A Perspective on High-Intensity Interval Training for Performance and Health

Abstract

Interval training is a simple concept that refers to repeated bouts of relatively hard work interspersed with recovery periods of easier work or rest. The method has been used by high-level athletes for over a century to improve performance in endurance-type sports and events such as middle- and long-distance running. The concept of interval training to improve health, including in a rehabilitative context or when practiced by individuals who are relatively inactive or deconditioned, has also been advanced for decades. An important issue that affects the interpretation and application of interval training is the lack of standardized terminology. This particularly relates to the classification of intensity. There is no common definition of the term "high-intensity interval training" (HIIT) despite its widespread use. We contend that in a performance context, HIIT can be characterized as intermittent exercise bouts performed above the heavy-intensity domain. This categorization of HIIT is primarily encompassed by the severe-intensity domain. It is demarcated by indicators that principally include the critical power or critical speed, or other indices, including the second lactate threshold, maximal lactate steady state, or lactate turnpoint. In a health context, we contend that HIIT can be characterized as intermittent exercise bouts performed above moderate intensity. This categorization of HIIT is primarily encompassed by the classification of vigorous intensity. It is demarcated by various indicators related to perceived exertion, oxygen uptake, or heart rate as defined in authoritative public health and exercise prescription guidelines. A particularly intense variant of HIIT commonly termed "sprint interval training" can be distinguished as repeated bouts performed with near-maximal to "all out" effort. This characterization coincides with the highest intensity classification identified in training zone models or exercise prescription guidelines, including the extreme-intensity domain, anaerobic speed reserve, or near-maximal to maximal intensity classification. HIIT is considered an essential training component for the enhancement of athletic performance, but the optimal intensity distribution and specific HIIT prescription for endurance athletes is unclear. HIIT is also a viable method to improve cardiorespiratory fitness and other health-related indices in people who are insufficiently active, including those with cardiometabolic diseases. Research is needed to clarify responses to different HIIT strategies using robust study designs that employ best practices. We offer a perspective on the topic of HIIT for performance and health, including a conceptual framework that builds on the work of others and outlines how the method can be defined and operationalized within each context.

Fig. 1

A conceptual framework for application of interval training in performance and health contexts. ¹Authors’ example modeled after common frameworks including elements from Seiler [20]; Casado et al. [21]; and Jamnick et al. [23]. ²Common three-domain classification based on work rate or physiological indicators [22, 31, 32]. ³World Health Organization 2020 guidelines on physical activity [25]. ⁴American College of Sports Medicine’s guidelines for exercise testing and prescription, 11th ed. [26]. ⁵Anaerobic speed reserve [39]. ⁶Extreme-intensity domain [22, 35]. ASR anaerobic speed reserve, CP/CS critical power/critical speed, GET gas exchange threshold, HIIT high-intensity interval training, HR_max maximal heart rate, HRR heart rate reserve, LT1 first lactate threshold, LT2 second lactate threshold, MAS maximal aerobic speed, MET metabolic equivalent of task, MLSS maximal lactate steady state, MSS maximal sprint speed, RPE rating of perceived exertion (out of either 10 or 20 depending on the scale), SIT sprint interval training, $\dot{V}{\text{O}}_{2max}$ maximal oxygen uptake

Fig. 2

Simplified depiction of sample high-intensity interval training (HIIT) and sprint interval training (SIT) protocols with reference to thresholds demarcated in common domain-based training models and physical activity and exercise intensity classifications [, –26, 31, 32]. Icons made by Prosymbols Premium (top left) and Freepik (middle and bottom left) from Flatiron (www.flaticon.com/free-icons/trail-running; www.flaticon.com/free-icons/run; www.flaticon.com/free-icons/chase)