VO2 Kinetics Explained: The Need-To-Knows For Cyclists
At the start of an intensive competitive event, or even within a training session, a rapid transition from a low to high metabolic rate occurs.
According to Westerblad et al. (2020) “the energy consumption of skeletal muscle cells may increase up to 100-fold when going from rest to high intensity exercise” (emphasis ours).
This article takes a look at so-called ‘VO2 kinetics’ - i.e. the responsiveness of your aerobic system - and the role this plays in your ability to respond to a rapid change in exercise intensity.
We will also look at ways of improving your VO2 kinetics so that you can better handle changes in exercise intensity, including acute strategies (e.g. warm-up routines) and longer-term strategies (e.g. training interventions).
Energy System Use
When exercising, energy is provided by the break-down of ATP (“adenosine triphosphate” - the body’s energy currency) to ADP (“adenosine diphosphate”). However, the body only has sufficient ATP stores to produce energy for a matter of seconds. This means that the body’s energy systems are immediately called upon to regenerate sufficient ATP to fuel the continuing effort.
The 3 main energy systems involved in supplying the required ATP are:
The phosphocreatine (PCr) system: This creates ATP by transferring a high-energy phosphate ion from creatine phosphate (‘PCr’ - stored in the muscles) to ADP to form ATP, without the need for oxygen.
The glycolytic system: This creates ATP via the breakdown of carbohydrate anaerobically (i.e. again without oxygen).
The aerobic system: This creates ATP via combustion of fat and carbohydrate (or more specifically lactate/pyruvate, which are the products of the glycolytic system) using oxygen.
These systems contribute to ATP production in tandem but have different “boot up” speeds, as well as different proportional contributions that change as efforts progress.
The response of the aerobic system to a change in exercise intensity - known as ‘VO2 Kinetics’ - is particularly important, as it impacts the amount of work that can be done at high intensities above the threshold power before fatigue hits. This can play a role in competition performance, as well as the effectiveness of training sessions.
Energy System Contribution
The figure below, taken from Gastin (2001), shows how the contributions from the different energy systems evolve over the course of a 90-second maximal effort. Note that the figure below is slightly out-dated, and doesn’t fully reflect current physiological understanding (in particular, that all energy systems continue to contribute to energy production to at least some extent). However, for the purposes of this article it presents the key message - namely that the different energy systems respond differently at the onset of exercise.
The aerobic system has the longest “boot up” speed, meaning it’s the slowest to react to a change in exercise intensity. This is because the aerobic system involves many steps to ultimately produce ATP, and it can take around 2 minutes for the aerobic system to be fully operational at its maximum capacity (Hill & Stevens, 2005).
The glycolytic system is faster to respond than the aerobic system, being fully operational within around 10-15 seconds, and the PCr system is even faster yet, kicking in within seconds (Gastin, 2001). These two anaerobic systems therefore supply the bulk of ATP during the first ~10-30 seconds of exercise.
While the contribution of the aerobic system ramps up more gradually, it does eventually overtake the anaerobic systems as the greatest energy contributor later into an effort (presupposing an effort is longer than 30-60 seconds). If the exercise intensity is high enough and the effort long enough, the energy contribution from the aerobic system will continue increasing until VO2max is reached.
By ‘high enough’, we mean above the maximal lactate steady state/second ventilatory threshold/muscle oxygen saturation threshold, which we’ll refer to as the ‘threshold power’ going forward. In effect, threshold power is the maximum power that can be sustained for an extended period where metabolic process (once ‘booted up’) can remain in a steady state.
The figure above also shows how the energy system contributions differ for endurance-trained and sprint-trained athletes, with endurance trained athletes having lower relative contributions from the anaerobic systems and a higher contribution from the aerobic system. Ultimately, for both sprint and endurance trained athletes, the aerobic system becomes the main contributor to energy production after approximately 30-seconds of exercise.
Disadvantages Of ‘Anaerobic’ Metabolism
As many cyclists are theoretically and/or experientially aware, there are serious limitations related to the sustained use of the anaerobic energy pathways, despite these systems providing energy very rapidly. PCr stores in the muscles are incredibly limited and are expended within a matter of seconds of near-maximal exercise. Moreover, the PCr and glycolytic systems are associated with the production of metabolic byproducts that appear to contribute to fatigue, meaning these systems are also not sustainable for extended periods (Cairns, 2006; Westerblad et al., 2002).
Furthermore, although the anaerobic energy systems are quick to supply energy, their total yield is far lower than the aerobic system. For example, 2 ATP molecules are created from one molecule of glucose by the glycolytic system. In contrast, the aerobic system is able to supply approximately 30 ATP molecules in the complete oxidation of glucose (the precise amounts vary due to efficiency limitations) (Rich, 2003).
Add to this the fact that the aerobic system’s byproducts (carbon dioxide, water and heat) are non-fatiguing, and you have what’s clearly the most desirable energy source for sustained high intensity.
Oxygen Deficit & Fatigue
Due to the latency of the aerobic system, the body is forced to rely on the ‘less optimal’ anaerobic systems in the early stages of exercise. The anaerobic systems effectively ‘plug the gap’ between the workload demand and the oxygen supply.
This gap is represented in the graph below (adapted from Gastin, 2001) by the grey space between the “Energy demand” line and the “Aerobic” line, and is sometimes referred to as the ‘oxygen deficit’. The larger the oxygen deficit, the more energy must be derived through anaerobic pathways.
This initial ‘oxygen deficit’ is seen both in efforts above and below the threshold power. However, the oxygen deficit becomes of increased importance in efforts above threshold, because the energy demands are so high that the fatiguing byproducts produced in this initial phase of exercise cannot be fully cleared.
This raises a question for the performance-driven cyclist:
Can you reduce the oxygen deficit and limit the need for anaerobically-derived energy during sustained supra-threshold efforts by ‘booting up’ the aerobic system faster?
This is a very good question, because improving the responsiveness to the aerobic system (i.e. improving 'VO2 on-kinetics’) would mean less reliance on the anaerobic systems during the early stages of the effort; meaning less depletion of PCr stores, and a lowered rate of production of fatiguing byproducts.
Ultimately, this should mean that a supra-threshold effort can be sustained for longer and/or at a higher intensity, or that the effort can be repeated sooner. These abilities are important for a wide variety of cycling disciplines, including hill climbs, road, crit, cyclocross, MTB XC, to name a few.
Improving VO2 on-kinetics might also be important for training. For example, training sessions that target improvements in VO2max often aim to accumulate time training close to VO2max. If the aerobic system can be made to respond more quickly, then more time might be accumulated close to VO2max for a given workload, potentially leading to a more effective training session.
Fortunately there are a few strategies that can be used to improve VO2 on-kinetics. These include acute strategies that can be used before or during a race or high-intensity training session, as well as long-term training strategies. We’ll look at these in turn.
Warm-Up Strategies
Arguably one of the best, and easiest ways to improve your VO2 on-kinetics is via a targeted warm-up routine.
There is clear evidence that a bout of hard exercise during a warm-up can help to speed up the VO2 kinetics so that oxygen consumption reaches VO2max (or close to VO2max) more rapidly. This is often referred to as a ‘priming’ effort.
To highlight one study, Jones et al. (2003) studied the effects of a 6-minute bout of heavy exercise above threshold power on subsequent performance in exhaustive bouts at 100%, 110% and 120% of power at VO2peak versus no prior exercise. They found that the prior exercise allowed for longer times to exhaustion and the achievement of VO2peak much quicker than without prior exercise.
The results from this study are copied below, where the open dots show oxygen consumption after a priming effort, and the black dots show oxygen consumption after no warm-up.
How to prime?
The majority of studies looking at the use of ‘priming’ within a warm-up have used a priming effort lasting 6-minutes with around 10-20 minutes of recovery (rest or very light pedalling) between this priming effort and the start of competition or an intensive interval training session.
However, other styles of priming effort can also work, provided that the efforts are above the threshold power, and are of sufficient duration that lactate levels are elevated somewhere between 2-5mmol/L at the start of competition/interval training (Burnley et al., 2005).
So instead of a continuous 6-min effort, you could, for example, include a series of shorter intervals within your warm-up, provided that these elevate your blood lactate levels in the required range.
It’s probably best to avoid severe-intensity exercise (e.g. sprints) that elevate blood lactate beyond 5mmol/L though, as these may impair performance (Burnley et al., 2005; Burnley et al., 2011).
When to prime?
A key question when it comes to the inclusion of a priming effort within your warm-up is when to include this. A study by Burnley et al. (2006) looked at this specific question, by examining VO2 kinetics in response to 2x 6-min high-intensity efforts. The efforts were repeated on multiple occasions, separated by differing recovery periods (ranging from 10-mins to 60-mins).
The VO2 kinetics during the second 6-min effort at shown below, where the open dots show oxygen consumption after a priming effort, and the closed dots show the ‘control condition’ where no priming effort was included.
The study shows that the benefits of a priming effort seem to last between 10-45 mins. So there’s some good scope for flexibility when it comes to fitting a priming effort in ahead of a competition or interval session.
The optimal length of time between your priming effort and the start of a competitive event or interval session is probably 15-20 minutes.
Individual variability
It’s worth noting that there are individual differences when it comes to the type of priming effort that works best. Some athletes may respond better to a priming effort of more moderate intensity and a longer duration (e.g. 10-12 minutes just below the lactate threshold) (Burnley et al., 2005). We’d therefore recommend testing out various priming routines in training and low-priority competition to see what seems to work best for you.
The optimal priming routine should result in an effort feeling easier than it would without such a priming effort (particularly after the first few minutes). This should provide a good subjective indication of whether a warm-up protocol is working for you, especially if you’re unable to test lactate levels.
Recommended priming routine
If in doubt, we would recommend the following warm-up routine as a good starting point:
Begin your warm-up by riding for 10-30 minutes gradually building up the intensity from around ~45-50% FTP (2-3/10 effort) up to around 65-80% FTP (3-4/10 effort level). This will help gently warm up your muscles and ligaments and reduce the risk of musculoskeletal injury.
Complete a ~6-min effort at around your 20-minute maximal power (or approximately 103-108% FTP, or an effort level of around 8/10).
Ride lightly (45-60% FTP or 2-3/10 effort level) for 10-15 minutes, then rest or continue to ride easily until your competition/intervals.
Try to time your warm-up so that the 6-minute effort is completed around 15-45 minutes before your competition or interval session. In practice this will mean starting your full warm-up routine around 40-60 minutes before your event.
As an aside, this is broadly the warm-up strategy Tom has used during the 2021 hill climb season to take 9 victories (including 7x course records) and a safe win in the National Hill Climb Championships, where rapid VO2 kinetics are a major performance factor.
It’s worth pointing out that a priming effort might not be beneficial for certain activities where the speed of the VO2 kinetics are less determinative of performance (namely those that do not begin with efforts above threshold). For long events (i.e. those lasting several hours, and beginning at a sub-threshold intensity) it might even be detrimental to perform a high-intensity priming effort as this may induce unnecessary fatigue.
Generally speaking, priming warm-ups are most strongly recommended for any events lasting between around 2-mins to 40-mins, or for events that begin with a hard effort off the start line (e.g. MTB XC or cyclocross). They are also useful for interval sessions that seek to accumulate time training close to VO2max.
Optimising Interval Pacing
Another benefit to understanding VO2 kinetics comes in the form of being able to create optimised pacing strategies for certain interval sessions.
Bailey et al (2011) conducted an interesting study comparing fast-start, evenly-paced and slow-start approaches to pacing to investigate which might be most effective in different scenarios (namely, a shorter 3 minute maximal effort and a longer 6 minute maximal effort).
In this study, the participants followed a predefined pacing strategy, before being asked to perform an all-out sprint in the final 60-seconds.
For the 3-minute effort, there was a clear benefit to beginning the effort with a hard start. By including a hard start, participants were able to perform a greater overall workload, and were able to spend more time training close to VO2max due to faster VO2 kinetics.
For the 6-min effort, the benefits of the hard start were less apparent, although the VO2 kinetics still tended to be slightly faster than the other pacing strategies.
Another study by Ronnestad et al. (2020a) compared 5x 5-min constant power intervals at 90% speed at VO2max, with 5x5-min intervals that started with 1.5M at 100% speed at VO2max, and then dropped down to 85% speed at VO2max (among elite XC skiers). Both sets of intervals had a similar average work output, yet the hard-start intervals accumulated more time above 90% VO2max (12.0-mins versus 10.8-mins). What’s also interesting is that the participants found the hard-start intervals subjectively easier than the constant power intervals.
Overall, for interval sessions aiming to accumulate time training close to VO2max, or for sessions seeking to reduce the subjective effort level, then hard-start pacing strategies might be good to adopt.
Enhancing VO2 Kinetics Via Training
So far we have focussed on acute strategies for improving VO2 kinetics. However, one way to improve the responsiveness of your VO2 on-kinetics over the long-term is through training.
Research suggests that, much like VO2max, VO2 kinetics can be limited by central and/or peripheral components. Central components relate to the ability of the body to extract oxygen from the air and deliver this to the muscles, and peripheral components relate to the ability of the muscles to take on and process oxygen.
Key factors dictating the responsiveness of VO2 on-kinetics appear to include factors such as the responsiveness of the heart to the onset of exercises (a ‘central’ factor), and the availability of substrates and responsiveness of enzymes necessary for aerobic metabolism (both ‘peripheral’ factors) (Grey et al., 2015). The specific limiters for a given athlete will be individual and will vary between athletes.
The speed of response of these components can be improved through training in several ways:
Long-term aerobic endurance training:
A number of studies support the use of endurance training at low and moderate intensities below the threshold power for enhancing VO2 on-kinetics (Murias et al., 2011; Grey et al., 2015).
These improvements are thought to be mediated via both structural changes (e.g. increased capillary density) and functional changes (e.g. quicker vasodilation, leading to better muscle blood flow) (Grey et al., 2015).
It’s well-known that high volume, long duration workouts stimulate a wide variety of important aerobic adaptations, so it’s not too surprising that these sessions are also helpful when it comes to VO2 kinetics.
We would recommend dedicating roughly 80% of your training sessions to this type of endurance training, aiming to keep your power largely between ~55-75% FTP or heart rate between approximately 60-70% Max HR. Subjectively, these sessions should feel like a 3-4/10 effort level (possibly reaching up to a 5/10 for particularly long sessions), and your breathing rate should remain ‘conversational’, meaning that you can string together several sentences easily.
High intensity workouts:
A number of studies also support the use of high-intensity training to develop VO2 on-kinetics, although these studies are all within untrained or recreational-level athletes, and only a very small number of interval designs have so far been investigated, so it’s hard to make solid recommendations as to the best forms of interval training.
Two studies (McKay et al., 2009; Berger et al., 2006) employed a ‘1-min on/1-min off’ interval design repeated several times per week over multiple weeks (3-6 weeks), and observed an improvements in VO2 on-kinetics. These studies used slightly different training intensities and numbers of repeats (ranging from 8-20 intervals per session).
The precise training intensity to target if you’re trying to replicate these sessions is not entirely clear from these studies, as intensity was specified as a percentage of the maximal power attained in an incremental ramp test (which is highly protocol-dependent). We’d therefore recommend pacing such efforts based on feel, aiming for the maximum power you think you can sustain across the block of intervals, and aiming for a higher number of efforts (e.g. 20+) if you are more well-trained. Overall, the session should feel like an 8/10 effort level.
Another study (Bailey et al., 2009) observed that 6 training sessions over two weeks, each including sprint interval training (4-7x 30S all-out sprints with 4-min recovery) also helped to improve VO2 on-kinetics.
While other styles of interval session have not yet been investigated, it seems reasonable to assume that, in general, sessions that help improve VO2max will also contribute to improved VO2 on-kinetics, given that many of the factors that are thought to contribute to VO2 on-kinetics are also factors that contribute to VO2max. That being the case, some of the traditional styles of VO2max intervals may also be beneficial.
Summary
Understanding the ‘boot-up’ time of the aerobic system, the impact that this can have on fatigue and performance, and strategies for improving the speed of the VO2 on-kinetics presents a sizeable opportunity for performance improvement in competition. It may also represents a useful strategy for enhancing the effectiveness of interval sessions; particularly those seeking to accumulate time training close to VO2max. This may be particularly important for time-crunched and high-level athletes, where it’s important to eek out as much training benefit as possible from a given session.
Generally-speaking VO2 kinetics will be most important for competitive events lasting 2-mins to 40-mins, or in events that begin with a supra-threshold effort off the start line. If you compete in such events, then you should definitely give your VO2 kinetics some thought!
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References
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