78]
https://pubmed.ncbi.nlm.nih.gov/30732854/
[79]
Medial Tibial Stress Syndrome in Novice and Recreational Runners
Oct 13, 2020 · MTSS consists of a lower leg running injury caused by overuse [12]. It is characterized by exercise-induced pain along the posteromedial border ...Medial Tibial Stress... · 3. Results · 4. DiscussionMissing: strains | Show results with:strains
[80]
Injuries sustained during high intensity interval training - PubMed
Injuries related to HIIT increased 144% from 2012-2016 compared to 2007-2011, correlating with a 274% increase in HIIT interest.
[81]
Acute effect of HIIT on testosterone and cortisol levels in healthy ...
May 22, 2021 · HIIT significantly increased cortisol after 60 min in controlled studies (p < 0.001), whereas no changes were found in pre-post intervention ...Missing: drawbacks | Show results with:drawbacks
[82]
Overtraining Syndrome: A Practical Guide - PMC - PubMed Central
Decreased glycogen stores may account for feelings of heavy legs and muscular fatigue in overtrained athletes. Increased uptake of tryptophan into the brain ...Missing: drawbacks | Show results with:drawbacks
[83]
(PDF) High-Intensity Interval Training - health benefits and risks
Despite these benefits, HIIT also presents risks such as a higher incidence of musculoskeletal injuries, potential adverse cardiac events in high-risk ...<|control11|><|separator|>
[84]
Warm Up, Cool Down | American Heart Association
Jan 16, 2024 · Warm-up widens blood vessels and raises muscle temperature. Cool-down keeps blood flowing and gradually decreases heart rate and body ...
[85]
https://www.researchgate.net/publication/382630883_High-Intensity_Interval_Training_-_health_benefits_and_risks_-_literature_review
[86]
Nutrient Timing: What to Eat Before and After a Workout - NASM Blog
A good way to start recovery is to consume a snack with carbohydrates and a moderate amount of protein, plus fluids and sodium, within 30 minutes after exercise ...When To Eat Before A Workout... · Workout Recovery Basics And... · What About Protein?
[87]
Guidelines for the delivery and monitoring of high intensity interval ...
Jan 24, 2019 · HIIT involves alternating high and light exercise. Guidelines include a framework for prescription using objective and subjective measures, and ...
[88]
High‐Intensity Interval Training for Patients With Cardiovascular ...
Oct 31, 2018 · HIIT has shown a relatively low rate of major adverse cardiovascular events for patients with coronary artery disease or heart failure when applied within CR ...Missing: strain | Show results with:strain
[89]
National Athletic Trainers' Association Position Statement: Fluid ...
To ensure euhydration before activity, an athlete should be mindful of individual cues, such as thirst, body weight, urine color, and voiding frequency.