Critical Speed: Swim Faster With Less Work

A new study says that a subtle, but important, change in the way we test ourselves in the pool and train could reap huge dividends with less yardage in the pool.


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We all know that swimming for the first time after a several-month hiatus feels like you’ve never swum before.

We also know that getting back to a good level of fitness takes a couple of weeks of hard work. But what can we do to get back to our pre-lockdown levels—and maybe improve?

In a recent study from England’s University of Bath, researchers found that—contrary to popular belief—shorter, high-intensity training sessions can create even better results than the standard big volumes that swimmers used to do.

But the benefits didn’t just come from fast and furious training sessions. They also came from the swimmers being tested regularly. In their study “Individualizing Training in Swimming: Evidence for Utilizing the Critical Speed and Critical Stroke Rate Concepts,” the authors (a few of whom are triathletes and triathlon coaches) found that swimmers’ results improved when using the “Critical Speed” testing protocol and assessing the “Critical Stroke Rate” for each athlete (more on these two below).

The researchers tested 12 highly trained swimmers for 15 weeks in the fall of 2019. While the study was limited to 12 subjects—and only high-level athletes—the researchers believe that their findings are likely to apply to other swimmers, including age groupers.

The test consisted of a three-minute, all-out swim to determine their Critical Speed (CS, measured in m/s, but can be converted to splits for use in workouts) and their ability to work above this threshold (D’, pronounced “D prime”—a number based off a very complex set of calculations). They later performed four 200m tests at CS to establish their Critical Stroke Rate (CSR, the highest stroke rate that a swimmer can maintain for an extended period).

After the tests, came the sessions: a block of three key high-intensity workouts per week. The main set was 5 x 3-minute intervals to deplete 60% of their D’; the second session targeted 3 x 3.5-minutes to drain 80% of their D’; and the third had a 10 x 150m (or 200m) swim at CS at a lower CSR (and programmed with a tempo trainer). Even the workout formats themselves, and terms like “depleting D’” may feel unfamiliar to triathletes, but that’s kind of the point.

The subjects repeated the test and training cycles for a total of eight testing visits and three HIIT blocks.

After the series of blocks (where the athletes swam 20.8 km per week on average, 15K performed at below CS and 5K at or above it), the swimmers showed an improvement in CS of 5.4%. They also showed an improvement in their 200m predicted time by 2.7%, and their personal best for their first and second specialty event by 1.2% and 1.6%.

While that may not seem huge, first bear in mind that these were already well-trained athletes, and the biggest thing of all: This was achieved by reducing their swimming volumes by almost 50% (from 40-45 km per week). The reduction in volume was not only a huge time saver, but is also beneficial to reduce injuries, burnout, and dropout—traditionally associated with the high-volume approach to swimming.

What is Critical Speed, exactly?

Critical Speed is the swimming (and also running) equivalent of the Critical Power (CP) protocol used in cycling. CP calculates the power output that a cyclist could theoretically sustain for an extended time. CS substitutes power with speed in both running and swimming, and for swimming it’s calculated as the average speed of the last 50m of a 3-minute all-out test. In a pool, one would need to take 10m splits of the final 50m and average them out to learn critical speed.

“In swimming, CS represents not only a physiological threshold but also a biomechanical one. As soon as you go above CS, your efficiency starts to reduce drastically,” said the study’s lead researcher Eva Piatrikova, who also is a sports scientist working with Olympic athletes including Slovakian Olympic triathlete (and superswimmer) Richard Varga.

If you’re more familiar with FTP (Functional Threshold Power), that’s a different protocol that tries to individuate the same values of the CP—just in a different way. FTP either uses 95% of the average power sustained over a 20-minute test or the average sustained over a full 60-minute one.

CP and CS, on the other hand, regularly use two to three different bouts of exercise (of 2 to 15 minutes in length) to calculate that value. Though also derived from power and speed files, CP and CS have always been seen as the scientific and more precise counterparts of FTP. On a power-duration curve (power over time on a Cartesian plane), the CP/CS is determined by the asymptotic value of power output/speed. If this doesn’t all make sense, that’s because CS and D’ involve some pretty heavy mathematical lifting that’s not quite ready for “consumer” use.

Performance Predictors

“The critical power/speed (CP/CS) concept represents a physiological boundary involving respiratory, metabolic and contractile physiological profiles which provide great scientific and practical utility,” said exercise physiologist Peter Leo, who was not involved in the study. “CP or CS is a physiological boundary or fatigue threshold where homeostasis can be achieved (below CP/CS) or not (above CP/CS). This concept enables important insights into central or peripheral fatigue mechanisms at different exercise intensities and the influence of oxygen delivery on cardiovascular and metabolic control mechanisms. Due to its physiological foundations, the CP/CS concept can be used to precisely predict endurance performance in various sports disciplines (swimming, cycling, running, rowing).”

In other words, the theory behind the study is strong and accurate, but we also still have a long way to go before triathletes can use CS to create workouts and sessions like the ones the test subjects did. Leo adds that if practitioners or coaches use the wrong protocol or approach, they end up overestimating CP/CS. As a result, the duration from 2 to 15 minutes is preferred for the tests.

“In swimming, that would correspond to one bout of 150-200 meters, one of 400-500 and one of 800-1000,” he said. “The minimum required in prediction trials is to apply the work-distance time model or at least three to use the hyperbolic approach.”

What are all these calculations aiming at?

All the protocols mentioned above are trying to individuate the threshold power or speed that athletes could theoretically sustain for a long time. In physiological terms, that’s known as Maximal Lactate Steady State (MLSS), and it’s the point at which lactate accumulation and lactate combustion in the muscles are somewhat in balance. Below that point, the power, speed, and intensity are more sustainable, and the metabolic system on balance.

Above that value, the lactate accumulation will spike exponentially, and the power/speed/intensity will not remain sustainable for long.

Research has also shown that this threshold (also known as lactate threshold, just threshold, or LT2) is not a fixed value, but a blurred line—further complicating what is already a fairly abstract concept.

Critical Speed vs. Critical Swim Speed (CSS)

Critical Swimming Speed (CSS) aims to determine the same CS’s value but through a different testing protocol. CSS, which is mostly used in the triathlon world (particularly in the U.K.) to test athletes in the pool, is typically split into two parts: 200m and 400m time trial tests. The calculation that results in gives athletes and coaches the prediction of the pace that athletes can theoretically sustain over 1,500m.

“They [CS and CSS] both test the threshold,” Piatrikova said. “If you go above that threshold, you are going to fatigue, build blood lactate, and reach VO2max, and so on. It’s a Maximum Lactate Steady State. The problem is that if you use CSS, you use time trials of 200 and 400 meter TT, which usually overestimate these thresholds by about 2-3%. If you want to get valid CSS, you’d have to do ideally 200 and 800-meter TT to get more accurate values. But the problem is that most people don’t have time for that.”

Piatrikova and her colleagues used the 3-minutes all-out because it’s shorter, still highly accurate and scientifically validated, and its results can be used in both a pool and an open-water environment.

What is the Critical Stroke Rate?

It’s defined as the biomechanics surrogate of the CS. It represents the highest stroke rate that a swimmer can maintain for an extended period and that swimmers naturally adopt when swimming at threshold. That also means that asking a swimmer to swim at CS with a lower stroke rate than their CSR will force them to choose a longer stroke length, which will improve their stroke efficiency and economy. We measure CSR as one stroke cycle in freestyle, in other words, from the right-hand entry to right-hand entry. You can measure the stroke rate from three complete cycles usually from the middle of the pool to avoid the impact of turns on stroke mechanics.

“Age groupers should focus on both CS and CSR as they are linked—you can improve CS through both physiological (i.e., shifting thresholds to higher speeds) and technical improvements (i.e., improved stroke length/distance covered per stroke). They are both inherently interlinked,” Willsmer said. “This means coaches can focus on multiple aspects of performance that can improve an athlete’s swim times in a triathlon.”

Are there benefits of this protocol and training regime for age groupers?

“Age groupers normally don’t have much time for training, so you want to maximize the little time they have for training and individualize their training regime,” Piatrikova said. “So, if you can reduce the volume, and get the same benefit if not bigger, that’s probably the best strategy. There’s plenty of research now saying that in swimming, a big volume at low intensity is not as effective as a smaller volume at high intensity. The reason is that swimming is a lot more complicated than cycling and running. The energy cost of swimming goes up exponentially, even if you want to improve only a few seconds over 100 meters. And that also brings changes into technique: When you’re practicing your technique at a low intensity you’re not adopting the technical skills you’d be using in a race”.

But this doesn’t mean that all the sessions should be performed at high intensities. Some will be still planned at lower speeds.

“In swimming, you just don’t want to do an unnecessary volume,” Piatrikova said. “Also because swimming always takes the smallest amount of time in a triathlon—in short, and long-distance alike—it’s more important that you nail the technique for the speed you’ll have in a race.”

Where do we go from here?

Even the test alone—finding your average speed in m/s over the final 50m of a 3-minute time trial—is already quite difficult to execute. If you want to try the protocol, you’ll need the help of another coach or athlete, who can set up 10m markers on the side of the pool and time you with a stopwatch, noting your time over each of those final 10-meter markers as you swim. And then you’ll need to do the CS and D’ calculations, which may require reaching out to the research authors themselves! Even if you don’t use the study’s models in your training, however, it’s another confirmation that in triathlon swimming bigger volumes do not necessarily pay dividends on race day. What’s more important is testing your fitness regularly—whatever the protocol—and working in high-intensity sessions at the correct-for-you speed and stroke rate.