You do not need to become obsessed with physiology to train well. Mostly easy, sometimes hard, still takes most athletes a very long way. But it does help to understand what coaches and scientists actually mean when they use terms like threshold, LT2, Critical Speed, Critical Power, or CSS.

"How do I use Citicial Speed (CS) run pace in Training Peaks? Currently, I have my hour-long threshold pace in there rather than CS."

This question reflects one of the most common misconceptions in endurance training: the idea that threshold is defined by how long you can sustain it, rather than by the physiological state itself, whether that is Critical Speed (CS), Functional Threshold Power (FTP), or Critical Swim Speed (CSS). When an athlete anchors threshold to an hour-long pace, they often end up training below the intensity needed to drive the intended metabolic adaptations. The rest of this article explains why that gap exists and how to correct it.

To be completely open, this confused me for a long time. Swimming seemed to treat 1500m pace as threshold pace, even though for competitive swimmers that will mean less than 20 minutes of work. Cycling was heavily anchored to hour power (even here it was being clarified and refined). Running, meanwhile, appeared to be shifting from half-marathon or 15 km pace toward something closer to 10 km pace through Critical Speed and Critical Power testing. It all felt inconsistent, and for a long time I struggled to understand how such different anchors could all be pointing to the same underlying physiological intensity.

So first, let’s define what threshold is.

Physiologically, threshold is often described in relation to Maximal Lactate Steady State (MLSS), the highest intensity at which blood lactate remains relatively stable over time. Lactate is best understood here as a useful marker of the underlying metabolic strain, rather than the sole cause of fatigue.

Once you move above that intensity, the disturbance begins to build progressively and the effort becomes far less sustainable. Stay at or just below it and you can maintain the effort for a substantial period.

In the early days of cycling power meters, Functional Threshold Power (FTP) was widely popularised as “the power you can hold for one hour.” That was a useful shorthand, not a biological rule. The real idea was always the highest power an athlete can sustain in a steady or near-steady physiological state, not a fixed 60-minute performance for every rider.

Some elite cyclists can hold that boundary for close to an hour, but that reflects exceptional durability, economy, and training status, not the definition itself. The clock is only a rough guide. The physiology is the real anchor.

Bottom line: whether you use pace or watts, the goal is to identify the same metabolic red line, the highest intensity at which internal strain remains relatively stable rather than progressively rising. In running and swimming, Critical Speed, Critical Power, and Critical Swim Speed are practical field estimates of that same boundary, giving you a useful proxy for MLSS without needing a lab test.

If you anchor your training on an "hour power" model across all three sports, you risk using a performance outcome as your reference point instead of the physiological boundary you actually want to target.

A 60-minute effort is highly aerobic, but it is not defined by aerobic steady-state metabolism alone. It is also shaped by W′, your finite work capacity above critical intensity, along with pacing, fatigue resistance, and sport-specific durability. That means an hour pace or power can sit slightly above or below your true metabolic threshold depending on the athlete and the sport.

When training is prescribed from that anchor, the aerobic stimulus often becomes less precise than intended. To drive the right adaptation, training should instead be anchored to your actual metabolic aerobic limit: MLSS, CP, CS, or CSS.

In previous articles, we explored how training systems drift when intensity is not clearly bounded. That same logic applies here.

We anchor training zones to LT2 (MLSS), or to practical field proxies such as Critical Speed (CS), Critical Power (CP), or Critical Swim Speed (CSS), rather than to maximal outputs like vVO₂max or race-based heuristics.

VO₂max defines your ceiling. LT2 defines the boundary between sustainable and non-sustainable work. For training purposes, that boundary matters more, because it determines how much quality work you can accumulate and repeat without progressively depleting W′, your finite reserve of work above critical intensity.

Anchoring zones to LT2 does not make training easier, it makes training repeatable. It keeps threshold work honest, reduces accidental intensity drift, and ensures each session targets the intended metabolic system rather than slipping into a no man’s land between zones.

If Threshold is a metabolic steady state, why can't we maintain it for the same duration in every sport? The reason lies in metabolic clearance vs. mechanical occlusion.

In Swimming (The Occlusion Problem): Water drag requires high force throughout the stroke. High muscular tension compresses the capillaries within the muscle (venous occlusion). This creates a "local" environment where Hydrogen ions (H+) and metabolic byproducts cannot be "washed out" into the bloodstream effectively. You reach cellular failure faster because the waste products are trapped in the muscle fiber.

In Running (The Membrane Problem): The "pounding" of running causes eccentric muscle damage. This microscopic tearing of the cell membranes leads to "calcium leakage." This leak disrupts the electrical signal required for muscle contraction, meaning your "battery" (W') drains due to structural breakdown, even if your aerobic engine is still "steady."

In Cycling (The Clearance Advantage): Cycling has a very high "duty cycle" efficiency. There is a clear "off" phase in the pedal stroke where tension drops and blood can flow freely to clear byproducts. This allows the cellular environment to stay cleaner for longer, which is why cycling threshold duration is typically the longest.

If you use a Stryd power meter, you will be familiar with Critical Power (CP). CP and CS are two sides of the same coin:

Critical Speed (CS): Measures your metabolic limit in terms of Pace (min/km).

Critical Power (CP): Measures your metabolic limit in terms of Work Output (Watts).

Power accounts for external variables like hills, wind, and terrain.

The 10km Anchor: In practice, your CP will often land close to your best 35–45 minute effort (often around 10k race intensity). This is why your Stryd "Auto-CP" often looks "aggressive" compared to a 1-hour power estimate; it is finding your actual physiological limit, not a theoretical clock-based duration.

Think of your fitness as a battery-powered engine. The relationship between your Engine (Threshold) and your Battery (W') defines your Phenotype.

Nerd Note: W' vs D' You may see some articles refer to D' (D-prime). This is the exact same concept as W', just measured in meters (distance) rather than Joules (energy). While Stryd uses W', pace-based systems often use D' to tell you how many "bonus meters" you have in the tank when you sprint.

The Diesel (High Threshold / Small Battery): Massive engine, tiny battery. Can cruise at a high pace all day but has no "kick." Surging just 5% above threshold causes rapid failure (within minutes).

The Rocket (Lower Threshold / Massive Battery): Smaller cruising engine, huge "emergency reserve." Can surge, sprint, and attack repeatedly. Dangerous in a finish-line sprint but struggles with 40-minute steady-state grinds.

Bottom line: Understanding your phenotype is a powerful diagnostic tool. While it helps you decide where to place "matches" in a race, its primary value is identifying if a low speed ceiling (small battery) is the bottleneck preventing your threshold from moving higher.

Most recreational athletes without a sprint background present as accidental "Diesels." Because they lack high-intensity background, their W' (Battery) is often severely underdeveloped. For these athletes, raising the "speed ceiling" through specific speed work is essential. Making your top-end speed faster makes your threshold pace feel comparatively easier.

Your Critical Power/Speed is not a fixed number; it is a perishable one. In sports science, this is called Durability. Your CS at the start of a race is different from your CS after you have burned 2,000 calories. High-level endurance training isn't just about moving your "fresh" threshold higher; it’s about making your threshold durable so it’s still there in the final hour of a race.

Bottom line: This is why we don’t just test CS, we build the ability to hold it when tired.

As you become more advanced, your Power-Duration Curve flattens out significantly, narrowing the gap between "Easy Pace" (Zone 2) and "Threshold Pace" (Zone 4/CS).

The Elite Profile: A world-class marathoner's Zone 2 pace might be only 15 to 20 seconds per km slower than their Threshold pace.

The Recreational View: Don't try to force this by running easy days too fast. True Zone Compression is a result of fitness, not a method of training. You want your easy pace to naturally "drift" toward your threshold over years of consistency.

When you go above threshold, you are "spending" your W' battery.

The Drain: In swimming, drag is so high that W' disappears almost instantly if you surge. In cycling, you can "meter" the drain precisely.

The Recharge: To refill the battery, you must drop below threshold. It is easiest to "recharge" in cycling (coasting). It is hardest in swimming and running (constant work/impact), which is why recovery intervals in those sports feel less "refreshing."

If your “threshold” is anchored to something like 15 km pace, short threshold sessions can miss the mark. They may be too slow to create a strong ceiling-raising stimulus, yet still hard enough to generate meaningful fatigue and consume recovery capacity. The result is a kind of no-man’s land: not hard enough to target the intended upper boundary of steady-state fitness, but not easy enough to be low-cost aerobic volume either.

Critical Speed and Critical Power solve that problem by anchoring training to the boundary that actually matters. They let you place threshold work right on the limit of sustainable metabolism, where the stimulus is both repeatable and specific.

That does not make hour pace or 15 km pace useless, it simply means they should be used deliberately as session intensities, not as the reference point for the whole system.

The mechanical demands of each sport dictate how long you can hold that engine at its limit:

Bottom line: Set CS/CP correctly, and your training stops drifting.

You do not need to become obsessed with physiology to train well. Mostly easy, sometimes hard, still takes most athletes a very long way. But it does help to understand what coaches and scientists actually mean when they use terms like threshold, LT2, Critical Speed, Critical Power, or CSS.

That understanding helps you cut through jargon. It helps you recognise when different terms are describing the same underlying idea, and when they are not.

It helps you become your own best coach, because you are less reliant on borrowed rules of thumb and better able to judge the purpose of a session for yourself. And it helps you avoid hype, because you are less likely to be distracted by labels, trends, or false precision.

In the end, the value is not complexity, but clarity. Understanding the physiology helps you cut through the jargon, avoid the hype, and make better decisions about your own training. That does not replace simple training. It just helps you understand why simple training works.