High-intensity interval training (HIIT) has become popular because time is our most precious resource and no one wants to waste it doing exercise that won’t deliver results. HIIT burns calories, strengthens muscles and improves health with workouts that take less than 20 minutes to complete, including the warm-up and cool-down phases. However, while HIIT can be highly effective, it’s important to understand how to use it correctly so clients experience results with a minimal risk of developing overtraining syndrome or overuse injuries. The high-intensity nature of HIIT means that it places a tremendous amount of stress on the muscular and cardiorespiratory systems; as such, most people should limit their HIIT workouts to no more than three per week, and rest for at least 48 hours between each session. This article explains how to use the Cardiorespiratory Training component of the ACE Integrated Fitness Training^® (ACE IFT) Model to design effective HIIT workouts for individuals and groups.

Adaptations Are Created by Overload 

The overload principle describes how progressively challenging levels of exercise intensity can result in physiological adaptations. There are two specific types of overload: mechanical and metabolic. Mechanical overload refers to the physical forces applied directly to the protein structures of muscle fiber. When muscles generate force against an external resistance, it can damage the fibers, which must then be repaired. For the purposes of this article, however, we’ll focus on metabolic overload, which occurs when exercising to the point of fatigue and depleting all available energy. The high-intensity intervals that characterize HIIT create a metabolic overload that challenges muscles to produce the energy needed to fuel muscle activity and to remove the metabolic byproducts created by muscle activation. 

Muscles metabolize energy to fuel contractions via three distinct pathways: immediate, long-term or intermediate. The immediate pathway draws from the limited amount of adenosine triphosphate (ATP), the chemical used to fuel muscle activity, that is stored in muscle cells. When this stored ATP is depleted, which happens very quickly, muscle cells can metabolize more ATP one of two ways: with oxygen (aerobically) or without it (anaerobically). 

The long-term pathway is generally utilized at lower levels of exercise intensity, when type I muscle fibers use aerobic respiration, which requires oxygen to help metabolize fat into ATP. The intermediate pathway occurs at higher levels of exercise intensity, when type II muscle fibers metabolize glycogen (the form in which carbohydrates are stored in muscle cells) into glucose and then into ATP, either aerobically or anaerobically. Aerobic glycolysis is the process of using oxygen to metabolize glucose into ATP, which can yield approximately 36 to 39 molecules of ATP per molecule of glucose. By contrast, anaerobic glycolysis does not require oxygen for ATP production and produces 2 to 3 molecules of ATP per molecule of glucose. It’s worth noting that variance of ATP production is based on whether the glycogen has to first be converted to glucose or if ATP is produced directly from glucose in the blood (Kenney, Wilmore and Costill, 2015). The byproduct of glycolysis is an accumulation of hydrogen ions, pyruvate and lactate, which can elevate blood acidity, causing the burning sensation felt during high-intensity exercise. 

Table 1 depicts the amount of ATP that can be supplied by different energy pathways. As exercise intensity increases, the yield of ATP per molecule of substrate decreases. Anaerobic glycolysis produces only 2 to 3 ATP per molecule of glucose, which explains why HIIT burns so many more net calories than lower-intensity exercise (in the so-called fat-burning zone), where 1 molecule of fat produces a significant amount of ATP. In other words, during lower-intensity exercise, where fats are the predominant fuel source, only 1 molecule of fat is needed to provide more than 100 molecules of ATP. In contrast, higher-intensity exercise, where carbohydrates are the predominant source of energy, generates just 2 to 3 molecules of ATP from each molecule of carbohydrate. This means that many more carbohydrate molecules would be needed to yield the same number of ATP produced from 1 molecule of fat. For example, if 1 molecule of fat was metabolized to 104 molecules of ATP, it would take 52 molecules of carbohydrate to yield the same number of ATP.

Table 1. Levels of ATP Supplied by the Different Energy Pathways

A System for Organizing Exercise Programs

The ACE IFT Model offers a systematic approach for designing exercise programs that help individual clients and groups achieve results ranging from improved health and stronger muscles to enhanced performance in competitions. The ACE IFT Model is organized into two main components—Cardiorespiratory Training and Muscular Training—with each component featuring three levels of progression.

The three phases of the Muscular Training component—Functional, Movement and Load/Speed Training—are designed to improve overall postural stability and joint mobility, movement pattern efficiency and muscle force production and speed, respectively, through the application of progressively challenging strength-training workouts. The three phases of the Cardiorespiratory Training component—Base, Fitness and Performance Training—can help clients burn calories, improve health markers and enhance their aerobic capacity by applying a structured approach for programming workouts using different levels of intensity. 

The Need for a New Way to Determine Exercise Intensity

Using an age-based formula (usually 220 minus age) for estimating maximum heart rate was once standard operating procedure for most exercise professionals, despite the fact that these formulas were not scientifically validated and produced margins of error that could be considered “unacceptably large.” When the ACE IFT Model was introduced in 2010, it featured a new way of designing cardiorespiratory exercise programs that was based on efficiency by identifying a client’s heart rate at the first ventilatory threshold (VT1). Unlike using prediction equations to determine an estimated target heart rate, the use of ventilatory thresholds offers an individualized approach for assessing and monitoring exercise intensity based on a person’s unique ventilatory response to increasing workloads during submaximal exercise. 

When clients are exercising at an intensity where they can talk comfortably, such as walking on a treadmill or doing a flat road during an indoor cycling class, their muscles are relying on aerobic respiration and they are generally working at an intensity below VT1. Increasing the incline or pace of a client’s treadmill or challenging a class to add resistance to their indoor cycling bikes increases the exercise intensity and the need for more energy, so working muscles will start using glycolysis to produce ATP. Glycolysis results in expiring (breathing out) a greater amount of carbon dioxide (CO₂), which explains why breathing becomes quicker during more intense exercise—the lungs are pushing out CO₂ while simultaneously trying to pull in more oxygen. This increased ventilatory rate limits the ability to speak. You might think of CO₂ as the exhaust product of muscles using glycogen to generate ATP; the faster the lungs can push it out, the more efficient the ability to sustain a high work rate.

The talk test can be used to determine the heart rate at VT1 and to create a three-zone training model in which zone 1 is below VT1 and relies on the aerobic respiration pathway for ATP. Zone 2 represents heart rates at and above VT1 but below VT2 and ATP is produced from glycolysis. Zone 3 training occurs when intensities are at and above VT2 and energy demands are met by stored ATP and anaerobic glycolysis. This link describes the specific protocol for applying the talk to test to an individual client, while this link presents a video demonstration of how to apply the test 

The second ventilatory threshold (VT2) corresponds with the onset of blood lactate accumulation (OBLA; also referred to as the respiratory compensation threshold), which represents an exercise intensity at which the muscles are using ATP at a rate that is faster than the rate at which the metabolic byproduct of anaerobic metabolism can be removed. Pyruvate and lactate, both byproducts of anaerobic glycolysis, can be utilized during lower-intensity exercise to produce ATP, which explains why active-recovery intervals are used during HIIT workouts. An indicator of OBLA is a client who is breathing so fast that they can only say one or two words at a time or can only grunt in response to your cues. This client will only be able to sustain this intensity for a brief period of time before needing a recovery interval to remove metabolic byproducts and to produce new ATP. 

That burning sensation in muscles is kind of like having a full recycling bin—you have to remove the current recycling before more recycling can take place. Over the long term of an exercise program, HIIT can train muscles to tolerate working at OBLA as well as improve the ability to quickly remove metabolic byproducts and produce new ATP.

Designing Cardiorespiratory Training Programs Using the ACE IFT Model

The three phases of the Cardiorespiratory Training component—Base Training, Fitness Training and Performance Training—correspond to the three metabolic pathways discussed above. Whereas the Base Training phase is performed at an intensity below VT1, the start of the Performance Training phase is identified by VT2 and can be determined with the VT2 threshold assessment. Between these two metabolic markers is the Fitness Training phase. 

Exercise programs in the Base Training phase should cause clients to breathe faster than normal yet be able to maintain a normal conversation. The Fitness Training phase helps clients become more efficient at using glycolysis for energy metabolism by improving the ability to convert glycogen into ATP and remove metabolic byproducts. When exercising in the Fitness Training phase, clients should be breathing rapidly and capable of only saying short phrases or a few words at a time. Finally, Performance Training occurs at the highest level of intensity, where muscles rely on stored ATP and anaerobic glycolysis for the energy to fuel muscle activation and the accumulation of metabolic byproducts elevates blood acidity, which causes discomfort. Clients exercising in the Performance Training phase are working so hard they will only be able to say one or two words, at most. 

HIIT should alternate between high-intensity work intervals in the Performance Training phase that last 10 to 90 seconds and recovery intervals in which the intensity is consistent with the Base Training phase. During the high-intensity intervals, breathing should be extremely fast to the point of being breathless, while breathing during the recovery intervals should return to a more controllable, sustainable rate.

Measuring the Intensity of Cardiorespiratory Training  

Most modern fitness trackers and smartwatches feature an integrated heart-rate monitor, which makes it easy to monitor heart rate during exercise. However, monitoring the breathing rate and using a 0 to 10 rating of perceived exertion (RPE) scale could be more efficient and doesn’t require clients to invest in or wear a tracker. The 0 to 10 RPE scale can be used to help clients determine intensity based on the feeling of exercise intensity, with 0 indicating an intensity level comparable to being at rest and 10 representing near maximal or maximal effort. High intensity may be considered an RPE of 7 to 10, while the lower-intensity, active-recovery intervals are performed at an effort level of 3 to 4. A review of the research by Eston (2012) found that using the RPE scale can be a reliable method for determining exercise intensity. 

Table 2 describes the Cardiorespiratory Training phases, metabolic markers, energy pathways and estimated length of time at intensity for various RPE levels.

Table 2. Rating of Perceived Exertion and Metabolic Markers of the ACE IFT Model

How to Program HIIT for Your Clients

HIIT should alternate between high-intensity intervals at an RPE of 7 to 10 and low-intensity recovery intervals at an RPE of 3 to 4. While there are several HIIT protocols, there is no single format that works for every individual, which explains why it’s important to know how to supervise clients (and teach them to self-monitor) to ensure they are receiving the appropriate amount of recovery. 

High-intensity work intervals result in metabolic byproducts, which can impede muscle activation. The purpose of the recovery interval is to allow byproducts to be removed, while allowing time for new ATP to be produced. As discussed earlier, type II muscle cells can become more efficient at removing metabolic byproduct, producing new ATP and become capable of storing more glycogen, which is how clients improve their ability to sustain high-intensity exercise for longer periods of time. Many clients will be interested in HIIT for weight loss, but this type of program could also result in muscle growth. Glycogen stored in muscle cells hold on to water (as glycogen is used for energy it releases the water, which is what causes sweat). As the amount of stored glycogen (and water) increases, the result is an increase in overall muscle size.

When designing HIIT workouts, begin by programming approximately two to eight minutes of HIIT intervals alternating with two to eight minutes of lower-intensity exercise to allow a complete recovery. For example, the standard Tabata protocol is 20 seconds of exercise at an RPE of 9 to 10, followed by 10 seconds of passive rest, which is repeated eight times for a total of four minutes. After a four-minute Tabata protocol, programming a series of lower-intensity body-weight or strength-training exercises allows clients and class participants to experience a complete recovery, which can be followed by another round of high-intensity intervals. 

Table 3 outlines three sample HIIT workouts that can be performed using equipment available in most fitness facilities. For best results, select exercises or equipment that involve more muscle mass; ergometers, which increase resistance the faster the exerciser moves, are particularly good options that make it easy to change speeds for interval training. For example, if given a choice, choose a stationary bike that features arm movements as well as pedaling. Another option is a rowing machine, which requires the arms and legs to work at the same time. Self-powered treadmills, where the running pace of the user determines the speed of the treadmill belt, are another good option, especially for longer intervals. Additional equipment options include jump ropes or heavy ropes, especially for shorter interval periods. Powered treadmills, stair steppers and elliptical runners are not recommended because they do not allow for easy transitions between speeds. 

Table 3. Three Sample HIIT Workout Protocols

Note: HIIT = High-intensity interval training; RPE = Rating of perceived exertion

Conclusion

Understanding how to apply the different phases of the Cardiorespiratory Training component of the ACE IFT Model can result in effective HIIT workouts. However, it’s important to remember that not every cardio workout needs to be performed to the point of fatigue. Steady-state training (SST) in the Base Training phase of the ACE IFT Model involves exercising at a consistent rate of intensity that corresponds to an RPE of approximately 3-4. Performing SST the day after an anaerobic HIIT workout uses a different energy pathway and allows a client to experience the benefits of exercise without placing undue stress on the body. SST remains an effective mode of exercise and should be considered as an effective option when added to clients' workouts to help develop overall aerobic conditioning.