W′, Critical Power, and the Physiology Behind High‑Intensity Training

Photo by Markus Spiske on Unsplash

Table of Contents

• Outline
• Introduction: performance metrics meet physiology
• AMP, AMPK, and the signal that tells cells to adapt
• Critical power and W′: two quantities, one practical model
• Why W′ matters beyond being “just a number”
• W′ as a fatigue tank: a practical mental model
• Practical example: why 400 watts for 20 seconds is not the same as 400 watts for 60 seconds
• Using W′ and time‑near‑depletion to structure progression
• Which training elements still matter
• How W′ relates to fiber types and VO2max
• Limitations of the W balance model
• Where the model works and where it struggles
• Future directions: toward more realistic fatigue models
• Testing considerations: how to get accurate CP and W′ values
• Practical session design using W′
• Common pitfalls and how to avoid them
• Where W′ fits in the athlete development roadmap
• Summary: how to use W′ intelligently
• FAQ
• Final thoughts

Outline

• What W′ and critical power represent
• How high‑intensity efforts trigger molecular adaptation
• W′ as a fatigue tank: the mental model
• Practical use: monitoring W′ and time near depletion
• Why power alone can mislead when designing intervals
• Limitations of the W balance model and where research needs to go
• Testing and prescription suggestions
• Conclusions and practical takeaways
• FAQ

Introduction: performance metrics meet physiology

Endurance coaches and athletes increasingly rely on concepts such as critical power and W′ to structure high‑intensity training. Those metrics are powerful, but they also hide a lot of physiological detail. Understanding how energy systems, cellular signaling, and fatigue mechanisms interact gives context to the numbers and reveals how to use W′ most effectively.

AMP, AMPK, and the signal that tells cells to adapt

During high intensity efforts the muscle’s high‑energy phosphate pool changes dramatically. When creatine phosphate and ADP get used up, AMP accumulates. That seemingly small chemical shift matters because AMP directly activates AMP‑activated protein kinase, or AMPK.

 

“When we activate AMP kinase, that’s kind of like that’s a master switch, which then goes downstream and turns on a whole bunch of other genes.”

 

AMPK functions as a cellular energy sensor. When AMP rises it signals the cell that energy availability is low, and it flips a regulatory switch that changes gene expression. One of AMPK’s downstream targets is PGC‑1α, a transcriptional coactivator that promotes mitochondrial biogenesis and the expression of genes involved in oxidative phosphorylation.

This chain of events explains why bouts of near‑maximal effort—where anaerobic systems dominate—can paradoxically stimulate improvements in the aerobic machinery of muscle, especially in fast twitch fibers that are otherwise more glycolytic. In short, short bursts of high intensity can trigger adaptation that increases the muscle’s aerobic capacity.

Critical power and W′: two quantities, one practical model

Critical power (CP) is the asymptote in a power‑duration relationship. It represents the sustainable aerobic power a rider can hold without accumulating a progressively worsening fatigue signal. W′ (W prime) is often described as the finite amount of work available above critical power—a battery or tank of intensity that is depleted while power exceeds CP and recharged while below it.

Visualizing the concept helps. Imagine a table that marks the threshold between “steady state” and “borrowed time.” Below that table, aerobic metabolism can keep up; above it, the body taps into finite reserves and fatigue accumulates. Training adjusts the height of that table—progression raises the threshold so the same absolute power becomes less fatiguing.

Why W′ matters beyond being “just a number”

W′ is more than a mathematical artifact. It is a useful performance metric that correlates with the fatigue mechanisms that drive adaptation. If those fatigue mechanisms also act as signals for cellular changes—through AMPK and other pathways—then time spent near W′ depletion could be the stimulus for improving aerobic power and raising critical power.

Translating that to practice: tracking how much W′ a rider uses in a session and how long they spend near the point of exhaustion may provide a more accurate way to prescribe and progress high intensity training than using power alone.

W′ as a fatigue tank: a practical mental model

Think of W′ as a tank that fills and empties. When power is above CP, the tank drains; when power falls below CP, the tank refills. That mental model clarifies why two workouts with identical total time at an absolute power can produce wildly different physiological effects.

Consider these points:

• Intensity and duration together determine depletion: Short spikes above CP drain only a little W′; sustained efforts drain more.
• Recovery kinetics matter: The rate at which W′ is replenished depends on the difference between current power and CP, and that repletion is non‑linear.
• Power alone is an incomplete prescription: Two sessions that look the same on a power meter (same total minutes at X watts) can load W′ very differently and therefore provoke different adaptations.

Practical example: why 400 watts for 20 seconds is not the same as 400 watts for 60 seconds

The power value tells only part of the story. If an athlete spikes to 400 watts for 20 seconds, the amount of W′ expended is small. Hold 400 watts for a full minute and the W′ drain is much larger. Do that same 400 watts with different recovery durations and the total time spent in a low W′ state shifts.

For example, ten 1‑minute intervals at 400 watts with one minute recovery are not equivalent to ten 20‑second intervals at 400 watts with 20 seconds recovery. Even though the absolute power and the summed number of seconds at that power might match, the physiology—depletion depth, recovery kinetics, metabolic stress, signaling activation—changes.

Using W′ and time‑near‑depletion to structure progression

One practical way to use W′ is to track the total W′ spent in a training week and the cumulative time spent in the range where W′ is largely depleted. For high‑intensity training days, this could become the key progression metric, rather than looking only at power zones.

Example progression scheme:

1. Week 1: 10 hours of training, 20 minutes spent with W′ near depletion.
2. Week 2: Reduce to 15–18 minutes at that near‑depletion zone to allow recovery while maintaining stimulus.
3. Week 3: Increase to 25–30 minutes in the near‑depletion zone across the week to provoke adaptation.

This kind of minute‑by‑minute progression allows precise overload while preserving recovery. It also gives a way to quantify the stimulus that’s most relevant to improving CP and aerobic capacity in fast twitch fibers.

Which training elements still matter

W′‑focused training should be one element in a broader plan. The best programs combine:

• Low intensity aerobic base work: Long, slow distance to develop slow twitch fibers and metabolic efficiency.
• Threshold and sweet spot training: Moderate volumes near CP to improve the sustainable power and fatigue resistance.
• W′‑targeted high intensity sessions: Shorter, more intense sessions that deplete W′ and drive fast twitch aerobicization and mitochondrial signaling.

The bulk of training volume should remain low intensity. W′ monitoring is specifically useful for the high‑intensity component and should not replace time‑in‑zone metrics for aerobic base work.

How W′ relates to fiber types and VO2max

High intensity work that repeatedly depletes W′ likely stimulates aerobic improvements in type II and type IIa fibers. Those fibers are more glycolytic but also plastic; under an AMPK/PGC‑1α stimulus they can become more aerobic. This could translate into modest gains in VO2max and an increase in the muscle’s ability to sustain higher power outputs before hitting CP.

Meanwhile, slow twitch fibers respond best to long duration, low intensity work. That remains the pathway to increase metabolic economy and endurance at subthreshold intensities.

Limitations of the W balance model

The W balance model is elegant and useful, but it rests on simplifying assumptions that limit its accuracy in many real‑world situations. Two key issues stand out:

1. Recovery and depletion kinetics are not perfectly modeled

Current W′ recovery models assume a particular shape and rate to how quickly the tank refills. In practice those kinetics vary with intensity, preceding activity, muscle fiber recruitment, and individual physiology. The model performs well for long intervals—say one to ten minutes—but less reliably when workouts mix very short sprints, Tabata formats, and long intervals within the same session.

When athletes perform short sharp efforts followed by brief recovery, observed W′ behavior can deviate from the model: some riders show very rapid depletion followed by slow recovery, others recover quickly and then reach an unexpectedly large negative W′ during subsequent efforts. Those mismatches suggest the recovery function in the model requires refinement.

2. The model assumes instantaneous aerobic contribution at CP

The W balance math treats aerobic supply as if it instantaneously equals critical power. Physiology disagrees. When power increases suddenly the aerobic contribution (VO2) ramps up over time. At the onset of a hard effort the actual aerobic contribution is lower than the steady state at CP. That means the model tends to underestimate anaerobic expenditure during sudden increases in power.

In practical terms, if someone jumps from rest to 500 watts, their immediate aerobic supply will be a fraction of CP. The remaining energy required comes from anaerobic processes and fast depletion of W′. The W balance model, assuming aerobic output equals CP instantly, will therefore misallocate some of that early work and provide inaccurate W′ depletion estimates.

Where the model works and where it struggles

The W balance framework is most reliable for relatively homogeneous interval structures with consistent work and rest durations—typical controlled sets of 1–10 minute efforts. It also works best when input values (CP and W′) are accurate. Tests with poor pacing or performed in different freshness states produce inconsistent model behavior.

The model struggles when:

• Intervals include very short, maximal sprints mixed with long efforts and variable rest.
• Recovery durations are extremely short, such as Tabata or repeated 20 second sprints with little rest.
• Input CP and W′ are derived from poorly executed tests or tests conducted on different days with varying freshness.

Future directions: toward more realistic fatigue models

Improving the utility of W′ metrics will require two parallel advances:

1. Better empirical modeling of recovery and depletion kinetics. Collect large datasets across a diversity of interval formats and athletes, then fit functions that capture the true non‑linear dynamics of W′ repletion.
2. Reductionist modeling of fatigue processes. Instead of a single lumped W′ number, develop models that partition contributions from anaerobic energy, aerobic lag, ionic disturbances, and neuromuscular fatigue. That will allow practitioners to know not just how much performance is lost but why.

As models become richer, they will require more inputs: metabolic markers, respiratory measures, and perhaps blood assays. A more mechanistic approach will be slower to run and harder to apply broadly, but for high performance sport it could provide actionable partitioning of limiting factors.

Testing considerations: how to get accurate CP and W′ values

The usefulness of any W′‑based prescription depends on accurate CP and W′ estimates. A few practical testing tips emerge:

• Prefer single‑session testing protocols when possible. Tests that combine multiple efforts into one session reduce variability caused by between‑day differences in freshness and motivation.
• Include efforts that span the time ranges you care about. If you want the model to predict responses for 1–4 minute intervals, include those durations in the test.
• Use maximal and well‑paced efforts. Underpaced or overly conservative test segments yield underestimated capacities and skew model outputs.
• Repeat tests periodically and after major training blocks. CP and W′ are dynamic and respond to training, fatigue, and health status.

Practical session design using W′

When applying W′ in sessions, the following principles help translate the concept into everyday workouts.

Design sessions by targeting W′ depletion and time near depletion

Program intervals so the cumulative time spent in the near‑depleted range aligns with the week’s plan. Use shorter repeats with controlled rest to dose minutes in that zone precisely. Progress by increasing total minutes or by lowering recovery enough to increase time near depletion.

Balance W′ days with threshold and base days

Reserve W′‑focused training for one to two days per week for most athletes. Keep the majority of volume at low intensity to build base, and include dedicated threshold efforts to boost sustainable power.

Monitor recovery and be conservative with mixed formats

Mixed interval sessions that include very short all‑out sprints and long intervals may produce unpredictable W′ behavior. Exercise caution when using W′ estimates to program such sessions until modeled kinetics improve.

Common pitfalls and how to avoid them

• Relying on power numbers only: Power alone obscures how much W′ is actually being used. Track W′ depletion and time in near‑depleted state for high intensity prescribing.
• Interpreting W′ as the only adaptation driver: W′ is one stimulus among many. Aerobic adaptations from steady state work and threshold sessions remain essential.
• Using inaccurate CP or W′ inputs: Bad test data leads to bad prescriptions. Reassess parameters regularly.

Where W′ fits in the athlete development roadmap

For athletes focused on improving the ability to produce and sustain very high powers—time trialists with repeated surges, criterium racers, and riders who rely on short, decisive attacks—using W′ as a training metric can be transformative. When combined with thoughtful testing, close monitoring of time near W′ depletion, and appropriate recovery, it becomes a surgical tool for driving fast twitch aerobicization and raising critical power.

For athletes with objectives that favor sustained subthreshold efforts, W′ should not be the primary metric. Their focus remains on steady volume and threshold work to develop slow twitch capacity.

Summary: how to use W′ intelligently

W′ is a useful abstraction: a quantifiable tank of high intensity work available above critical power. It links to the cellular signaling pathways—AMP, AMPK, PGC‑1α—that drive mitochondrial adaptations. Using W′ to monitor cumulative depletion and time spent near exhaustion provides a promising way to dose high intensity training with precision.

However, the current W balance model simplifies kinetics and assumes instantaneous aerobic contributions that do not match human physiology in all situations. Improvements to recovery and depletion functions and a move toward more mechanistic, reductionist models will increase accuracy and expand practical application.

In practice:

• Use W′ to structure and progress high intensity sessions, focusing on cumulative time near depletion.
• Keep W′‑oriented work to one or two sessions per week and maintain a foundation of low intensity base and threshold work.
• Prioritize accurate testing and be cautious with mixed sprint/long interval formats.

FAQ

What exactly is W′ and how should it be interpreted?

W′ is the finite amount of work available above critical power. Interpret it as a tank that drains when power exceeds critical power and refills when below. It represents a combination of anaerobic energy stores and fatigue mechanisms rather than a single physiological entity.

How does W′ depletion stimulate adaptation?

Depleting W′ engages high intensity metabolic pathways and increases cellular signals like AMP that activate AMPK. AMPK then influences transcriptional programs such as PGC‑1α that promote mitochondrial biogenesis and increased aerobic capacity, particularly in fast twitch fibers.

Can I use W′ for all types of training?

W′ is most useful for quantifying and prescribing high intensity work. It is not helpful for long slow distance or pure aerobic base training because those efforts typically do not touch W′.

Why does the W balance model sometimes give weird results?

The model assumes a fixed recovery curve and that aerobic contribution instantaneously equals critical power. In real physiology, recovery kinetics differ by intensity and athlete, and aerobic contribution rises over time. Mixed interval formats with very short sprints or variable rest create mismatches between model predictions and observed fatigue.

How should I test to get reliable CP and W′ values?

Use well‑paced, maximal efforts within a single testing session when possible. Include durations relevant to the work you will prescribe. Avoid poorly paced or overly conservative tests and repeat testing periodically to account for changes in fitness and fatigue.

How much time should I spend near W′ depletion each week?

There is no one‑size‑fits‑all answer. A practical approach is to track total minutes spent near W′ depletion and progress that value slowly—adding a few minutes per week—while monitoring recovery and performance. For many athletes, one to two sessions focused on W′ depletion per week is appropriate.

Will focusing on W′ increase VO2max?

High intensity work that depletes W′ can increase aerobic capacity in fast twitch fibers and may contribute to modest VO2max gains. However, VO2max improvements also respond to sustained aerobic and threshold training. Both approaches complement each other.

Final thoughts

The intersection of physiology and performance modeling offers powerful tools to guide training. Understanding what W′ actually represents, how cellular signaling responds to high intensity work, and where current models fall short enables smarter session design and clearer progression. W′ is not a magic number but, used thoughtfully, it becomes a precise instrument for shaping the high intensity stimulus that drives meaningful adaptation.