Article Page
HIIT vs Continuous Endurance Training: Battle of the Aerobic Titans
Micah Zuhl, Ph.D. and Len Kravitz, Ph.D.

The fitness industry is currently experiencing a surge of interest and growth in high intensity interval training (HIIT). This method of training involves repeated bouts of high intensity efforts that range from 5 seconds to 8 minutes followed by recovery periods of varying lengths of time. Billat (2001) points out that as early as 1912 Hannes Kolehmainen, famous Finish Olympic long-distance runner, was employing interval training in his workouts. As the knowledge of HIIT increased, exercise scientists demonstrated that this type of exercise not only provides performance benefits for athletes and improves the health of recreational exercisers, but it may also be a suitable alternative to endurance training, or continuous aerobic exercise. To improve cardiovascular fitness the belief has always been to increase the volume of exercise, whether it's longer runs, bike rides, or extended time on an aerobic machine (e.g., stairstepper, elliptical, cycle, treadmill). The breadth of current research has revealed that HIIT improves numerous physiological parameters, often in less time when measured against high volume continuous exercise (Daussin et al., 2008). Therefore, the purpose of this article is to discuss and compare the cardiovascular, skeletal muscle, and metabolic adaptations to HIIT versus continuous endurance exercise. Continuous aerobic training is defined as exercise (e.g., running, cycling, swimming, etc.) lasting greater than 20 minutes and held at steady intensity during the entire bout. Additionally, research examples of HIIT and continuous endurance training workouts are included in this article.

Cardiovascular Physiology 101: Basic Reponses and Adaptations of Aerobic Training
Before comparing HIIT and continuous endurance training, a brief review of the cardiovascular responses and adaptations to chronic aerobic exercise is warranted, because it is central to both programs. During aerobic exercise the performance of the heart is based on heart rate, the amount of blood pumped per beat (stroke volume), and heart contractility, or the forcefulness of each heart contraction. Combined, these variables increase blood flow and oxygen supply to meet the demands of the exercising muscles. The contraction of the skeletal muscle also increases venous blood flow return to the heart, which increases ventricle blood filling (called the preload). This boosted preload contributes to the heart's enhanced stroke volume during exercise, which is a major determinant of aerobic performance (Joyner and Coyle, 2008).

Heart muscle structure adaptations are common with progressively increasing amounts of endurance training. These adaptations include thickening of the heart muscle and increased left ventricle size, which contribute to improved heart function during exercise. Consistent bouts of endurance exercise such as 30-60 minutes of continuous running or cycling performed 3-7 days per week leads to several other cardiovascular adaptations including the following:
1. Increased cardiac muscle mass
2. Increased stroke volume
3. Increased disposal of metabolic wastes
4. Increased oxidative enzymes and efficiency
5. Faster diffusion rates of oxygen and fuel into muscle
6. Increased left ventricle dilation and chamber volume
7. Increased carbohydrate sparing (thus greater use for fat as fuel)
8. Increase in mitochondria (energy factory of cell)
9. Increase in cell regulatory mechanisms of metabolism
10. Increased fat oxidation
11. Increased expression of fatigue-resistance slow twitch muscle fibers
(Joyner and Coyle, 2008; Pavlik, Major, Varga-Pintér, Jeserich, & Kneffel, 2010)

HIIT vs. Continuous Endurance Exercise: Cardiovascular Adaptations
Recent work shows that the cardiovascular adaptations to HIIT are similar to and in some cases superior to those of continuous endurance training (Helgerud et al., 2007; Wisløff, Ellingsen, & Kemi, 2009). Helgerud et al. showed that 4 repetitions of 4-minute runs at 90-95% of heart rate max (HRmax) followed by 3 minutes of active recovery at 70% HRmax performed 3 days per week for 8 weeks resulted in a 10% greater improvement in stroke volume when compared to a long, slow distance training group. Additional research by Slordahl et al. (2004) demonstrated that high intensity aerobic training at 90-95% of maximal oxygen consumption (VO2max) increased left ventricle heart mass by 12% and cardiac contractility by 13%, which is comparable to cardiovascular changes observed in continuous aerobic exercise.

Maximal oxygen consumption (VO2max) is considered the uppermost ability of the body to consume, distribute and utilize oxygen for energy production. It is commonly called maximal aerobic capacity and is a good predictor of exercise performance. Improvements in cardiovascular function will increase one's VO2max. Some research suggests that VO2max improvements with HIIT are superior to those with endurance training. Daussin et al. (2007) measured VO2max responses among men and women who participated in an 8-week HIIT and a continuous cardiovascular training program. VO2max increases were higher with the HIIT program (15%) as compared to the continuous aerobic training (9%). Improving cardiovascular function and increasing VO2max are major goals of patients that suffer from cardiovascular disease. For this reason some cardiac rehabilitation centers are beginning to include interval training sessions with heart disease patients (Bartels, Bourne, & Dwyer, 2010). Results show similar improvements as traditional low intensity exercise, but in a shorter time and fewer sessions.

HIIT vs. Continuous Endurance Exercise: Skeletal Muscle Adaptations
Increased mitochondria (the energy factory of the cell) size and number is becoming a hallmark adaptation to HIIT (Gibala, 2009). This is referred to as an increase in mitochondria density, and has been thought for many years to only occur from chronic endurance training. Mitochondria use oxygen to manufacture ATP (the energy molecule of the cell) at high levels through the breakdown of carbohydrates and fat during aerobic exercise. With increased mitochondrial density there is more energy available for the working muscles to produce greater force, and for a longer period of time (i.e., such as running longer at a higher intensity). In a 6-week training study, Burgomaster et al. (2008) showed similar increases in oxidative enzyme levels (proteins in mitochondria that accelerate biological reactions to liberate ATP) among subjects who performed a HITT program with four to six 30-second maximal cycling sprints (followed by 4.5 minute recovery bouts) on 3 days/week and subjects who completed 40-60 minutes of continuous endurance of steady cycling at 65% VO2max on 5 days/week. An increase in these mitochondrial oxidative enzymes leads to more effective fat and carbohydrate breakdown for fuel. Related work performed by MacDougall et al. (1998) demonstrated increased skeletal muscle oxidative enzyme levels of citrate synthase (36%), malate dehydrogenase (29%), and succinate dehydrogenase (65%) among healthy male undergraduate students engaging in 7 weeks of HIIT cycling sprints. Three days per week the subjects performed between four to ten 30-second maximal cycling sprints followed by a four-minute recovery. The higher levels of mitochondrial enzymes seen among the subjects led to improved skeletal muscle metabolic function.

There has been a spike of current research explaining the complex molecular pathways that lead to increased mitochondrial density. HIIT can lead to analogous physiological changes that are observed in traditional endurance training, yet this is accomplished through different message signaling pathways.

Molecular Biology Focus Paragraphy. Signaling Pathways of Continuous Endurance Training and HIIT
Source: Laursen 2010. In this model calcium-calmodulin kinase (CaMK) and adenosine monophosphate kinase (AMPK) are signaling pathways that activate peroxisome proliferator-activated receptor-g coactivator-1alpha (PGC-1 alpha). PGC-1alpha is like a “master switch'' that is believed to be involved in promoting the development of the skeletal muscle functions shown. High-volume training appears more likely to operate through the CaMK pathway and high-intensity training appears more likely to signal via the AMPK pathway.

HIIT vs. Continuous Endurance Exercise: Metabolic Adaptations
Increasing mitochondrial density can be considered a skeletal muscle and metabolic adaptation. One focal point of interest for metabolic adaptations is with the metabolism of fat for fuel during exercise. Because of the nature of high intensity exercise, the effectiveness of this type of training for fat burning has been examined closely. Perry et al. (2008) showed that fat oxidation, or fat burning was significantly higher and carbohydrate oxidation (burning) significantly lower after 6 weeks of interval training. Similarly, but in as little as two weeks Talanian et al. (2007) showed a significant shift in fatty acid oxidation with HIIT. In their research review, Horowitz and Klein (2000) summarize that an increase in fatty acid oxidation is a noteworthy adaptation observed with continuous endurance exercise.

Another metabolic benefit of HIIT training is the increase in post-exercise energy expenditure referred to as Excess Post-exercise Oxygen Consumption (E.P.O.C.). Following an exercise session, oxygen consumption (and thus caloric expenditure) remains elevated as the working muscle cells restore physiological and metabolic factors in the cell to pre-exercise levels. This translates into higher and longer post-exercise caloric expenditure. In their review article, LaForgia, Withers, & Gore (2006) note that exercise intensity studies indicate higher E.P.O.C. values with HIIT training as compared to continuous aerobic training.

Final Verdict: And the Winner of the Battle of the Aerobic Titans is…
The major goals of most endurance exercise programs are to improve cardiovascular, metabolic, and skeletal muscle function in the body. For years continuous aerobic exercise has been the chosen method to achieve these goals. However, research shows that HIIT leads to similar and in some cases better improvements in shorter periods of time with some physiological markers. Incorporating HIIT (at the appropriate level of intensity and frequency) into a client's cardiovascular training allows exercise enthusiasts to reach their goals in a very time efficient manner. And, since both HIIT and continuous aerobic exercise programs improve all of these meaningful physiological and metabolic functions of the human body, incorporating a balance of both programs for clients in their training is clearly the 'win win' approach for successful cardiovascular exercise improvement and performance. GO HIIT and GO Endurance!

Side Bar 1. HIIT Program Development
When developing a HIIT program the duration, intensity, and frequency of the interval must be considered along with the length of the recovery interval. Duration of the work (high intensity effort) bout should be between 5 seconds to 8 minutes. Power athletes tend to perform shorter work intervals (5 sec - 30 sec) while endurance athletes will extend the high intensity work interval (30 sec - 8 min) (Kubukeli, Noakes & Dennis, 2002). Intensity during the high intensity work bout should range from 80% to greater than 100% of maximal oxygen consumption (VO2max), heart rate max, or maximal power output. The intensity of the recovery interval ranges from passive recoveries (doing very little movement) or active recoveries (which is more common) of about 50-70% of the above described intensity measures.

The relationship of the work and recovery interval is also a consideration. Many studies use a ratio of exercise to recovery, for example a ratio of 1:1 could be a 30-second interval followed by 30 seconds of recovery. A ratio of 1:2 would be a 30-second interval followed by a 1-minute rest. Typically, the ratios are designed in order to challenge a particular energy system of the body.
Read through the sample workouts that follow. These workouts have been used in previous research studies to induce both cardiovascular and skeletal muscle changes. Each component of a training session is included.

Sample 1: Track workout
Warm-up: Light 10-min run around track.
Interval: 800-meter runs at approximately 90% of maximal heart rate (based on estimation heart rate max = 220-age). Each 800-meter interval should be timed.
Rest Interval: Light jog or walk for same amount of time it took to run each 800 meter
Work/Rest ratio: 1 to 1 ratio. The time for the interval (800 meter) and rest interval should be the same.
Frequency: Try to complete 4 repetitions of this sequence.
Cool Down: 10-min easy jog.
Comments: The distance of the interval can be adjusted from 200 meter to 1000 meter. Also, the length of the rest interval can be adjusted.
Adapted from Musa et al. (2009).

Program 2: Sprint training workout
Warm-up: 10 min of light running.
Interval: 20-second sprints at maximal running speed.
Rest interval: 10 seconds of rest between each sprint. Light jogging or walking
Work/Rest ratio: 2 to 1 ratio. The work interval is 20-sec and rest interval is 10-sec.
Frequency: 3 groups or sets of 10-15 intervals. Take 4 min of rest between each set
Cool Down: 10 min easy jog
Comments: This is a sprint workout. The first few intervals should be slower allowing muscles to adapt to the workout. It is important to be safe and careful avoiding muscle damage during maximal sprinting exercise. The warm-up session is very important.
Adapted from Tabata et al. 1996.

Program 3: Treadmill workout
Warm-up: 10 min of light jogging.
Interval: Set treadmill incline at 5% grade and speed at 3 mph. During each high intensity interval increase speed to 5 mph - 6.5 mph, while keeping grade at 5%. The length of the interval should be 1 min.
Rest Interval: 2-minute rest interval with the walking speed set to 3 mph. Do not adjust incline.
Work/Rest Ratio: 1 to 2 ratio. The work interval is 1-minute and the rest interval is 2-minutes
Frequency: 6-8 repetitions of this sequence.
Cool Down: 5 - 10-minutes of easy jogging
Comments: This is a hill running interval session. Incline, running speed, interval length, and rest interval can be adjusted during the interval session.
Adapted from Seiler and Hetlelid, 2005.

Side Bar 2: Four Great Endurance Programs Ideas
The following 4 endurance exercise programs are adapted from the research investigations reviewed by LaForgia, Withers, & Gore, 2006. Perform an adequate warm-up (~10 min of light exercise) and cool-down (~5-10 min of low-intensity exercise) for each program. All of the workouts below can be performed on any aerobic mode.

1) Maximal lactate steady state exercise.
The maximal lactate steady state (MLSS) workout is the highest workload an exerciser can maintain over a specified period of time. MLSS exercise work bouts can last between 20 and 50 minutes. Tell the client to work at his/her maximal steady state of exercise for the desired time (between 20 and 50 minutes).

2) Alternating aerobic modes endurance exercise.
Alternate aerobic modes (i.e., treadmill and elliptical trainer) every 20 to 40 minutes of aerobic exercise keeping the exercise intensity &Mac179;70% of heart rate max. Keep the time on each mode of exercise that same. Number of alternating modes is dependent on fitness level of client.

3) Step-wise endurance exercise.
With step-wise endurance exercise the client progresses from 10 minutes (at &Mac179;50% heart rate max) to 10 minutes (at &Mac179;60% heart rate max) to 10 minutes (at &Mac179;70% heart rate max) on any aerobic mode. For a slight modification, the personal trainer may have the client step-wise up in intensity and also step-wise down on this workout. Thus, after completing the 10 min at a 70% heart rate max the client would switch to 10 minutes at 60% heart rate max and then 10 minutes at 50% heart rate max.

4) Mixed-paced endurance exercise.
On the selected mode of exercise randomly vary the endurance duration (i.e., 5 min, 10 min, 15 min blocks of time) and the intensity of exercise. For instance, a 45 minute endurance treadmill workout could begin with 10 min at 50% heart rate max, then sequence into 5 min at 70% heart rate max, then 15 min at 60% heart rate max, then 10 min at 75% heart rate max, and finish with 5 min at 50% heart rate max.

Fact Box 1: Seven Remarkable Endurance Feats of Interest
1. The official International Association of Athletics Federations world Marathon record for men is 2:03:59, set by Haile Gebrselassie of Ethiopia on September 28, 2008 at the Berlin Marathon.

2. The women's record holder in the marathon is Paula Radcliffe of the United Kingdom in a time of 2:15:25.

3. The predicted human capability of the marathon based on physiological characteristics as describe by Joyner (1991) is 1:57:58. This equals a 4:30 per mile pace.

4. The longest certified road race in the world is the 3100 mile Self-Transcendence Race in New York City that takes place around a half-mile city block in Queens, NY. Only 30 runners have completed the race, which requires each contestant to complete 2 marathons per day for 50 days.

5. The longest bicycle race is the Tour d'Afrique, which is 12,000 km (7500 miles) and 120 days traveling from Cairo, Egypt to Cape Town, South Africa.

6. One of the longest swims ever was recorded by Martin Strel in 2009. The Slovenian man swam the length of the Amazon River (3,272 miles) in 66 days.

7. Another outstanding swimming feat was a performance by Benoit Lecomte, who swam across the Atlantic Ocean from Cape Cod to France, which is 3,736 miles. He averaged six to eight hours per day of swimming.

Side Bar 3: Four Important Questions and Answers Regarding HIIT
1. How many times per week can HIIT be completed?
A: Research says that three times per week may produce the best results while limiting injury (Daussin et al., 2008; Helgerud, et al., 2007; Musa, et al., 2009; Perry, et al., 2008). Interval training is very demanding and it is important to be fully recovered between sessions.

2. Barefoot running has grown in popularity in the past several years. Is it safe to perform HIIT barefoot?
A: According to Pauls and Kravitz (2010) it is important to progress slowly into barefoot running regardless of intensity. The best method may be to perform other daily living activities such as walking, cleaning, or gardening before beginning to run. Once you are consistently barefoot running there should be another progression with HIIT. Begin with one or two barefoot intervals and increasing to three or four over a several weeks is the best recommendation.

3. If a client has been inactive for several months is it safe to start an exercise program with HIIT?
A: There should be a careful progression of activity when re-starting any exercise program. Beginning with HIIT may increase the chance for injury and muscle soreness. A better approach would be to start with continuous aerobic exercise at a low intensity level. Once the client is able to run for thirty consecutive minutes at a moderate intensity he/she can then progress slowly into interval training.

4. Is it O.K. to before a high intensity interval training session?
A: When exercise intensity is greater than 80% the rate at which the contents of stomach empty slow down (de Oliveira & Burini, 2009). This is due to changes in blood flow along with hormonal and neurotransmitter activation. To reduce the chance of gut problems during exercise eat foods that are low in fiber, lactose, and nutritive sweeteners several hours before a training session, and be sure to drink plenty of fluids (de Oliveira & Burini, 2009).
Bartels, M.N., Bourne, G.W., and Dwyer, J.H. (2010). High-intensity exercise for patients in cardiac rehabilitation after myocardial infarction. Physical Medicine and Rehabilitation. 2(2), 151-155.

Billat, L. V. (2001). Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part I: aerobic interval training. Sports medicine, 31(1), 13-31.

Burgomaster, K.A., Howarth, K.R., Phillips, S.M., Rakobowchuk, M., Macdonald, M.J., McGee, S.L., and Gibala, M.J. (2008). Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans. Journal of Physiology, 586(1), 151-160.

Daussin, F.N., Zoll, J., Dufour, S.P., Ponsot, E., Lonsdorfer-Wolf, E. et al. (2008). Effect of interval versus continuous training on cardiorespiratory and mitochondrial functions; relationship to aerobic performance improvements in sedentary subjects, American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 295, (R264-R272.

de Oliveira, E.P. and Burini, R.C. (2009). The impact of physical exercise on the gastrointestinal tract. Current Opinion in Clinical Nutrition and Metabolic Care, 12(5), 533-538.

Gibala, M. (2009). Molecular responses to high-intensity interval exercise. Applied Physiology, Nutrition, and Metabolism, 34(3), 428-432.

Helgerud, J., Høydal, K., Wang, E., Karlsen, T., Berg, P., et al. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and Science in Sports and Exercise, 39(4), 665-671.

Horowitz J.F. and Klein S. (2000). Lipid metabolism during endurance exercise. American Journal of Clinical Nutrition. 72(2 Suppl), 558S-563S.

Joyner M.J. (1991). Modeling: optimal marathon performance on the basis of physiological factors. Journal of Applied Physiology. 70(2), 683-687.

Joyner, M.J. and Coyle, E.F. (2008). Endurance exercise performance: the physiology of champions. Journal of Applied Physiology, 586 (1), 35-44.

Kubukeli, Z.N., Noakes, T.D., and Dennis, S.C. (2002). Training techniques to improve endurance exercise performances. Sports Medicine, 32(8), 489-509.

LaForgia, J., Withers, R.T., and Gore, C.J. (2006). Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. Journal of Sports Science, 24(12), 1247-1264.

Laursen, P. B. (2010). Training for intense exercise performance: high-intensity or high-volume training? Scandanavian Journal of Medicine and Science in Sports, 20 Suppl 2, 1-10.

MacDougall, J.D., Hicks, A.L., MacDonald, J.R., McKelvie, R.S., Green, H.J., and Smith, K.M. (1998). Muscle performance and enzymatic adaptations to sprint interval training. Journal of Applied Physiology, 84(6), 2138-2142.
Musa, D.I., Adeniran, S.A., Dikko, A.U., and Sayers, S.P. (2009). The effect of a high-intensity interval training program on high-density lipoprotein cholesterol in young men. Journal of Strength and Conditioning Research, 23(2), 587-592.

Pauls, C. and Kravitz, L. (2010). Barefoot running: An exciting new training dimension to consider for certain clients. IDEA Fitness Journal, 7(4), 18-20.

Pavlik, G., Major, Z., Varga-Pintér, B., Jeserich, M., & Kneffel, Z. (2010). The athlete's heart Part I (Review). Acta Physiologica Hungarica, 97(4), 337-353.

Perry, C.G.R, Heigenhauser, G.J.F, Bonen, A., and Spriet, L.L. (2008). High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle. Applied Physiology, Nutrition, and Metabolism, 33(6), 1112-1123.

Seiler, S., and Hetlelid, K.J. (2005). The impact of rest duration on work intensity and RPE during interval training. Medicine and Science in Sports and Exercise, 37(9), 1601-1607.

Slørdahl, S.A., Madslien, V.O., Støylen, A., Kjos, A., Helgerud, J., and Wisløff, U. (2004). Atrioventricular plane displacement in untrained and trained females. Medicine and Science in Sports and Exercise, 36(11), 1871-1875.

Tabata, I., Nishimura, K., Kouzaki, M., Hirai, Y., Ogita, F., Miyachi, M., and Yamamoto, K. (1996). Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Medicine and Science in Sports and Exercise, 28(10), 1327-1330.

Talanian, J.L., Galloway, S.D., Heigenhauser, G.J, Bonen, A., and Spriet, L.L. (2007). Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. Journal of Applied Physiology, 102(4), 1439-1447.

Wisløff, U., Ellingsen, Ø., and Kemi, O. J. (2009). High-intensity interval training to maximize cardiac benefits of exercise training? Exercise Sport Science Review, 37(3), 139-146.

Micah Zuhl, Ph.D. is an assistant professor in the Department of Health Sciences at Central Michigan University. His research interests include the GI tract and exercise, sports performance and disease prevention through exercise. Micah is a recreational cyclist, runner, and hiker.

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and was chosen as the American Council on Exercise 2006 Fitness Educator of the Year.