Man and machine: endurance and the durability factor

Another very recent study highlighting the importance of durability in endurance performance comes from a team of French researchers who have been investigating the link between durability and critical speed/critical power⁽⁸⁾. Critical power (CP) and critical speed (CS) determine the maximum power or speed you can maintain for a given distance, and are key markers for helping athletes to determine the correct (fastest) pace that can be held over a race distance without blowing up! CP and CS can also be used as fitness monitoring tests – for example maximum pace and distance covered in the 3-minute all out test.

In this study, the researchers set out to investigate whether an hour of heavy-intensity running carried out prior to a 3-minute CS test induced a decline in CS and other performance parameters. Seven trained runners completed a set of separate 3-minute all-out tests to measure their 3-minute CS. Multiple tests were carried out and then averaged to provide a more accurate assessment of CS and to confirm the reliability of the test – ie that it gave a similar result for the same athlete even when carried out on different occasions.

On a subsequent visit, the runners ran for 1 hour at 85% of their CS (ie moderately hard) to induce fatigue while wearing a portable GPS device (to measure the distance covered). The average speed during the fatiguing run was 3.6 (0.4) metres/sec (8.0mph), representing 78% of the athletes’ CS and 86% of maximal heart rate After this 1-hour run, they then performed another 3-minute all-out test to assess CS under fatigue. What they found was that the prior 1-hour run caused the athletes’ CS to decline by 6% - a clear illustration of the durability effect. Another finding was that D′ (a measure of the ability to run above critical speed during a CS test) declined by a massive 68% following the 1-hour run. In addition, total running distance, maximal speed, and time to reach maximal speed were also significantly reduced. These findings clearly demonstrate that concepts such as CS and CP are durability dependent. This in turn means that your critical speed for a race at the start of the race is not the same as later in the race, because it will decline through the race.

Training durability

If durability is so important, how can athletes train it to minimize ‘drift’ as a race or event unfolds? Is there a specific type of training that endurance athletes can do that targets building durability? For some answers to this, we can turn to a 2023 study also carried at the University of Jyväskylä, which is one of the first intervention studies to examine how low- (LIT) and high-intensity (HIT) endurance training affect durability⁽⁹⁾. Specifically, they investigated the ‘time of onset’ and magnitude of deterioration in key physiological characteristics over time during prolonged exercise’.

What they did

In this study, a mix of sedentary and recreationally active men (16) and women (19) completed a 10-week training program of consisting of one of the following two types:

·        Low-intensity cycling (LIT) - average weekly training time 6.8 hours per week.

·        High-intensity cycling (HIT) – 1.6 hours per week.

LIT - consisted of cycling under first lactate threshold (easy-moderate intensity). The weekly 5 - 6 training days included long (1.5 - 4 hours), medium (1 - 1.5 hours), and short (45 - 60 minute) sessions. Weekly training hours progressed individually based on perceived exertion from 4.5 up to a maximum of 12.5 hours. Subjects did their training mostly outdoors with their own bike with dual-sensing power meters.

HIT - consisted of 2 - 3 weekly indoor training sessions using Wattbike Trainers, or indoor trainers with their own bicycles equipped with power meters. Training consisted of high-intensity work intervals 3 - 7 minutes long with recovery periods that were ¾ of the work interval duration. In the first training week, there were 15 minutes of cumulative high-intensity time in a training session, and it progressed individually up to 30 minutes per session.

Before and after the training program, durability was assessed during a 3-hour cycling session at 48% of pre-training maximal oxygen uptake. In particular, durability was assessed using:

·        Measures of ‘physiological strain’ - defined to be the absolute responses of heart rate and its variability, blood lactate levels, and rate of perceived exertion.

·        The absolute magnitude of the physiological drift that occurred

·        The onset of drifts - i.e. how quickly factors like alterations in energy expenditure, heart rates, rate of perceived exertion, and breathing rates etc changed.

What they found

The key finding was that both types of training resulted in improved durability – ie less drift in key physiological measures over the 3-hour cycling sessions. Overall, there was a reduction in the magnitude of drift for the factors measured of 1.2% and 2.4% for the LIT and HIT programs respectively. However, it was also noted that the improvements in durability were really quite small. The onset of drift was delayed by around 25 and 29 minutes on average for the LIT and HIT groups respectively.

However, when the data was statistically analyzed, it turned out that the (small) gains in durability were similar in both groups, which mean that both the low and high-intensity training programs produced small and similar gains, and neither was significantly superior to the other (see figure 2). The authors concluded that ‘despite the fact that durability was enhanced in the participants, the 10-week intervention did not alter drifts and the onset of these drifts by a large amount, even though it attenuated physiological strain’.

Figure 2: Durability changes following LIT and HIT training

Left hand (A/C/E) plots = LIT program; right hand (B/D/F) plots = HIT program. Top = heart rate; middle = perceived exertion; bottom = lactate.  All plots show the drift in the factors measured over the 3-hour bike session, comparing pre-training with post-training. Higher levels of durability are indicated by a flatter slope to the line (less change over time). Lower durability is shown by a steeper slope. Note how the line slopes are barely changed between pre and post-training, indicating only minimal durability improvements.

Implications for athletes

As Andrew Sheaff explained in his article, your physiological capacities are not fixed, but are subject to the effects of fatigue. So while a higher lactate threshold, economy, and aerobic capacity will improve your performance, it’s important to understand that the ability to sustain those performance parameters as much as possible is also critical. From the data above, we can see that the declines or ‘drift’ in performance measures due to the durability effect are significant. This means that a) athletes need to take them into account when developing training and pacing regimes and b) athletes should endeavor to improve durability where possible.

In the study above, we can see that while both LIT and HIT 10-week programs improved durability, the gains were small in absolute terms. This suggests that training durability to significantly improve it is likely to be a long-term process measured more in months and years rather than weeks. So persistence is key! Interestingly, the improvements in durability were similar in both the low and high-intensity programs even though the HIT program involved just a quarter of the training time. This suggests that per unit of time invested, higher-intensity training is likely to improve durability more than lower intensity, and therefore that endurance athletes who are seeking to improve durability should be sure to include regular HIT sessions in their program.

A final point is to be aware that when fitness testing or setting pacing strategies (eg running at a % of critical speed), the durability factor will come into play. Therefore, to ensure consistent and reliable data, you should undertake testing or set your pacing strategies in a fresh state – not after you have been training or undertaking other forms of exercise. Bear in mind too that if you have a strategy to set a certain pace later on in a race, a calculation made when you are fresh will almost certainly be too fast later on in a race when the durability effect has kicked in. Always test your strategy first in training!

References

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