The Swimming Paradox

The Swimming Paradox

Have you ever done a tough swim workout that felt hard, but you barely got any faster? You’re not alone. Unlike cycling or running, swimming presents a unique challenge, something I’ll call the Swimming Paradox.

In cycling or running, the math is simple: push a certain wattage or pace, and your body gets a predictable physiological stimulus. In swimming, that link between effort and speed is far less stable.

This means a swimmer can be working extremely hard but, due to technical slips (hips dropping, head lifting, stroke breaking down), much of that effort is lost to drag instead of forward motion. The pool clock then tells a story that doesn’t match the reality of what the body went through.

Every small technical adjustment can keep you balanced and moving forward, while one slip can send all your effort into fighting the water rather than propelling through it. For developing swimmers, this instability means RPE (“feel”) often works better than pace for gauging intensity. 

But what about when technique is more stable and you want training to target physiology with more precision?

Coming Across the DSU System

As part of a recent swim coaching education program, I came across a fascinating methodology from the Danish Swimming Union (DSU).

It was originally developed under national coach Paulus Wildeboer and is built on the physiological framework of Dr. Jan Olbrecht (The Science of Winning). At first glance, it struck me as a very practical way of reconnecting pace to physiology; without needing a complex lab setup.

The DSU system is not something I’d say everyone should adopt. Rather, it’s a model that seems particularly valuable for experienced swimmers who want to ensure that the hard work they put in is always matched by the right adaptation.

I’m still exploring the system myself, but what I like about it is how it addresses the “Swimming Paradox” head-on with both a demanding field test and built-in safety checks that prevent technique breakdowns and anaerobic contribution from skewing the results.

Technique First for Developing Swimmers

Before going further into the DSU test, it’s worth highlighting again that for newer or slower swimmers, time is far better spent improving efficiency and skills than chasing longer sets or faster times. Every gain in body position, balance, and propulsion makes later training vastly more effective. If your strokes are inconsistent, the pool clock won’t give you a true picture of progress.

The 3000m Test

The system is anchored by a challenging 3000m Time Trial (TT) — designed to measure the highest aerobic effort a swimmer can sustain. Alongside this test, coaches apply three safety checks to make sure the data reflects both physiology and technique.

A Note on Lactate Testing

One of the ongoing debates in swimming science is the value of lactate ramp testing. Given how strongly technique influences pace — and how inconsistent that link between effort and speed can be. I’ve always been cautious about relying on a step test to precisely identify LT1 and LT2 swim paces. A single change in swim form mid lap can throw the numbers off.

Where I do find lactate testing valuable is in the way the DSU system applies it: as a secondary check at the end of a fixed distance effort. Instead of chasing exact inflection points from a ramp protocol, the DSU approach uses lactate to validate whether the 3000m pace was metabolically honest. This feels like a more pragmatic use of physiology in swimming — less about pinning down a perfect curve, more about confirming that what we saw in the water was supported by the body’s response.

How the Test Generates Your Training Paces

This is the clever part of the DSU system: once you have your average 100m pace from the 3000m TT (validated by the safety checks), that number becomes the anchor for all your training zones.

A Real Example

Let’s say an athlete completes the 3000m TT in 54:00 minutes. That equals an average pace of 1:48/100m. Using the official DSU tables, the training targets are specified exactly for different interval lengths.

Easy swimming, focus on DPS, relaxation, technique:

Steady endurance:

Strong aerobic effort, just under LT1:

Threshold effort (around LT2). Sustainable in sub‑60 minute events; for longer races true race pace is lower (AE2–AE3)

Note: The DSU tables also provide LC (Lactate Capacity) and VO₂max target times.

LC vs VO₂: Same Times, Different Purpose

One detail that stands out in the DSU charts is that LC and VO₂ share the same target times. What differentiates them is not speed, but set design and training effect:

• LC (Lactate Capacity): Workouts are structured to deliberately accumulate high lactate levels, then train the body to tolerate and clear it. This is where swimmers learn to deal with the painful “burn” of racing.

• VO₂max: Sets are designed to push oxygen uptake to the maximum. The same swim times are used, but with different interval lengths, rest periods, and volumes to emphasise aerobic power rather than lactate tolerance.

This subtle distinction highlights the sophistication of the DSU approach: it recognises that training effect comes not only from pace but from how the session is constructed.

Two Tables a Training Toolkit

The DSU protocol uses two main training tables, you can download a copy with the following links:

• AEC (Aerobic Capacity) – builds the fuel tank during base training.
  

• AEP (Aerobic Power) – tunes the engine during competition prep.

This division resonated with me because it mirrors the way endurance coaches think in other sports, but tailored very directly to swimming’s technical and metabolic demands.

The Six Key Intensities (DSU Model)

Note: In the DSU charts, LC is listed before VO₂. This ordering reflects the training philosophy rather than a strict mapping to a 5‑zone endurance model. In a classical model, VO₂max work would normally be seen before lactate tolerance. But the DSU framework distinguishes them by training effect: LC emphasises tolerance/clearance of high lactate, VO₂ targets oxygen uptake. Since their target times are identical, the DSU charts simply present LC before VO₂ to group the lactate‑specific work ahead of aerobic power work.

DSU vs CSS: First Impressions

Most swimmers are familiar with the Critical Swim Speed (CSS) test, which uses a 400m/200m time trial. It’s simple and effective for estimating threshold pace, but it doesn’t account for whether that pace is achieved with good sustainable technique or an honest aerobic contribution.

By contrast, the DSU/Wildeboer System is more demanding — but it checks technique and lactate to ensure the training speeds are physiologically valid.

• CSS is quick, easy, and perfect for recreational swimmers.
  

• DSU is more complex but better suited to advanced long-distance and open-water athletes who want the certainty that their pace really matches their physiology.

Closing Thoughts

For me, the DSU system feels less like a “replacement” for CSS and more like a next step for those who want to dig deeper. I find it intriguing that a national federation built such a structured approach to balance physiology and technique. Others must exist, but like this one, they are certainly not common knowledge.

• For developing swimmers: Don’t worry about whether you can complete the 3000m under an hour. Focus instead on efficiency, body position, and skills. Every gain in technique makes the metabolic work that comes later more effective.
  

• For advanced swimmers: The DSU test offers a clear, honest way to ground your training in race-relevant paces. If you’re chasing long-distance goals, it’s an inspiring challenge worth attempting.

Whether your next step is improving your feel for the water or benchmarking yourself with a 3000m TT, the DSU framework provides valuable tools to help guide progress.

And for my own Fast Lane swimmers — Åsa, Jesper, and the Johans — don’t be surprised if your Christmas present this year looks a lot like a 3000m time trial (with a side of lactate testing). Happy laps.