Original episode & show notes | Raw transcript
High-Intensity Interval Training (HIIT) has become a cornerstone of modern fitness, promoted as a time-efficient method to achieve significant gains in performance. However, its popularization has often stripped away the scientific nuance of how, why, and for whom it works best. This guide delves into the physiological principles of HIIT, drawing from the foundational research and its practical applications for athletes.
At its core, HIIT involves repeated bouts of high-intensity work interspersed with periods of rest or low-intensity recovery. The podcast highlights that “high intensity” is a relative term and that HIIT protocols can be structured in several distinct ways, each with different physiological consequences:
Short Bursts, Short Rest: This involves very short, near-maximal efforts with equally short rest periods (e.g., 15 seconds on, 15 seconds off). Due to accumulating fatigue, the power output in the work intervals will naturally decline over a longer set.
Maximal Sprints, Long Rest: This protocol uses a short, truly all-out effort (e.g., 30 seconds) followed by a long rest period (e.g., 4 minutes) that allows for near-complete recovery. This is designed to maximally stress the anaerobic system with each repetition.
The “Tabata” Protocol: This specific, famous protocol involves 20 seconds of work at an extremely high intensity, followed by 10 seconds of rest, typically repeated eight times.
The key advantage that modern cyclists have in applying these protocols is the power meter, which allows for precise measurement and execution of work intervals, a tool unavailable in many other sports for decades.
Much of the modern HIIT craze can be traced back to a seminal 1996 paper by Dr. Izumi Tabata and his colleagues. To understand its significance, it’s important to consider the training context of the time, which was often “pace-based”—training at a target race pace, which was not always intense enough to stimulate maximal adaptation.
The Study Design: Dr. Tabata took moderately trained subjects and split them into two groups for a six-week training program (five days per week).
Group 1 (Moderate-Intensity Endurance): Performed 60 minutes of continuous cycling at 70% of their VO2 max. This is metabolically similar to what many cyclists would call “Sweet Spot” or “FTP” work.
Group 2 (HIIT): Performed the now-famous Tabata protocol: 8 sets of 20 seconds of work at a punishing 170% of VO2 max power, with 10 seconds of rest between each set.
The Key Measurements:
VO2 Max: A measure of maximal aerobic capacity—the maximum amount of oxygen the body can utilize during intense exercise.
Anaerobic Capacity: The body’s ability to produce energy without oxygen, crucial for short, powerful efforts. This was measured via accumulated oxygen deficit, which quantifies how much energy was supplied by anaerobic pathways by measuring oxygen consumption during and after the effort. The heavy breathing you experience after an all-out sprint is your body “paying back” this oxygen debt.
The Groundbreaking Results:
The endurance group improved their VO2 max but saw no change in their anaerobic capacity.
The HIIT group significantly increased their anaerobic capacity (by 28%).
Most surprisingly, the HIIT group increased their VO2 max by the same amount as the endurance group, despite the total workout time being drastically shorter (4 minutes of intervals vs. 60 minutes of steady-state).
This study was revolutionary because it demonstrated that a very short, intermittent protocol could simultaneously improve both the aerobic and anaerobic energy systems in moderately trained individuals.
The results of the Tabata study raise a critical question: how can a predominantly anaerobic workout produce such powerful aerobic adaptations?
The Untrained State: For individuals who are new to training, virtually any stimulus is effective because their entire physiological system is a “weak link.” Every part of the chain, from breathing to muscle cell metabolism, has significant room for improvement.
Stimulus and Recovery: The magic of the Tabata protocol isn’t just in the 20-second work intervals; it’s also in the 10-second recovery.
The work intervals are intensely anaerobic. They flood the system with metabolic byproducts and stress the pathways that burn carbohydrates for quick energy.
The aerobic adaptation is driven by what happens during and after these efforts. The aerobic system is forced to work overtime to recover from the anaerobic stress—replenishing energy stores, clearing lactate, and “paying back” the oxygen debt. It is this repeated, intense recovery demand that stimulates an increase in VO2 max.
A Look Inside the Muscle Cell: A second paper discussed in the podcast provides a fascinating glimpse into the cellular mechanisms at play.
The Protocol: Researchers had subjects perform a single session of all-out 30-second sprints with 4 minutes of rest.
The Science: They studied the sarcoplasmic reticulum (SR), a structure within muscle cells that stores and releases calcium. Calcium is the fundamental trigger for muscle contraction, and its presence is a powerful signal for the cell to adapt.
Findings in Untrained Subjects: The intense sprints caused the ryanodine receptors (the “gates” that release calcium from the SR) to fragment. This led to a “calcium leak” into the muscle cell, even during rest. This prolonged calcium signal is a potent stimulus for mitochondrial biogenesis—the creation of new mitochondria, which are the aerobic powerhouses of the cell.
Findings in Trained Subjects: When highly-trained endurance athletes performed the same protocol, their muscle cells showed no receptor fragmentation. Their cells were already so well-adapted to handling large amounts of calcium that this specific stimulus was no longer strong enough to trigger the same adaptive response.
The findings from both studies converge on a critical point for experienced athletes: as you become more trained, the “catch-all” benefits of HIIT diminish. The body adapts, and to continue improving, the training stimulus must become more targeted.
The End of “Anything Works”: Once an athlete is well-trained, FTP work no longer significantly improves VO2 max, and VO2 max work does little to improve FTP. The physiological systems have adapted, and you must now stress a specific “weak link” to force further improvement.
The Teeter-Totter Effect: In elite athletes, the relationship between systems can become a trade-off. A block of training focused heavily on anaerobic capacity might slightly increase 5-minute power but could cause a small dip in FTP. Conversely, a large aerobic base-building block might slightly reduce top-end sprint power. This highlights the need for careful, periodized training that focuses on specific goals at different times of the season.
Targeting the Right System: If the goal is to improve VO2 max, a trained athlete should use intervals that maximally stress the aerobic system (e.g., longer 3-5 minute intervals at VO2 max power). If the goal is to improve anaerobic capacity for race-winning attacks, short, maximal sprints with long recoveries are more appropriate. Using a Tabata-style protocol may not be the optimal tool for a trained athlete seeking a specific adaptation.
In summary, HIIT is a scientifically-backed, powerful training modality. For those beginning their fitness journey, it offers remarkable dual benefits for both aerobic and anaerobic systems. However, for the dedicated, well-trained athlete, it serves as a powerful lesson in the principle of specificity: to reach the highest levels of performance, you must precisely identify your physiological limiter and apply the right tool for the job.