Original episode & show notes | Raw transcript
Over-under intervals are a popular and structured form of workout in endurance sports, particularly cycling. The basic structure involves alternating between periods of work slightly above a physiological threshold (like Functional Threshold Power, or FTP) and periods slightly below it, with no true rest in between. A common example might be alternating between 105-110% of FTP for the “over” portion and 85-95% of FTP for the “under” portion.
These workouts are often prescribed with the belief that they confer unique physiological advantages. The purported benefits typically revolve around lactate:
Enhanced Lactate Clearance: The primary claim is that by producing high levels of lactate during the “over” phase and then forcing the body to process it during the “under” phase, these intervals “teach” the muscles to become more efficient at clearing lactate.
Increased Lactate Tolerance: An older, though less accurate, view suggests these intervals improve the body’s ability to “tolerate” the buildup of lactate and its associated metabolic byproducts, thereby reducing fatigue.
Race-Specific Simulation: They are thought to mimic the variable demands of racing, where an athlete must surge to cover an attack and then settle back into a hard, but sustainable, pace.
This document will deconstruct these claims by examining the underlying physiology, drawing upon the scientific studies and concepts discussed in the podcast, to determine if over-unders truly possess special adaptive properties.
To understand over-unders, we must first understand lactate.
Historically, lactate was viewed as a metabolic waste product, a dead-end result of anaerobic glycolysis that directly caused the burning sensation in muscles and led to fatigue. In this model, the “lactate threshold” was seen as the point where lactate production overwhelmed the body’s ability to remove it, leading to an inevitable and rapid decline in performance. The goal of training, therefore, was to either reduce lactate production or increase lactate tolerance.
Modern physiology has completely reframed our understanding of lactate. It is not a waste product, but a crucial metabolic intermediate and a preferred fuel source for highly oxidative tissues like the heart, liver, and slow-twitch muscle fibers.
Lactate Shuttle Hypothesis: Proposed by Dr. George Brooks, this theory posits that lactate is produced in one location (e.g., glycolytic fast-twitch muscle fibers) and shuttled via the bloodstream to another location (e.g., oxidative slow-twitch muscle fibers or the heart) to be used as fuel.
Lactate is Not the Cause of Fatigue: The acidosis (drop in pH) once attributed to lactic acid is now understood to be caused by the release of hydrogen ions (H+) during the rapid breakdown of ATP, not from lactate production itself. Lactate production actually consumes a hydrogen ion, slightly counteracting acidosis. Lactate concentration is therefore a correlate of fatigue, not its cause. It’s an innocent bystander at the scene of a metabolic crime.
Under this modern paradigm, the goals of training shift to enhancing lactate oxidation (clearance) and improving the overall aerobic capacity of the muscle. The central question for over-unders becomes: does the specific act of producing and then clearing high levels of lactate trigger a superior adaptation for lactate oxidation compared to other forms of training?
Lactate clearance refers to the removal of lactate from the bloodstream, primarily through its uptake and oxidation (use as fuel) by various tissues.
If the goal of over-unders is to maximize lactate clearance during the “under” phase, it’s critical to know what intensity is best for this process.
A key study explored this very question: “Blood Lactate Clearance During Active Recovery After an Intense Running Bout Depends on the Intensity of Active Recovery.”
Methodology: Participants performed a 5-minute, high-intensity effort (90% of VO2max) to generate high blood lactate levels. They then recovered at different intensities: passive rest (0% of lactate threshold), 40%, 60%, 80%, and 100% of their lactate threshold (LT), as well as a self-selected pace.
Key Finding: The fastest rate of lactate clearance occurred at 80% of LT. The second fastest was at 100% of LT. Clearance rates were significantly slower at 60%, 40%, and passive rest.
Why? This result is a function of two competing factors:
Concentration Gradient: For lactate to move from the blood into the muscle to be oxidized, there must be a favorable concentration gradient (higher concentration in the blood than in the muscle). Lower intensity exercise creates a larger gradient.
Metabolic Rate: The muscle must be working hard enough to create a demand for fuel. Higher intensity exercise increases this metabolic rate.
At 80% of LT, these two factors find a sweet spot. The intensity is high enough to create a significant demand for fuel (driving oxidation), but low enough to maintain a large concentration gradient, allowing lactate to flow readily from the blood into the muscle cells. At 100% of LT, the metabolic rate is higher, but the local lactate production within the muscle reduces the gradient from the blood, slightly slowing clearance.
Implication for Over-Unders: If maximizing lactate clearance is the goal, the “under” portion of the interval should be performed at an intensity roughly corresponding to 80-90% of FTP, which aligns with the “sweet spot” training zone.
The movement of lactate across cell membranes (into the blood, out of the blood, and into mitochondria) is not passive. It is facilitated by a family of proteins called Monocarboxylate Transporters (MCTs).
MCT1: Has a high affinity (low Km) for lactate, meaning it functions efficiently at lower lactate concentrations. It is often associated with oxidative, slow-twitch fibers and is thought to be primarily responsible for lactate uptake.
MCT4: Has a lower affinity (high Km) for lactate, making it more effective at higher concentrations. It is often associated with glycolytic, fast-twitch fibers and is thought to be primarily responsible for lactate efflux (export).
A central claim for over-unders is that they specifically upregulate the expression of these MCTs. However, the story is more complex:
Bidirectionality: Both MCT1 and MCT4 are bidirectional. The direction of lactate flow depends solely on the concentration gradient, not on the transporter type.
Training Status: In untrained individuals, almost any form of exercise will increase the expression of both MCT1 and MCT4.
MCT1 and Mitochondria: In trained individuals, the expression of MCT1 is very strongly correlated with mitochondrial content (as measured by citrate synthase activity). There is virtually no correlation between MCT4 and mitochondrial content.
This suggests that the upregulation of MCT1 is not an isolated adaptation but is part of a larger program of generalized aerobic adaptation. When the body builds more mitochondria, it also builds the transport machinery (MCTs) needed to supply them with fuel. The idea that you can specifically target MCTs by “bathing the cell in lactate” is not supported; the trigger appears to be the same signal that triggers mitochondrial biogenesis, primarily PGC-1α.
If over-unders are not special for targeting MCTs, what does improve lactate clearance and overall aerobic performance?
The ultimate rate-limiter for lactate oxidation is not the number of transporters, but the total oxidative capacity of the muscle. This is determined by mitochondrial density. More mitochondria mean more surface area and more enzymatic machinery to burn fuel, including lactate.
Research shows a clear, almost linear relationship between training volume and increases in mitochondrial content. While intensity plays a role, the total volume of work is the most powerful driver for building more mitochondria.
Therefore, the most effective way to improve lactate clearance is to engage in consistent, high-volume endurance training that stimulates mitochondrial biogenesis.
Even if you could increase MCT expression independently of mitochondrial growth, it would likely not improve performance. In a well-trained athlete, threshold performance is not limited by the ability to transport fuel into the mitochondria. It is limited by the supply of oxygen and the total oxidative capacity of the mitochondria themselves.
This is analogous to the findings in VO2max research, where performance is limited by the cardiovascular system’s ability to deliver oxygen, not the muscles’ ability to use it. Simply providing more fuel (lactate) to the same number of mitochondria with the same oxygen supply will not increase the maximal aerobic ATP production rate, which is what defines threshold power.
The “over” portion of an over-under interval involves working above threshold, which engages the anaerobic system. Could these intervals be a superior way to train anaerobic capacity?
The famous Tabata study provides insight. It compared two groups:
Endurance Group: Moderate intensity training (70% VO2max).
High-Intensity Intermittent Group: Very high-intensity efforts (170% VO2max for 20s) with short rest (10s).
The high-intensity group saw a massive 28% increase in anaerobic capacity, while the endurance group saw none. This demonstrates the principle of specificity: to improve anaerobic capacity, you must perform dedicated anaerobic capacity work.
While over-unders do involve work above threshold, they are typically not intense enough or structured to provide the same potent stimulus as true anaerobic capacity intervals. They can be considered moderately effective for this purpose, more so than steady-state threshold work, but less effective than all-out, dedicated efforts.
The central argument of the podcast is that over-unders do not possess unique or magical adaptive properties. The physiological adaptations they are purported to target—namely, enhanced lactate clearance via MCT upregulation—are not special.
Lactate Clearance is a Function of General Aerobic Fitness: The ability to oxidize lactate is fundamentally tied to mitochondrial mass. The most potent stimulus for building mitochondria is overall training volume. The molecular signals (like PGC-1α) that trigger mitochondrial growth also trigger the expression of its associated machinery, including MCTs. You cannot isolate one from the other.
Specificity of Training Rules: If the goal is to improve anaerobic capacity, dedicated anaerobic work is superior. If the goal is to improve threshold power, raising VO2max and performing high-volume aerobic work are the most critical components.
This does not mean over-unders are a useless workout. They are a perfectly valid and effective form of training with several practical applications:
Psychological Variety: They break the monotony of steady-state intervals.
Race Simulation: They are excellent for preparing for the specific demands of variable-pace events.
A “Blended” Stimulus: They provide a strong aerobic stimulus while also touching on anaerobic systems, making them a time-efficient workout.
Ultimately, over-unders are simply one tool in a vast toolbox. They are effective because they represent a hard, structured workout. Their effectiveness comes from the stress they apply to the aerobic system, not from a unique ability to “teach” the body to clear lactate. An athlete can achieve elite levels of performance with or without them, as long as the fundamental principles of training—progressive overload, volume, and specificity—are respected.