Original episode & show notes | Raw transcript
This guide synthesizes and explains the advanced concepts for training VO2 max presented in the Empirical Cycling Podcast. The central philosophy, as stated in the episode, is that “The ultimate goal of any area of physiology is to discover the fundamentals of how a given function works, thus empowering to modify outcomes as desired.”
This is not a list of “miracle intervals,” but rather a framework for understanding the physiological levers you can pull to elicit specific, long-term adaptations for improving your maximal oxygen uptake (VO2 max).
Before modifying workouts, we must understand the primary physiological limiter we are trying to influence.
VO2 max, the maximum volume of oxygen your body can utilize per minute, is a product of two main factors (as described by the Fick Equation):
Cardiac Output (Q): The amount of blood your heart pumps per minute. This is a central factor. (Cardiac Output = Heart Rate × Stroke Volume).
Arteriovenous Oxygen Difference (a-vO2 diff): How much oxygen is extracted from the blood by the muscles. This is a peripheral factor.
While peripheral factors like mitochondrial density and capillary density are important and trainable, the podcast’s core thesis, based on extensive physiological research, is that the primary long-term limiter of VO2 max in trained athletes is central: specifically, the heart’s maximum stroke volume. Stroke volume is the amount of blood pumped with each beat.
You can only use the oxygen you can deliver. Therefore, to raise your ultimate ceiling, you must improve the delivery mechanism.
To increase stroke volume, we must focus on inducing a specific adaptation called eccentric hypertrophy. This is a beneficial enlargement of the heart’s chambers (particularly the left ventricle), which allows the heart to hold and pump more blood with each beat.
This adaptation is primarily driven by cardiac preload:
Preload: The stretch on the heart muscle at the end of its filling phase (diastole). It’s determined by the volume of blood that has returned to the heart right before it contracts.
Frank-Starling Law: A fundamental principle of cardiac physiology stating that the more the heart muscle is stretched by preload, the more forcefully it contracts. This increases the ejection fraction and stroke volume.
Afterload: The resistance the heart must overcome to push blood out. High afterload (e.g., from high blood pressure or heavy weightlifting) tends to cause concentric hypertrophy (thickening of the heart walls), which is not the primary goal for improving stroke volume.
The Goal Summarized: To improve your long-term VO2 max, you must design training that maximally and repeatedly increases cardiac preload. This sustained, high-preload state is the direct stimulus for eccentric hypertrophy, which leads to a larger stroke volume, and thus a higher VO2 max.
The podcast outlines a strategy to shift the focus of intervals away from hitting external numbers (like power) and toward creating the internal physiological state (max preload) that drives adaptation.
This is presented as the single most important takeaway. Performing intervals at a higher-than-normal cadence (e.g., 100-110+ RPM) directly manipulates cardiac hemodynamics to increase preload.
The Mechanism: The Muscle Pump. Your leg muscles, when contracting, squeeze the veins running through them. Since veins have one-way valves, this action actively pumps blood back towards the heart, a mechanism known as the “muscle pump.”
The Evidence: A study cited in the podcast (“Cycling Cadence Alters Exercise Hemodynamics”) showed that at the same power output (200 watts), increasing cadence from 70 to 110 RPM had profound effects:
Heart Rate: Increased slightly (161 to 169 bpm).
Cardiac Output: Increased dramatically (18.2 to 24.5 L/min).
Stroke Volume: Increased significantly (114 to 145 ml/beat).
The Conclusion: A mere 8 bpm increase in heart rate from higher cadence yielded a 31 ml increase in stroke volume. In contrast, a similar heart rate increase achieved by raising power output only yields a ~5 ml increase in stroke volume. High cadence is a direct hack to increase stroke volume and, by extension, cardiac preload. It is a more potent stimulus for eccentric hypertrophy than simply pushing harder at a lower cadence.
Traditional VO2 max intervals are often prescribed at a specific percentage of FTP (e.g., 120%). The podcast argues this is flawed because:
Athlete Variability: An athlete with a large anaerobic capacity may take many minutes at 120% FTP to reach their true VO2 max, accumulating needless fatigue. An athlete with a low anaerobic capacity may not be able to hold the power at all.
The Goal is Internal: The goal isn’t to hold a power number; it’s to maximize time at peak preload.
The Method: Pace your intervals by maximal perceived exertion. Start each interval harder than you think you can sustain for the full duration and simply hang on. Your power will naturally decline, but your internal physiological stress (heart rate, breathing, and most importantly, preload) can remain maxed out.
The Rationale: This approach guarantees you spend more time in the “adaptive zone.” It self-regulates to your fatigue level while keeping the stimulus on the central cardiovascular system as high as possible.
The Method: Use shorter rest periods, often a 1:1 or 2:1 work-to-rest ratio (e.g., 4 minutes on, 2 minutes off).
The Rationale: By not allowing for full recovery, you don’t fully pay back the oxygen debt (known as EPOC, or Excess Post-exercise Oxygen Consumption). You begin the next interval in a partially fatigued state, which causes your VO2 and heart rate to rise to maximum levels much more quickly. This increases the total time per session spent at the desired maximal preload state.
The Method: During the workout, the best feedback is subjective: are you “gasping like a fish on a boat”? After the workout, analyze your data. Look for a heart rate plateau where your heart rate holds steady within a few beats of its maximum for that session.
The Rationale: This shifts the focus from hitting a peak number to sustaining a maximal state. A sustained plateau is a much better indicator that you held your central system at its limit than a brief spike.
The Method: The classic 5x5 minute structure is not sacred. The goal is to maximize total time at high preload.
If an athlete finds 5-minute efforts so daunting that they subconsciously pace them too conservatively, it’s better to do more, shorter intervals (e.g., 7x3 minutes).
If an athlete can maintain a maximal state for longer, then longer intervals (e.g., 4x6 minutes) are appropriate.
The Rationale: This individualization ensures that psychological limiters don’t prevent the athlete from achieving the necessary physiological stimulus.
The podcast presents a strong critique of using short, intermittent intervals (like 30/15s or 40/20s, often called HIIT) for the primary purpose of improving VO2 max in well-trained athletes. This is centered on a takedown of a 2020 study by Rønnestad.
The Common Claim: Studies show HIIT is superior to traditional long intervals.
The Podcast’s Rebuttal:
Flawed Study Design (A “Straw Man”): The Rønnestad study compared a HIIT group to a “long interval” group doing 4x5 minutes. However, the long interval group’s intensity was effort-matched, resulting in a power output that was likely below their functional threshold. The study was not comparing two effective protocols; it was comparing a hard protocol to an ineffective one and unsurprisingly found the hard one worked better.
The Physiological Problem with HIIT: The primary argument is that the short “on” periods (e.g., 30 seconds) do not provide enough sustained time at maximal diastolic filling to be a potent stimulus for eccentric hypertrophy. The heart experiences a brief stretch-and-relax cycle, but it doesn’t get the prolonged, high-pressure “soak” at maximum volume that is thought to trigger the remodeling process in trained hearts.
Confounding Factors: The measured “improvements” from HIIT in these studies can often be attributed to other adaptations, such as increased anaerobic capacity or improved lactate shuttling, which can improve performance on a ramp test without necessarily increasing the central cardiac capacity.
In short, while HIIT is a time-efficient and potent training tool for many purposes, it may not be the most effective tool for the specific goal of driving long-term, central VO2 max adaptations in already trained athletes due to the lack of sustained preload.