Original episode & show notes | Raw transcript
This document provides a detailed exploration of the physiological principles of interval training, as discussed in the provided podcast transcript. The central theme is a shift away from a rigid, numbers-only approach to training towards a more nuanced understanding of how prescribed workouts stimulate adaptation within the body.
The core of this advanced approach lies in differentiating between stress (the external work we perform) and strain (the internal physiological response to that work). Understanding this relationship is fundamental to designing training that is not only effective but also sustainable, minimizing fatigue while maximizing physiological improvement.
At its most basic level, an interval is a structured period of work designed to overload a specific physiological system, signaling the body that it must adapt and become more resilient to that particular challenge. The podcast emphasizes that before prescribing or performing any set of intervals, one must answer three crucial questions:
What specific part of your physiology is this interval intended to change? Are you targeting neuromuscular power, anaerobic capacity, maximal oxygen uptake (VO2 max), lactate clearance, or muscular endurance? The design of the interval—its intensity, duration, and rest period—will differ significantly based on the answer.
How does this workout fit into the larger training plan? This question addresses both short-term planning (how does today’s workout account for daily fatigue?) and long-term progression (how will this type of interval evolve over a multi-week training block to ensure continuous improvement?).
What are the specific demands of your goal event? While a deeper topic, this question provides the ultimate context. The physiological systems you choose to develop should align with what is required to succeed in your target races or events.
This is the most critical concept presented. In exercise physiology, these terms have precise meanings that are essential for intelligent training design.
Stress is the external, objective, and measurable workload applied to the body. It is the physical work you are doing.
Examples:
Power Output: Measured in watts on a cycling power meter.
Pace: Minutes per mile/kilometer during a run.
Heart Rate: Beats per minute (though it’s a response to stress, it’s often used as an external target).
Cadence: Revolutions per minute.
Stress is the input you apply to the system.
Strain is the internal, subjective, and physiological response to the applied stress. It is what your body is actually experiencing on a biological level as it tries to meet the demands of the workout.
Components of Strain:
Metabolic State: The rate of ATP turnover, the contribution of different energy systems (phosphocreatine, glycolytic, aerobic), and the accumulation of metabolic byproducts like lactate and hydrogen ions.
Cellular Signaling: The activation of specific genetic pathways (like PGC-1α for mitochondrial biogenesis) that trigger long-term adaptations.
Oxygen Consumption (VO2): The actual amount of oxygen your muscles are using.
Neuromuscular Fatigue: Reduced ability of the nervous system to recruit muscle fibers.
Perceived Exertion (RPE): The subjective feeling of how hard the effort is.
Strain is the internal consequence of the input.
The crucial insight is that stress and strain do not have a 1:1 relationship. The same external stress (e.g., a 5-minute interval at 350 watts) can produce a vastly different internal strain depending on various factors:
Fatigue: If you are tired from poor sleep or previous training, 350 watts will induce a much higher strain than when you are fresh.
Duration: The strain of the first minute at 350 watts is very different from the strain of the fifth minute at 350 watts.
Fitness Level: As you become fitter, it takes a higher external stress to produce the same level of internal strain.
Effective coaching and self-coaching involve manipulating external stress to achieve the desired internal strain, which is the true catalyst for adaptation.
A common mistake is to assume that an interval performed at a specific “zone” (e.g., “VO2 max power”) means you are utilizing that energy system exclusively or immediately. The podcast provides an excellent example to debunk this.
Consider a 1-minute interval at your predetermined “VO2 max power” of 350 watts:
Seconds 0-10: The immediate energy demand is met by the phosphocreatine (PCr) system. This is an anaerobic, instantaneous energy source. Your oxygen consumption has barely begun to rise.
Seconds 10-45: As PCr stores are depleted, anaerobic glycolysis rapidly increases to produce ATP, leading to a significant production of lactate. Your heart rate and respiration are climbing steeply, but your body is still in “oxygen debt” and has not yet reached its maximal oxygen uptake.
Seconds 45-60 (and beyond): Only towards the end of the first minute (and likely into subsequent minutes of the interval) does your aerobic system ramp up to its highest rate. You may only reach a true state of VO2 max after 90 seconds to 2 minutes of sustained effort.
Implication: The power target is a tool to elicit a certain physiological state. The initial part of the interval, while heavily anaerobic, is a necessary prerequisite to stress the aerobic system to its maximum. This also explains why short, repeated intervals (e.g., 30-40 seconds on, 15-20 seconds off) can be effective for accumulating time at or near VO2 max without the same level of global fatigue as a single, long 5-minute effort.
Power Targets: Are excellent for providing an objective measure of stress and tracking long-term progress. However, rigidly adhering to a power target on a day when you are fatigued can lead to excessive strain, poor adaptation, and potential overtraining.
Perceived Exertion (RPE): Prescribing intervals based on RPE (e.g., “an effort you could hold for 5 minutes”) allows the athlete to auto-regulate. On a good day, this may result in a higher power output. On a bad day, the power will be lower, but the internal strain will be appropriate for the intended adaptation. This is why the host advocates for increased use of RPE, as it better manages the internal strain on the athlete.
For maximal efforts like sprints, RPE is paramount. The instruction “go all out” ensures maximal neuromuscular recruitment, which is the goal. The power number produced is an outcome of that effort, not the target itself.
The podcast uses a powerful example concerning sweet spot training (intervals around 90% of FTP). The effectiveness of an interval is relative to your current capacity.
Concept: Time to Exhaustion (TTE) is the maximum duration you can sustain a given power output.
Example: An athlete has an FTP TTE of 45 minutes. Their TTE at sweet spot (90% FTP) is likely much longer, perhaps 80-90 minutes. If this athlete performs a workout of 2x20 minutes at sweet spot, they are accumulating 40 total minutes of work—well within their current capacity of 80-90 minutes.
Conclusion: This workout does not provide a sufficient overload stimulus to improve their FTP. It is a maintenance workout at best. To drive adaptation, they would need to progress by extending the duration of the intervals (e.g., 2x30 min, 1x50 min) to push the boundaries of their current TTE.
Fatigue is not just a feeling; it’s a physiological state that fundamentally alters the stress-strain relationship.
When to Avoid High-Stress Intervals: For workouts where the adaptive signal depends on achieving peak output (sprints, short anaerobic efforts), high fatigue is counterproductive. Fatigue impairs neuromuscular recruitment, meaning you cannot generate the high-quality stress needed to trigger adaptation. You simply accumulate more fatigue with little to no benefit.
When Fatigue Can Enhance Adaptation: For other types of training, fatigue can be a tool. A long endurance ride performed in a slightly glycogen-depleted state can amplify the cellular signaling for mitochondrial growth and fat oxidation efficiency, even if the power output (stress) is slightly lower. The strain on the metabolic system is higher, which drives the desired adaptation.
The central message of the podcast is to evolve from being a “power zone follower” to becoming an “intelligent physiological stress applier.” This means:
Prioritize the Why: Understand the intended physiological adaptation of every workout.
Respect the Stress-Strain Relationship: Use power and other objective metrics as tools to guide your application of stress, but use RPE and self-awareness to monitor the resulting internal strain.
Embrace Dynamics: Recognize that your body’s response to a workout changes from day to day and even minute to minute.
Use Progression Intelligently: Ensure your workouts provide a sufficient overload stimulus by progressively challenging your duration and intensity capacities.
By integrating these concepts, an athlete can train more effectively, achieving better results with less cumulative fatigue and a greater understanding of their own physiology.