Critical Velocity

There are lots of different tests that endurance athletes can use to gauge their fitness and determine appropriate training intensity zones: VO2max tests, lactate threshold tests, time trials of various lengths, perceived effort calibration tests, the list goes on. Today I’m going to tell you about another protocol, one that has been around for a while but has lately been the object of heightened interest among exercise scientists.

It’s called the 3-minute all-out test and it’s pretty much what it sounds like. I say “pretty much” because it might sound like a 3-minute time trial, and it’s not that. In a traditional time trial, you pace yourself, aiming to cover as much distance as possible in the allotted time, which of course requires that you hold back in the beginning. In the 3-minute all-out test, the idea is to start as fast as you can and hang on. A truly maximal exercise output is only sustainable for 8 to 10 seconds, so athletes are expected to lose steam as they go, and invariably they do.

The rationale for this protocol is the existence of a critical output (measured as power or pace) that represents the highest output an athlete can sustain in a stable metabolic state. When this threshold is exceeded, the athlete’s internal state becomes unstable and exhaustion is hastened. “Hastened” doesn’t mean instantaneous, mind you. The average trained endurance athlete can sustain their critical power/velocity for 20 to 30 minutes.

The design of the 3-minute all-out test is quite clever. You might think that if you take off at a dead sprint and maintain full gas as long as possible, you will collapse in a heap within 3 minutes. But that’s not what happens. Instead the athlete slows down progressively and involuntarily until they’ve slackened enough for their metabolism to stabilize, at which point they are able to continue for some time. In other words, the athlete slows down until they reach critical power/velocity, making the 3-minute all-out test an effective way to pinpoint this value.

Some athletes level off sooner than others when performing this test, but almost everyone stops losing speed by the 2.5-minute mark. Therefore, average power/velocity in the last 30 seconds is used as an estimate of CP/CV. Validation studies on the protocol involving runners and cyclists have found that its results closely match those obtained from traditional, lab-based CP/CV tests.

One limitation of these validation studies is that they were also done in a laboratory environment. To address this limitation, scientists at the Sports Performance Research Institute New Zealand conducted another validation study to determine whether the 3-minute all-out test remains a reliable predictor of CP/CV when done in the field. Fifty-three cyclists and triathletes performed a pair of 3-minute all-out tests on their own indoor bike setups. A subgroup of ten of these athletes also visited an exercise lab and performed a sequence of constant work-rate cycling bouts at the power output estimated as their CP based on the results of the 3-minute all-out field tests. Oxygen consumption, carbon dioxide emission, and blood lactate were measured throughout these constant work-rate bouts to determine if the athletes were truly in a stable metabolic state.

The researchers found that the 3-minute all-out field test overestimated critical power by more than 16 percent. The reason appeared to be that subjects defied instructions and paced the test, slowing down more rapidly than is seen in the lab but not as much. The authors of the study speculated that two main factors caused the subjects to pace the test despite being told to go all-out. The first is that, whereas at home they were able to see time elapsed and time remaining, in the lab subjects are not given this information. It’s much harder to sustain a truly maximal effort for a full 3 minutes when you how much longer you have to keep going! The second reason is that, in the lab, experimenters shout encouragement at subjects as they pedal, whereas the subjects of this field study were not observed or encouraged.

On the basis of their findings, the authors concluded that “the [3-minute all-out test] should not be used to estimate [critical power] in endurance-trained cyclists.” But there’s another conclusion they might have drawn, which is that, if you wish to use this protocol on your home trainer, don’t monitor elapsed time (I recommend programming a 2.5-minute interval followed by a 0.5-minute interval into your device and then focusing your attention elsewhere until you hear that second chime) and psych yourself up by having your spouse shout encouragement at you or by doing the test with a partner or blasting your favorite workout music. Unless I’m missing something, these measures ought to take care of any discrepancies between the field test and the lab test.

Now the question becomes, what do you do with the result? Well, if you use the 80/20 Endurance zone scale, it’s pretty straightforward. Critical power is slightly lower than functional threshold power (FTP). Since FTP is used to calculate training intensity zones, you need to use your CP (your average power in the final 30 seconds of the 3-minute all-out test) to calculate your FTP, then enter the latter into the 80/20 Endurance Zone Calculator to get your current power zones.

The tricky part is that the precise mathematical relationship between CP and FTP varies by fitness level, with the fittest athletes having the smallest gap between the two. For the fittest athletes, CP is 102 percent of FTP. If your weight-adjusted average wattage in the last 30 seconds of the 3-minute all-out test exceeded 4.0 watts/kg, use this formula to estimate your FTP: CP ÷ 1.02 = FTP. If your weight-adjust power was between 3.0 and 3.9 W/kg, divide your CP by 1.03 to get your FTP. And if your weight-adjusted power was 2.9 W/kg or less, divide by 1.04. The same math applies to running, though you will need to use speed rather than pace to do the calculation and then convert to pace for the Zone Calculator.

Although I’ve mostly made peace with no longer being able to train due to long covid, I would love to be healthy enough to try the 3-minute all-out test for myself. I was experienced enough as an athlete to have been able to judge immediately if the results were valid (for me). As it is, I need you to guinea pig the test for me. Give it a go and email me with your results and observations. Thanks!

In last week’s blog post I mentioned that two new studies related to the phenomenon of VO2max had been published recently, and I described one of them, which showed that sustainable power declines more shallowly with increasing time in cyclists with higher VO2max scores. Today I’d like to tell you about the other study I alluded to, which sheds just as much light as the first on the phenomenon in question, but from a different angle.

This one was conducted by Benedito Denadai and Camila Greco of Paulista State University in São Paolo, Brazil, and published in the journal Current Research in Physiology. It was premised on the observation that, in any group of runners of different abilities, VO2max is a very good predictor of performance in races of any distance, whereas in a group of elite runners, VO2max has less predictive power at any distance. More specifically, in any mixed group of runners there will be a wide range of VO2max values, and those with higher values will tend to perform better in races of all distances. But among elite runners, VO2max values are relatively homogenous, and although some elites will perform better than others at either middle distances, long distances, or ultra-distances, few will perform better than others at all distances and the small differences in VO2max values among these runners fail to account for individual superiority at any distance.

This suggests that other components of fitness besides VO2max also make an important contribution to race performance, and that these components differ by race distance. The purpose of Denadai and Greco’s study was to identify these distance-specific contributors to race performance in elite runners. To fulfill this purpose, the two scientists conducted a retrospective analysis of data from past studies using elite runners as subjects. With the aid of sophisticated statistical tools that I don’t understand, they were able to evaluate the relative strength of each fitness component’s contribution to performance in races of various distances.

Here’s what they found: For 1500m specialists, velocity at VO2max (or vVO2max) is the strongest predictor of performance. A high vVO2max comes from having a high aerobic capacity and good running economy. At the 3000m distance, vVO2max and blood lactate response to exercise were coequal predictors of performance. Specifically, the velocity at which a runner’s blood lactate level reached 4 mM predicted performance as accurately as did their velocity at VO2max. This is not surprising, because the ability to attain high velocities at low blood lactate levels is also rooted in aerobic capacity. For runners specializing in the 5000m and 10,000m track events and the marathon, velocity at lactate threshold (2 mM) is the best predictor of performance. While related to velocity at 4 mM, this component of fitness is slightly different, having more to do with the ability to avoid producing lactate through aerobic metabolism at a high rate than the ability to metabolize lactate itself.

Velocity of lactate threshold is also the best known predictor of performance in elite ultrarunners, according to Denadai and Greco, but I say “known” because research on athletes in this category is sparse. I’d be willing to bet that respiratory exchange ratio (RER) is a stronger predictor of performance at ultra-distances than it is at shorter distances. RER is the velocity at which carbohydrate metabolism overtakes fat metabolism as the primary source of muscle energy and it comes from having a high fat-oxidation capacity.

Overall, the findings of this study underscore the need for limited specificity in training. To a great extent, fitness is fitness in running regardless of which race distance you specialize in. We see this in the fact that all elite runners have a high VO2max. Whether you race the 1500, 10K’s, marathons, or ultras, your training should focus on developing your aerobic capacity. However, fitness is not exactly the same across the spectrum of race distances. At each distance, athletes need a little more of certain fitness components, and a little less of certain others, than they do at other distances.

This is where specificity comes in. The hardest workouts a runner does in their heaviest period of training should simulate the specific demands of their event. For 1500m runners, short intervals (1-3 minutes) run at or near vVO2max fit the bill. For 3000m runners, such workouts should be coupled with somewhat longer intervals at a slightly lower intensity. For 10,000 specialists, long intervals and tempo efforts run between critical velocity and lactate threshold velocity are the best peak workouts. Marathoners should couple these workouts with sustained efforts run between half-marathon and marathon pace, and ultrarunners, of course, should make multihour long runs the hardest workouts they do in their heaviest period of training.

Ain’t science neat?

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