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How HIIT Helps Endurance Athletes Improve Performance

Fresh off the successes of the Japanese speed-skating team’s four-medal performance at the 1992 Winter Olympics, head coach Kouichi Irisawa asked his associate Izumi Tabata, PhD, to analyze the training techniques that had boosted the team’s performance. What Tabata learned would have implications far beyond the speed-skating community.

Loosely stated, Tabata found that approximately 4 minutes of extremely intensive exercise 4 days per week improved the body’s ability to deliver oxygen to muscle (VO2max) at about the same rate as 60 minutes of medium (but challenging) intensity 5 days per week (Tabata et al. 1996).

Tabata was hardly the first to examine the benefits of high-intensity exercise, but two decades later, Tabata-style training has captured the imagination of the health and fitness profession and is a program-design darling.

High-intensity interval training, or HIIT, does more than improve VO2max: It has been shown to increase muscles’ ability to use oxygen efficiently to burn stored and circulating fuel for work. Efficiently processing glucose (carbohydrates) and lipids (fats) during exercise contributes to athletic performance and overall health and fitness (Gibala & McGee 2008).

Research has also noted that HIIT improves the processing of exercise byproducts and boosts mitochondria biogenesis—the process of increasing the number of mitochondria (Jacobs et al. 2013).

Achieving these benefits in less time than was previously considered necessary essentially rules out the excuse that people do not have enough time to exercise. That makes HIIT modalities such as high-intensity cardiovascular intervals and intensive strength training appealing to time-challenged clients looking to improve overall health and stamina.
All these benefits are great in their own right, but one of the most fascinating things about HIIT is its ability to help athletes improve their performance in endurance events. How can a few
minutes of intense exercise make a difference in events that last for hours? Let’s find out.

HIIT and Endurance Performance

Let’s start by looking at traditional endurance training, which increases the duration of steady-state but low-intensity training (LIT) to improve oxygen and fuel use in muscle fibers. Longer was always better during the running and triathlon booms from the mid-1970s through the end of the 20th century—heavily influenced by legendary marathon performances and popular events like the Hawaii Ironman Triathlon® and the Tour de France®.

Even now, LIT protocols account for more than 70% of most endurance programming, with HIIT and moderate-intensity training (MIT) filling out the balance of annual training plans. Recreational endurance athletes can spend anywhere from 5 to 15 hours a week pursuing their passion, mostly logging mileage to build and maintain an aerobic base. HIIT programming is much more rare, used mainly closer to race day for effective pacing strategies (Seiler & Tonnessen 2009).

The LIT performance training model thrived for more than 50 years with little challenge to its supremacy until recent research into HIIT expanded our knowledge of how higher intensity can improve many of the physiological pathways and energy production systems common to LIT aerobic training.

Aerobic and Anaerobic Systems Contribute to Each Other

Our bodies have three basic energy systems with three separate purposes and three distinct cellular pathways based on how fuel provides energy. One system is aerobic, and the other two are anaerobic.

The aerobic system is the one we use daily for low-level functions at a low to medium heart rate (under 50%–70% of HRmax) for long periods of time. This is the system most used in endurance events as well. By contrast, the anaerobic systems are reserved for higher levels of physical effort, and function at much higher heart rates. The anaerobic system is divided into two subsystems based on duration (Wilmore, Costill & Kenney 1999).

Anaerobic and Aerobic Systems


The aerobic system, also known as the oxidative system, is a complex consumer of energy. It uses oxygen to break down the fuels (fats and carbohydrates) we ingest to function. Because lots of oxygen is needed to process fuel, work efforts must remain low to allow all of the chemical processes to take place properly.

This system can operate longer than the two anaerobic systems and remains relatively self-perpetuating at the muscle-cell level because of continued oxygen supply, stored fuel capacity and the possibility of refueling during longer efforts. These factors explain why the aerobic system gets the most attention from endurance athletes and coaches (Magness 2014).

Anaerobic System 1: Speed and strength

The two anaerobic subsystems are categorized by type of fuel used, length of time and the amount of energy they produce. The quickest and most powerful system for short intensive efforts is called the ATP-PCr System, which we will call the speed-and strength system. It is used in sprinting and weightlifting and has a lifespan of about 3–15 seconds before recovery is needed to replenish the muscle cells. These times are so short that the endurance world largely ignores the speed-and-strength system (Willmore, Costill & Kenney 1999; Seiler & Tonnessen 2009).

Anaerobic System 2: Glycolytic

The second anaerobic system is called the glycolytic system, so named because it uses glucose (carbohydrates). This system can sustain higher levels of work for brief periods of time, with its lifespan depending mostly on the buildup of fatiguing exercise byproducts called metabolites.

The glycolytic system can produce energy for anywhere from 30 to 120 seconds, but it also works in concert with the aerobic system to promote work at higher intensities (over 60%–70% of HRmax). Depending on a coach’s or athlete’s philosophy, glycolytic-system exercise can be an important training component, especially during race-pace development (Willmore, Costill & Kenney 1999; Seiler & Tonnessen 2009).

HIIT programming consists largely of the latter two systems and includes sprint, strength and short-rest, interval-based cardiovascular training. Peak efforts lasting 3–120 seconds might not seem like enough time to give these systems an equal or higher priority than aerobic training, but research has documented the effectiveness of HIIT (Laursen 2010; Steele et al. 2012).

Research Supporting HIIT’S Benefits for Endurance

Multiple studies have found that the improvements mentioned above can be trained in very short periods of time with HIIT. Gibala and McGee (2008) postulated that HIIT’s high demands stress both the oxidative and glycolytic systems, triggering a significant increase in muscle fiber recruitment. Both type 2 (fast-twitch glycolytic fibers) and type 1 (slow-twitch oxidative fibers) are recruited and trained during HIIT. This dual stimulus of short-duration, high-intensity work boosts mitochondrial mass and increases oxidative enzyme activity, effects normally associated with LIT. This is crucial because cell mitochondria are critical to providing fuel to muscle for energy (Gibala & McGee 2008; Burgomaster et al. 2005).

High-intensity efforts stimulate the body to recruit, challenge and train more motor units—the contraction-signaling structures within a muscle group (Steele et al. 2012). Having more of these highly trained units is advantageous, especially as intensity increases, because muscle fibers and their respective motor units are rotated during lower intensities in alternating rest/work cycles. Athletes with the most potentially active motor units will have more that can be cycled in to delay fatigue (Magness 2014).

While HIIT can enhance slow- and fast-twitch muscle fibers, LIT is almost exclusively a slow-twitch recruiter. All this does not mean HIIT should be performed exclusively for endurance or health in general. After all, there are several distinct advantages to LIT: Lower intensities help with recovery, improve peripheral circulatory adaptations and may improve stroke volume—the amount of blood pumped by the heart—better than HIIT does (Seiler & Tonnessen 2009). Furthermore, LIT at longer training distances may improve overall psychological preparation for longer racing distances.

All of these factors suggest the need for a mix of HIIT and LIT to create an effective overall program that will help your clients, especially if they are training for endurance events.

Setting Up Programming

High-intensity programming can consist of a mix of resistance training, intensive interval/short-rest cardiovascular training and sprinting. All of these modalities improve an athlete’s ability to maintain a consistent speed with efficient technique over a long duration. The ideal combination of HIIT and necessary LIT sessions is unknown because HIIT is rarely used for endurance training.

The novelty of HIIT makes programming somewhat more complex than LIT for a number of reasons. The coach or trainer must consider the athlete’s psychological and physiological makeup, time availability and performance goals, and has to monitor responses to training consistently enough to determine proper dosage. Trying to stimulate mitochondria growth via a 30-minute HIIT session presents many more unknowns than doing this in a 2-hour LIT session.

Ultimately, the key is determining how to accurately train the three energy systems mentioned above while artfully managing fatigue to avoid the pitfalls of overtraining.

Anaerobic System Training

Developing the speed-and-strength system is probably the newest aspect of endurance training, because short-duration exercises long seemed incompatible with this type of training. Yet the benefits to endurance have been established, with research showing significant improvement in endurance markers (Steele et al. 2012; Sunde et al. 2010; Ronnestad & Mujika 2014).

Resistance training to failure has been shown to increase many endurance-based characteristics. VO2max improves in most untrained subjects and some endurance-trained subjects. Oxygen is used more efficiently during work efforts, and time to exhaustion increases. The vascular system (arteries and veins) improves (Steele et al. 2012). Significantly, most athletes reach their peak potential VO2max at some point in their career, and further improvements in performance can be made via other mechanisms stimulated by resistance training and other forms of HIIT (Magness 2014).

Heavy resistance training can effectively target and strengthen the eccentric contraction (lengthening of a muscle during deceleration of movement) and also the concentric contraction
(shortening of a muscle during acceleration of a movement). Performing the endurance activity through LIT can do the same, but training with heavy weights will develop more muscle fibers and motor units within the muscle systems being targeted (Sunde et al. 2010; Sedano et al. 2013).

The Role of Work Economy

Resistance training’s contribution to endurance shows up in a number of ways linked to work economy, which describes how efficiently an athlete consumes oxygen during a specific task at a specific rate. It’s more commonly known as running economy or cycling economy in most research, but the principle applies to other sports as well. Stronger muscles provide several functions that increase efficiency of movement through improvements in elastic energy, stability, neuromuscular activity and force production.

In the case of running, having stronger, more elastic muscles in the legs allows an athlete to better absorb the energy with the eccentric contraction of a foot strike, briefly store that energy and then passively release it in a springlike manner. This limits the need to actively contract muscles concentrically to lift the foot back up. All of the muscles involved from the bottom of the foot up through the ends of the fingers take part in this process of storing and releasing energy, and the stronger and more elastic the muscles are, the less oxygen has to be used for the act of running (Steele et al. 2012; Magness 2014).

Similar principles apply to swimming, cycling and rowing, but because in these cases the water, bicycle and craft bear significant portions of the athlete’s weight, the contractions are mostly concentric. This makes stability, neuromuscular activity and force production more important contributions from strength training (Nader 2006).

For our purposes, stability is defined as necessary support at specific joints to allow efficient movement. A diverse, multiplanar resistance training program can effectively target muscle
groups needed to provide support around a joint (e.g., the lumbopelvic hip complex). This strengthening can allow an area to respond to the rapidly changing force mechanics from an alternating arm stroke in swimming or an alternating pedal stroke in cycling, for example. It can also allow the spine to remain taut to provide effective leverage during a rowing stroke (NASM 2008).

Neuromuscular development and force development are also closely linked to heavy resistance training. Resistance training can produce neuromuscular adaptations by increasing the number of motor units, allowing force production to be spread across a larger area of active muscle fibers. The increase in connections creates more sources of power during the force production of a foot strike, row stroke or ski-pole action (Taipale et al. 2013).

Choosing Resistance Exercises

When choosing resistance training exercises, it’s probably best to select compound free-weight exercises that target the main movers of a specific sport. Running should emphasize hip extensor muscle exercises like deadlifts, hip thrusts and glute-and-hamstring raises. Single-leg deadlifts, single-leg glute thrusts (donkey kicks) and hip thrusts, and lunges should be included too. Pulling and pushing sleds is also effective strengthening exercise for runners.

Cycling should emphasize hip flexor muscle groups like squats, step-ups, lunges, single-leg squats and roman chairs.

A final and perhaps more valuable component of resistance training is the development of durability within the HIIT system itself. A stronger body can be a faster body, especially when
various types of HIIT cardiovascular intervals are included in a training plan. The improvements in elasticity, stability, neuromuscular connections and force production help the athlete maintain proper technique during high-demand intervals.

HIIT Cardiovascular Intervals

HIIT cardiovascular intervals are short to moderate-duration high-intensity work sessions lasting from 20 seconds to 5 minutes, with brief rest periods to stimulate the anaerobic/aerobic system (Gibala et al. 2012). The rest intervals are usually the same length as or shorter than the work intervals. Longer sessions lasting 10–30 minutes can be included, but should be designed to stay at a specific pace until exhaustion (Carmichael & Rutberg 2012).

The idea is to train aerobically for a cumulative length of time just below the point where the body slips into the anaerobic system. The short rest intervals let the body recover slightly so it can continue working at a higher level. Known as threshold training, this technique is designed to improve endurance distances at higher rates of speed via improved fuel usage.

A crucial benefit of training at higher intensities is that it causes the body to process metabolites, the negative byproducts of exercise.

Lactate is one of the most important metabolites because it can be recycled and used for fuel as long as the amount does not overwhelm the body’s recycling capacity. If the system is overwhelmed, fatigue and muscle failure increase. HIIT training intervals are designed to train the body to become better at recycling lactate. This is called lactate buffering, and the training is called lactate-threshold training (Carmichael & Rutberg 2012).

HIIT cardiovascular intervals at lactate threshold are usually designed to train at just the right intensity to allow lactate to be used as fuel. The harder we work, the more lactate we produce, which means there is a threshold where there’s too much lactate in the bloodstream to recycle. It’s a complex process, but effective, especially if an individual is more suited for this type of training (Magness 2014; Carmichael & Rutberg 2012; Pierce, Murr & Moss 2007).

Additionally, this kind of training makes it psychologically easier for athletes to tolerate performances at uncomfortably high levels. Part of the goal of each training session is to be able to work as hard as possible for as long as possible while the metabolites build up.

What the optimal interval rest and work ratios and session lengths are is anyone’s educated guess at this point. Most endurance sports have interval training workouts programmed, but these account for only 20%–30% of the annual plan. Moving to a higher percentage of HIIT training for endurance is new territory.

To date, no research has precisely quantified the amount or balance of training needed to convert shorter HIIT sessions into a specific event that is four to six times longer, but several
coaches of elite-level recreational age-group athletes have developed unique approaches toward doing so.

Monitoring Response and Recovery

HIIT training is more complex than the standard LIT-based model because the three energy systems are usually challenged throughout the training and because intensities are fairly high. As a result, recovery periods must be built in to allow the body to adapt to each training period.

HIIT training models usually have significant breaks built into them. The “Run Less, Run Faster” model (see the sidebar “HIIT Model: Run Less Run Faster”) uses cross-training and rest days to let runners recover from three running workouts, while TCC (see the sidebar “HIIT Model: Time-Crunched Cyclist”) gives cyclists 2 or 3 days off per week, depending on the rotation of the schedule. The recovery week in the Ironman model (see the sidebar) is very low intensity, with no workout performed at more than 60% of peak heart rate.

These programs are designed to be maintained for 12- to 16-week blocks at most, with a recommended 3–4 weeks of LIT training in between these blocks.

Recovery periods are important because people recover at different rates from different types of modalities. Using various forms of monitoring—like heart rate variability, resting heart rate, rating of perceived exertion, and baseline training and competition tests—can prove quite helpful (Hoolihan 2014). Given the newness of HIIT training for endurance and the relative unknowns surrounding it, these measurements can be critical for highly competitive amateur and elite athletes.

With enough planning, research and monitoring, HIIT can be an effective and efficient alternative for endurance events.

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