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Triathlon transition: the price of fatigue
How does the effort level in the bike leg of a triathlon affect subsequent running performance? SPB explains why the answer depends on an athlete’s ‘durability’, and what that means for your pacing strategy
Over the last two years or so, there’s been a quiet revolution in how sports scientists regard the fundamental elements of fitness that determine endurance performance. These key elements are as follows:
· Aerobic capacity: VO2max - the maximum rate at which your muscles can utilize oxygen to create the energy needed for movement.
· Lactate threshold: the sustainable % of your VO2max you can use before fatiguing lactate begins to accumulate in the bloodstream, forcing you to slow down.
· Muscle economy: how efficiently your muscles can use oxygen to sustain a sub-maximal pace. Higher efficiency means less oxygen is required, which equates to better performances over longer distances.
The importance of these elements in determining fitness and performance has long been understood. But the revolution that is changing our understanding is the discovery that these three elements in athletes are not ‘fixed’ but can and do change during a bout of exercise, especially when the duration of the exercise becomes lengthy.
Therefore, unlike a car engine, which turns in the same maximum speed and miles per gallon regardless of whether it’s been running for ten minutes or ten hours, VO2max, lactate threshold and muscle economy of your body tend to decline as exercise progresses(1). This propensity for the key measures of fitness not to be fixed in stone but to change (decline) as exercise progresses is called ‘durability’; when durability is high, the declines in VO2max, lactate threshold and economy are relatively minimal and only set in after an extended period of exercise. Where durability is low, larger declines in these key performance markers are observed in a shorter period of time.
The importance of durability
The concept of ‘durability’ is more technically defined as ‘the time-dependent onset of physiological drift and degradation in performance characteristics during prolonged exercise’. All you need to know is that just like VO2max, lactate threshold and economy, good durability matters for performance. Indeed, recent research shows that a high level of durability is a key metric in how endurance athletes perform, with elite endurance athletes showing higher durability levels than amateur or recreational athletes(2). High durability - ie ability to resist physiological changes – has been shown to have clear performance advantages. For example, a marathoner with high durability can resist the inevitable performance deteriorating changes and maintain performance capacity for prolonged periods of time(3). Also, the greater heart drift commonly seen in amateur and recreational marathoners is associated with a decrease in running speed at the later stage of the race, as well as worse total time(4).
Durability in triathlon
The study of durability as a factor in endurance performance is relatively young. However, as this recent article explains, research into durability has looked its effects on how lactate threshold changes in runners (see figure 1)(5), how it impacts critical power (the fastest pace that can be held over a race distance without blowing up!) in cyclists(6), and the kind(s) of training that can favorably affect durability(7).
What’s noticeable however is that most of the research to date has looked at durability and how it impacts constant-effort performance during a single type of exercise (running, cycling etc). But what is not known is how one mode of exercise (eg cycling) can affect durability in another mode (eg running) – exactly the scenario that triathletes have to deal with as they transition from the bike leg to the running leg. What is also unknown is how fluctuating effort levels during endurance exercise affect durability. This scenario also applies to triathletes since permitted drafting means that the bike leg can involve steady pacing tucked into the pack interspersed with repeated, explosive surges, where one or more riders attempts a breakaway(8). This kind of cycling (sometimes referred to as ‘stochastic cycling’) places heavy demands on the anaerobic energy systems, causing significant additional muscle fatigue before the running even begins!
Figure 1: Durability of first lactate threshold over time
In this 2024 study on recreational runners, the researchers found clear durability effects over the course of a 90-minute run, with the lactate threshold decreasing 5.8% in females and 5.3% in males. This meant that while these athletes started their exercise at an intensity just below that required to produce a rise in blood lactate, the lack of durability meant they drifted into that threshold, causing a rise in lactate, even though the exercise intensity remained unchanged.
New research
What is the precise impact of triathlon bike leg on subsequent durability in running performance and what are the implications for triathletes? For some answers on this unexplored topic, we can turn to new research by a team of British scientists from the University of Loughborough(9) Published in the ‘International Journal of Sports Physiology and Performance’, this study set out to investigate if, and by how much, a previous bout of stochastic cycling impacts the running durability in triathletes transitioning from the bike to the run leg.
Eight highly trained male international and national level junior and under-23 triathletes were recruited for the study. These were aerobically very fit athletes with maximum oxygen uptake figures (VO2max) averaging 66mls/kg/min. All of the athletes underwent three different lab testing sessions on three different occasions:
· Visit 1: baseline testing, which involved an incremental ramp test on a treadmill followed by a time-to-failure (TTF) test to establish their baseline performance when they ran in a fresh condition.
· Visit 2: this consisted of a similar incremental ramp test as above, but this time on a cycle ergometer to measure cycling performance in a time-to-failure test. In addition, 10-second all-out sprint power was measured.
· Visit 3: this was the simulated race trial. In this trial, the triathletes completed a simulated sprint triathlon cycling leg, but instead of using a steady power output equating to their average typical output over a bike leg of this distance, the power protocol was highly variable (stochastic), fluctuating low-moderate intensity riding mixed with very high intensity surges (using 110% of sustainable aerobic output) along with a number of all-out 10-second sprints. Following this very demanding protocol, the triathletes moved directly onto the treadmill (to simulate the bike-run transition) and replicated the running tests performed during visit 1 but this time in a fatigued state.
Following these trials, the researchers analyzed the collected data to see if and how the key metrics of endurance performance – aerobic capacity, lactate threshold and muscle economy – changed from the ‘fresh’ run to the fatigued run following the bike leg.
What they found
The results showed that on average, the simulated sprint triathlon bike leg performed before the run had a massive negative impact on performance compared to that same run performed fresh:
· Aerobic capacity - the first finding was that VO2max dropped significantly in the fatigued run compared to the fresh run, which also meant that the athlete’s maximum pace at which they were working flat out also dropped.
· Lactate threshold - the running speed at which athletes hit their second lactate threshold/critical intensity (the pace at which lactate rapidly accumulates forcing a slowdown) dropped very significantly in the fatigued state compared to the fresh state. Basically, this means that the maximum sustainable pace dropped – obviously a huge disadvantage in any endurance event.
· Muscle economy – economy worsened in the fatigued state, indicating that the athletes had to work at a much higher relative percentage of their (already diminished) maximum capacity just to maintain a baseline pace!
· Durability – possible the most interesting finding was that within the above finding, there was a huge amount of individual variability; some triathletes experienced only minor declines in their lactate thresholds and running performance (ie they had high levels of durability) while others experienced catastrophic declines after the exact same cycling protocol.
Overall, the findings showed a big drop in running performance when the athletes ran in the fatigued state. However, this averaged finding disguised very large differences in durability. The durable triathletes saw only a slight decline in running performance between their fresh and fatigued state. However, the triathletes with poor durability experienced something of a running performance collapse in their fatigued state compared to fresh!
Practical advice for triathletes
This new study has profound implications for the way triathletes need to prepare for and pace a triathlon running leg. That’s because most triathletes base their training zones and race-pace targets on testing in a ‘fresh’ state. However, your optimum critical pace when fresh is NOT the same as after a gruelling bike leg! To assess the correct pacing strategy (for example using a 5km run test), you will need to conduct some ‘fatigued’ run testing – eg by performing a 5km time trial immediately after a hard and variable bike workout that contains surges and sprints. If you set your projected running pace based on fresh run testing, the likelihood is that you will blow up during the run leg and be forced to slow down. And while it’s harder to execute in training, the same principles apply to a lesser extent (because there’s less muscle overlap) to setting your correct biking pace after a gruelling swim.
Another implication that follows is your back-to-back bike-run sessions to train your muscles for the T2 transition. While performing a steady-state bike ride followed by a run is useful for speeding the process of getting your ‘running legs’ back, it does not properly prepare you for a transition after a bike leg in an actual race. To do this, you need to combine steady-state riding at below lactate threshold with a number of surges and sprints. For example, you could perform 4 x 10-minute blocks, with each block consisting of alternate 45-seconds just below lactate threshold and 15 seconds of surging or sprinting then move straight to running.
An additional implication thrown up by this study is that your running ability between fresh and fatigued is likely to be disproportionately affected by surges and sprints on the bike compared to steady-state riding. If your goal therefore is to bag the fastest time possible, smoothing out your effort on the bike is paramount to help ensure the impact of fatigue in the run leg is minimized. Each time you accelerate to pass a rider or push hard up a short hill, you are replicating the kind of effort that negatively impacted running performance. This is where a power meter can prove invaluable; keeping your power output from spiking too much and slowing a little up a hill can protect your running legs for later!
The final implication lies in the finding that not all athletes suffered a crash in running performance; in the athletes that had excellent durability the key metrics running performance declined only very slightly. Therefore, the question is how can you train your own durability to resist this fatigue-induced decline? In a recent article, we looked at evidence that training to improve durability is likely to be a long-term process measured more in months and years rather than weeks. Therefore persistence and building up training volumes over an extended time period is key. Including low-intensity, long, steady training sessions will help improve the structural adaptations required to allow your muscles to efficiently clear the metabolic by-products of hard bursts of effort more rapidly. This can potentially help improve durability in the longer term. However, research also suggests that per unit of time invested, higher-intensity training is likely to improve durability more than lower intensity(7). This means that endurance athletes who are seeking to improve durability should also be sure to include some regular high-intensity sessions in their programs.
References
1. Sports Med 2021. 51, 1619–1628
2. J.Physiol. 2024 Sep;602(17):4113-4128. doi: 10.1113/JP284205. Epub 2023 Aug 22.
3. Sports Med 2022. 52, 2283–2295
4. Front. Psychol 2020. 10, 3026
5. Eur J Appl Physiol. 2024 Oct 9. doi: 10.1007/s00421-024-05631-y
6. Int J Sports Physiol Perform. 2025 Mar 25;20(6):866-868
7. Front Physiol. 2023 Feb 16;14:1128111
8. J Sci Med Sport. 2008 Jul;11(4):424-32
9. Int J Sports Physiol Perform. 2026 May 7:1-9. doi: 10.1123/ijspp.2025-0469. Online ahead of print
Andrew Hamilton
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