WHEN DID IT BECOME A HIIT?
Imagine running through the woods, accelerating up slopes and around bends and decelerating only when you feel the need to, basing your 1- to 2-hour training on terrain and self-judgment alone. This type of running revolutionized athletic interval training in the 1930s and today is known as Fartlek running or “speed play.” It was developed in Sweden by Gosta Holmer (12). Later, two Germans, Woldemar Gerschler and cardiologist Herbert Reindel, decided Fartlek running did not offer enough precision when developing a training regimen. In particular, it did not give the opportunity to measure progression. Consequently, they developed the first interval training program based on heart rate (HR) responses (12). Their program stressed running short bouts (100–450 m) at high intensity until an HR of 180 BPM was reached. The participant was then given 90 seconds of rest for HR to decrease to 120 BPM. If the HR did not decrease to this level, the workout was terminated. From their work, predictive relationships were developed between average workout speed and race pace. The wide adoption of interval training led to the steady improvement in world record performances from the 400 m to the marathon. In the 1990s, interval training for clinical populations was popularized by the work of Katharina Meyer from Germany. She demonstrated that, after coronary bypass surgery, patients benefited more from interval training than continuous exercise (8). During the last decade, interval training has shown benefit in almost every population.
INTERVAL TRAINING TODAY
To prevent chronic disease, the American College of Sports Medicine (ACSM) recommends accumulating 150 minutes of moderate-intensity exercise or 75 minutes of vigorous-intensity exercise a week. Less than half of Americans meet the current activity guidelines (15). Lack of time is the leading perceived barrier to exercise (11). Accordingly, exercise programs have leaned toward workouts that are time efficient. Interval training has been suggested as an answer for time-crunched Americans trying to achieve cardiovascular health.
Interval training involves alternating periods of hard work with periods of either relative or complete rest. The work load usually is expressed relative to power output or speed, and recovering to a target HR is emphasized. It is different from the more commonly prescribed moderate-intensity continuous exercise (MICE), which involves maintaining a constant intensity throughout the duration of the exercise period. Because of the oscillation of intensity, interval training offers a break from the redundant and often monotonous regularity of continuous training. Interval training usually refers to high-intensity interval training (HIIT), where work intervals often exceed 90% of HR reserve (HRR). However, moderate-intensity interval training (MIIT) has been incorporated into fitness programs where high-risk individuals seek the benefits of interval training but are not cleared for HIIT. Early research has shown that interval training may be more enjoyable and mentally engaging than MICE (2). This is important when considering that adherence to a long-term change in physical activity behavior is only about 50% after 12 months (6). Claims that an individual may achieve similar health and fitness benefits from HIIT as MICE, often in less than half the time, have launched extensive research into studying the benefits of interval training.
The physiological adaptations and benefits of interval training are largely dependent on the intensity and duration of work intervals. Studies have demonstrated that HIIT may improve V˙O2max, endothelial function, muscle oxidative capacity, resting muscle glycogen content, H+ ion buffering, and time to exhaustion (3,4,10). In addition, it has been demonstrated that abdominal and total fat loss is significantly greater with HIIT compared with MICE (14).
The enhanced metabolic effect of HIIT may be caused by the recruitment of both type I and II muscle fibers, which leads to increased glycogen usage during exercise. This in turn leads to enhanced glucose uptake after the workout is terminated. Studies have demonstrated significant decreases in blood glucose for up to 24 hours after high-intensity low-volume HIIT and a significant increase in skeletal muscle GLUT4 protein after 2 weeks of HIIT training (7). The GLUT4 protein is primarily responsible for the uptake of glucose into skeletal muscle and is a frequent marker of the effect of training on carbohydrate metabolism. This makes HIIT especially appealing to individuals at risk for type 2 diabetes mellitus.
Prescription of interval training can be tailored to almost every population and can be done anywhere at almost any time. Depending on the capabilities of the individual, trainers can manipulate work interval intensity and duration, relief interval intensity and duration, exercise modality, sets and repetitions, and recovery intensity and duration between sets. In general, the duration of the hard interval is the primary variable that is manipulated. Relatively long work intervals paired with short recovery intervals are perceived as very demanding. Conversely, short work intervals paired with long recovery intervals are comparatively easy.
To avoid injury, it is best to establish an initial base fitness level. This is accomplished by participating in aerobic training 3 to 5 times a week for 20 to 60 minutes per session at a moderate intensity and strength training at least two nonconsecutive days per week for several weeks. Once a base level of fitness is established, it may be wise to start with MIIT and then progress to the more challenging HIIT. During MIIT, work intervals are conducted at 60% to 80% of HRR (rating of perceived exertion of 13–14); during HIIT, work intervals are conducted at 90% of HRR (rating of perceived exertion >15). With both MIIT and HIIT, rest intervals should be at very low intensity to allow for adequate recovery.
Depending on the goals of the individual, prescription usually will fall within three categories: 1) long intervals of 3 to 15 minutes, 2) moderate intervals of 1 to 3 minutes, or 3) short intervals of 10 seconds to 1 minute. Keep in mind that higher intensity intervals recruit more type II muscle fibers, which is beneficial for glucose metabolism. Even very short duration bursts, repeated several times, can result in substantial accumulation of work and provoke adequate adaptations (7).
Many public gyms commonly are prescribing HIIT based on the concept of “Tabata training.” Tabata training was developed initially by Dr. Izumi Tabata for Olympic speed skaters. Tabata training typically was conducted on cycle ergometers with the goal being to complete 8 rounds of 20 seconds at 170% of V˙O2max interspersed with 10 seconds of recovery (13). Today, most Tabata classes are completed at an intensity much lower than 170% of V˙O2max, but the same 20:10 work-to-rest ratio is used. This kind of high-intensity exercise is only appropriate for individuals medically approved to complete vigorous activity.
There are many popular models of HIIT. Four universally seen and commonly used protocols are summarized in the Table. The Wingate test is used to measure anaerobic capacity and is used commonly as a popular training protocol for HIIT. Individuals are asked to pedal as hard as they can for 30 seconds against a predetermined constant force based on their body weight. After the all-out bout, the participant gets 4 minutes of recovery. This protocol is repeated four to six times. The Wingate protocol is very strenuous and often elicits unpleasant feelings, which decreases training compliance. A more conventional model is repeated bouts of 60 seconds of exercise conducted at more than 90% of HRR, followed by 60 seconds of active recovery. This is repeated up to 10 times or 20 minutes. A popular model used in cardiac rehabilitation programs consists of 4 minutes of work intervals at 85% to 95% of maximal heart rate (MHR) separated by 3 minutes of easy intervals at 60% to 70% MHR. This is repeated up to four times (16).
For elite endurance athletes, training based on the ventilatory threshold (VT1) and respiratory compensation threshold (VT2) is most beneficial. A common approach is to spend 75% of time below VT1, 10% between VT1 and VT2, and 15% above VT2 (5). Incorporating some sort of HIIT training into endurance training regimens allows for adaptation to occur through different molecular signaling pathways, which translates to enhanced athletic performance. Regardless of population, modality, or interval-to-rest ratio chosen, proper warm-up and cool-down are required to prepare the cardiovascular system for the amount of stress that will occur with any interval training session.
The modality for HIIT should be specific to the individual’s goal (e.g., runners run and cyclists cycle). Traditionally, HIIT has been done on the treadmill, leg-cycle ergometer, arm-crank ergometer, swimming pool, or track. However, more recently, protocols are focusing on resistance training by using body weight, bands, or plyometrics. Replacing traditional cardio equipment with body weight exercises such as high knees, squats, jumping lunges, and side-step shuffle can remove the exerciser from the traditional gym setting and keep their mind busy, concentrating on switching between exercises. Studies have shown that combing upper- and lower-body aerobic HIIT programs will elicit enhanced exercise tolerance and muscle hypertrophy in both leg and trunk muscles (9), suggesting that a combination upper- and lower-body HIIT workout may be the best to provoke total body adaptations.
Interval training has been shown to benefit almost every population. It is safe for low-risk individuals as well as moderate-risk individuals cleared by their physician. High-risk individuals and those with known cardiovascular disease should only participate in interval training under direct medical supervision or when working with an ACSM Registered Clinical Exercise Physiologist® (1).
HIIT can provide a time-efficient, beneficial, and enjoyable workout to a variety of populations. However, because of the versatility of HIIT protocols, research cannot be standardized. Therefore, future studies on interval training need to look more specifically at the best combination of intensity, duration, frequency, and timing of work-to-rest intervals to quantify the best protocol within each population.
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