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.

High‐intensity interval training and cardiorespiratory fitness in adults: An umbrella review of systematic reviews and meta‐analyses

“…7 While historically practiced by athletes, the idea of utilizing HIIT for improving health in the general population has gained public attention in recent decades. 8,9 As recently discussed by Coates et al, 10 there is no universal definition of HIIT despite widespread use of the term. With regards to exercise prescription, HIIT is commonly characterized as being performed at an intensity that elicits ≥80-100% of peak heart rate.…”

“…3 A particularly intense variant, "sprint interval training" (SIT), is typically distinguished as requiring an "all-out" or "supramaximal" effort equal to or greater than the pace that elicits ≥100% VO 2 max. 3 Coates et al 10 noted that such definitions do not align with common thresholds used to distinguish intensity classifications in a health (i.e., light, moderate, vigorous) or performance context (i.e., moderate, heavy, severe), and there may be value in broadening the definitions to include multiple indicators as opposed to a single metric. Building on the work of others, Coates et al 10 proposed a conceptual framework that outlined how HIIT can be defined and operationalized in various contexts.…”

“…3 Coates et al 10 noted that such definitions do not align with common thresholds used to distinguish intensity classifications in a health (i.e., light, moderate, vigorous) or performance context (i.e., moderate, heavy, severe), and there may be value in broadening the definitions to include multiple indicators as opposed to a single metric. Building on the work of others, Coates et al 10 proposed a conceptual framework that outlined how HIIT can be defined and operationalized in various contexts. Irrespective of the explicit definition used, studies comparing HIIT to traditional moderate-intensity continuous training (MICT) commonly employ protocols in which the total exercise volume is either matched or not matched, typically based on estimated total exergy expenditure.…”

“…11 A common limitation of many studies that have compared the increase in CRF after HIIT versus MICT is relatively small sample sizes and short durations. 10,12,13 Systematic reviews and meta-analyses based on these studies have often focused on specific groups, such as healthy populations, [14][15][16] individuals with overweight/obese, [17][18][19][20] and elite athletes. 21 These reviews and meta-analyses also varied in comparison groups (e.g., non-active or active controls), settings (e.g., community or laboratories settings), or HIIT modalities with limited samples, impacting the generalizability of the findings.…”

Fractal correlation properties of heart rate variability as a marker of exercise intensity during incremental and constant-speed treadmill running

“…Training intensity distribution frameworks divide exercise into at least three predetermined intensity zones (Esteve-Lanao et al, 2007; Seiler & Kjerland, 2006). In one model, the moderate, heavy, and severe exercise intensity domains are delineated by the lactate threshold (LT) and maximal metabolic steady state (MMSS), facilitating exercise monitoring and testing (Coates et al, 2023; Jamnick et al, 2020). Methods to identify the LT and MMSS often require multiple invasive laboratory visits, translate poorly to real world settings, and/or incorrectly assume that these physiological attributes are static (Jamnick et al, 2020; Maunder et al, 2021); therefore, simple and translatable methods that consider how these attributes may shift in response to prolonged exercise, previous exercise, or other relevant external and internal stimuli are needed.…”