EVIDENCE-BASED PRACTICAL ADVICE FOR ENDURANCE ATHLETES

Swim training: what is ‘critical velocity’ and why is it critical for performance?

Andrew Sheaff explores a cheap, quick, and easy alternative to monitoring and planning your swimming performance, and explains why this alternative can be just as effective as more invasive testing protocols

While access to advanced physiological testing can certainly aid performance, most athletes don’t have the opportunity or the financial resources to utilise these resources.  Even if one is able to be tested on a one-off basis, this isn’t especially useful as physiological testing is most effective when employed regularly over a period of time. These obstacles are particularly evident in swimming as the principle of specificity informs us that swimmers need to be tested in the water, requiring even more specialised equipment.

Physiology and performance

Some of the foundational exercise science studies have established the importance of lactate threshold for endurance performance(1,2). These original studies have been further supported with subsequent research(3).  Specific to swimming, the best predictors of performances for a 400m swim race were shown to be the speed achieved at 85% of V02max and the speed associated with the accumulation of 4mM of blood lactate(4). In this particular study, both VO2max and lactate threshold were tested by researchers using fairly extensive equipment, likely unavailable to the typical recreational and even professional athlete.

While there are other similar physiological metrics that can be used such as respiratory compensation point, anaerobic threshold, or maximal lactate steady state, these tests all require specific equipment as well as individuals with the necessary expertise to conduct these tests. But though this information is clearly valuable to any swimmer, access will be an issue for most. However, while physiological testing may be out of the question, a concept known as ‘critical power’ can be used provide the same information – with none of the required equipment.

What is critical power?

Anyone with any training background can recall swimming, running, or cycling along at a pace they felt like they could sustain forever. Aerobically speaking, they may very well have been able to sustain that speed. At the same time, we’ve all had experiences where we’ve exercised at a level of output that we certainly couldn’t (and didn’t!) sustain. Somewhere between these two extremes exists our critical power, or the theoretical pace that we can sustain indefinitely(5). This pace is thought to represent an important parameter of aerobic metabolism, inclusive of all of the systems that contribute to aerobic performance.

While the concept of critical power is theoretically intuitive, it needs to be practically useful to be relevant to swimmers. Fortunately, critical power is not just a theoretical concept; it can be expressed mathematically as the result of several simple calculations. As power is quite difficult to measure in the water, velocity can be used as a surrogate for power. In this respect, critical velocity is a concept much like critical power, the only difference is that velocity, as opposed to power, is measured.

Critical velocity is the metric that is used in swimming research, and it’s the metric most useful to aspiring swimmers. When several (distance, time) data points are plotted graphically, we can calculate the slope of the line that best fits these data points, and this slope is equivalent to the critical velocity of a given swimmer. We’ll explore how to calculate your critical velocity below in a manner that is quick and easy. Critical velocity is particularly valuable because it takes multiple performance measures into account. This allows for coaches and swimmers to differentiate between individuals who are similar in one measure yet differ greatly in another(6). With that in mind, let’s look into the research on critical velocity in the pool.

Critical velocity in swimming

Due to the obvious utility of critical velocity in the pool, critical velocity has been a popular topic in the swimming research, and much is known about its relationship to other physiological parameters. As we’ll explore below, critical velocity has been established as a reliable correlate of aerobic performance, can be used to create predictable physiological responses in training, and improves as a result of long-term aerobic training. As a result, it can be used to control training intensities and monitor training progress(7). We’ll explore all of these benefits below.

Critical velocity has been consistently shown to be related to multiple markers of aerobic function. Research has demonstrated that critical velocity is similar to the maximal lactate steady state (MLSS), which measures the fastest speed a swimmer can achieve without increasing lactate levels(8). This marker is considered the gold standard for measuring aerobic performance. However, it appears that while critical velocity and MLSS are strongly correlated, they may not be the same, and likely can’t be used interchangeably(9). For our purposes it’s not important to precisely match the MLSS; what is important is that critical velocity can be used as a surrogate marker for aerobic performance.

In addition to the research involving MLSS, it’s been shown that there is no significant difference between critical velocity(10), the velocity at 4mmol/L of blood lactate concentration(10), or the speed achieved during a 30-minute swim test(10,11). Further, critical velocity was significantly correlated with the velocity associated with the onset of blood lactate accumulation(12) and oxygen uptake (VO2) at anaerobic threshold(13).

Beyond its relationship to metabolic thresholds, critical velocity represents a technical threshold as well. Critical velocity represents a transition point characterised by a significant increase in stroke rate(14), with stroke parameters changing unpredictably once critical velocity is achieved(15).

Using critical velocity

While critical velocity may not strictly represent the results of invasive physiological testing, the results are clearly correlated and correlated strongly. This should give swimmers confidence that using critical velocity in place of more extensive testing is a sound way to monitor and prescribe training. Having established this link, what happens when critical velocity is used to set training intensity?

A group of swimmers performed six sets of 5-minute swims with 2.5-minute recovery periods between repetitions on two occasions – once at 95% of critical speed and once at 105% of critical speed. During the 95% trial, swimmers were able to maintain stable blood lactate, perceived exertion, peak torque, stroke length, and stroke rate throughout all six sets. However, when at swimming 105% of critical speed, lactate, perceived exertion, and stroke rate all increased, while muscle torque and stroke length decreased(16). Swimmers performing ten x 200-metre (m) swims with a 8:1 work to rest ratio were able to sustain a speed comparable to critical velocity(17). The speed was slightly slower and faster when performing similar sets of 5 x 400m and 20 x 100m, respectively.

This is important to note; shortening the repetition distance can allow swimmers to achieve slightly faster speeds under the same physiological stress, because longer repetitions at critical velocity tend to lead to greater lactate accumulation(6). It appears therefore that training sessions approaching critical velocity are sustainable when recovery periods are included. However, if swimmers wish to perform sessions above critical velocity, they will need to significantly increase recovery periods between each repetition – or shorten the repetition duration. In figure 1, you’ll find a series of training sessions inspired by research findings(6,16,17).



When it comes to long-term training adaptations, critical velocity can be improved in only 4-6 weeks through the use of either continuous endurance training or interval training(18. Indeed, interval training bouts of 2-5 minutes have been shown to increase critical velocity(19). After nine weeks of an aerobic training phase, critical velocity was shown to improve in swimmers(20).

Putting it into practice

Hopefully you’re sold on the utility of using critical velocity to monitor your training and plan your training. The first step to using critical velocity to enhance your training is to actually calculate your own critical velocity. The actual swim testing needs to consist of 2-5 maximal-effort freestyle swims over distances ranging from 50m to 2000m. In general, the consensus is that an accurate estimation of critical velocity can be achieved with swims over less than four distances. However, it’s important to note that critical velocity will be most accurately estimated when longer distances are used(21).

Because there can still be some confusion as to which distances to use, and how the testing sessions should be conducted, I have included some guidelines in figure 2. The most important aspect of the process is consistency. Consistency in testing conditions will ensure the results generated are reliable. When considering how to time your efforts, a training partner or friend who can use a stopwatch would be your best bet. If that option isn’t available, using a digital pace clock can be effective. The less precise your timing method, the longer distances you should employ, as the results will be less impacted by small errors.



Once you have your time trials completed, you’ll need to convert your times into seconds. Once you’ve converted your performances into seconds, you can use the link below to calculate the slope of the line. Ensure to place the time values as x-coordinates and the distance as y-coordinates(5,22).

https://www.easycalculation.com/statistics/regression.php

As a quick example, we’ll calculate the critical velocity of a theoretical swimmer with swim times equivalent to the world records over 100-m, 400-m, and 1500-m, which are listed in figure 3. His critical velocity of 1.696 m/s would result in a 3:54 400m swim, a swim he could theoretically sustain indefinitely! Figure 4 shoes a graphical representation of the critical velocity concept using these specific results. You can check to ensure that you’ve understood the process by typing the information into the link above. If you’re savvy enough, this calculation can also be performed in most spreadsheets. For those in need of a summary, a brief overview of the process necessary to calculate your critical velocity can be found in figure 5.



Figure 4: Linear plot produced by using data above



As a quick aside, the y-intercept, or the number that the regression line crosses y-axis, does have physiological significance. It is thought to represent the anaerobic work capacity, although research is much less conclusive about the nature of this value, what determines it, how it responds to training, and its relationship with performance(5,22).

Applying critical velocity in training

Once you’ve calculated your critical velocity, monitoring training is simple and effective. As importantly, there are multiple ways you can track your progress on a short-term or long-term basis. In the short-term, you can compare training sessions guided by critical velocity to gauge your performance. If you’re continuing to improve your performance during critical velocity sets, you can be confident your critical velocity is improving. Included less frequently, testing how long you can sustain critical velocity is a great way to evaluate improvement. Lastly, actually re-testing critical velocity will let you know where you stand, although sufficient time should have elapsed between tests to allow for physiological changes to occur. For an overview, see below.

Monitoring training

*Short Term

  • Compare daily and weekly training sessions.
  • Strive to increase repetition distance or total volume, or decrease rest period during critical velocity sets.

*Long Term

  • Re-test critical velocity every 2-3 months.
  • Test sustainability at current critical velocity every month.

Critical velocity sets are challenging. As such, critical velocity sets should be used judiciously. When used on a weekly basis, these sets can be used 1-2 times per week. Other non-critical velocity sessions should be scheduled during the week to allow for recovery, or to target other training components such as speed, skill, or strength. If you’re looking to create an overload week, three critical velocities can be implemented during these overload weeks. However, no more than two overload weeks should be scheduled in a row, and adequate recovery should be implemented after overload weeks.

What’s particularly valuable about critical velocity is that while it is related to physiological measures, it is also a direct measurement of performance. In short, as improvements in critical velocity are a direct result of improvements in race specific distances. If critical velocity improves, you can expect performances in competition as well!

Designing training sessions

While the sample training sessions provided above were a great starting point, repeated four or five sets can get pretty monotonous! Fortunately, you can learn to design your own training sets following some simple guidelines, while using the sample training sessions as a baseline.  Please refer to the figure below for guidelines as to how to best design training sets.


Summary

Physiological testing can be valuable to athletes of all types. Unfortunately, access is restricted for most due to time, money, and availability. This is particularly true of swimmers due to the challenge of specific testing in the water. By using critical velocity, swimmers can accurately monitor and design their training to enhance their performance. All that is required is a pool, stopwatch, and a lot of effort!

Key learning points

  • Critical velocity is a theoretical concept with a physiological basis that can be defined as the maximal velocity that can be sustained indefinitely.
  • Critical velocity is related to measures of endurance performance such as lactate threshold, velocity at maximal lactate steady state, and the velocity associated with the onset of blood lactate accumulation.
  • Critical velocity can be calculated quickly and easily without any sophisticated equipment.
  • Critical velocity training sets have a predictable physiological response and critical velocity can be improved with consistent training.
  • Critical velocity can be used to effectively monitor training and design training sets.

References

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  21. Int J Sports Med. 2006 Feb;27(2):117-23.
  22. Sports Med. 2017 Mar;47(Suppl 1):65-78.

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