We hear lots about high intensity interval training these days, but one of the most fascinating things about HIIT is its ability to help athletes improve their performance in endurance events.
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 maximal HR) 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).
Aerobic (or Oxidative) System Functions
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 maximal HR). 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 cardiovasculair 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.