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: 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!
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