Shownotes

Jack’s physiological profile

01:13 -

• I came from a track running background where training was heavily lactate-based, which shaped me into a very glycolytic, fast-twitch athlete. Moving into triathlon, I found that tempo work (largely absent from my earlier 800m preparation) suited me well at first. 
• Over time, though, I have come to recognise that my more natural genetic profile leans slow-twitch: a flatter power-duration curve that fits 70.3 racing well.
• Because of that, I now have to be deliberate about reintroducing top-end, glycolytic work ahead of sprint-distance races, especially given the repeated surges that technical bike courses demand in paratriathlon guiding.

Mikael's physiological profile

02:15 -  

• From a young age I always identified as an endurance athlete (never explosive), even back in my football days. I have responded well to high volume and to threshold or slightly sub-threshold work. 
• Where I have consistently struggled is with higher intensities (VO2 max and glycolytic efforts). That, however, is not a reason to avoid that kind of training. 
• Not training it only deepens it as a weakness. The key has been learning to adapt that type of work to my profile rather than cutting it out altogether.

Physiological profiling

03:57 -

• At its core, physiological profiling is about the power-duration curve or speed-duration curve: how much power or speed I can produce at short durations compared to long ones, and how flat or steep that curve is.
• Physiologically, this tends to reflect muscle fibre composition (fast-twitch versus slow-twitch dominance). When I use those terms, I am not implying a muscle biopsy. 
• I am simply describing whether an athlete is naturally explosive with limited endurance, or the reverse. Someone with good natural endurance but without a big punch at short efforts. 
• It is also worth adding that the curve maps best all-out efforts across different durations, and the rate of drop-off as distance extends tells you a great deal about the profile.
• Profiling goes beyond placing someone on a simple fast-twitch/slow-twitch spectrum. It also means identifying the key physiological markers - LT1, LT2, VO2 max, VLamax, fat max, and fractional utilisation of VO2 max.
• There is an important nuance here too: a high VO2 max does not automatically mean fast-twitch. An advanced athlete can have an excellent aerobic ceiling and still sit firmly on the slow-twitch end of the profile.
• Those markers matter a great deal in practice. My general rule of thumb is that a slow-twitch athlete can tolerate training that drifts naturally toward LT1 without much concern. A fast-twitch athlete doing the same is a riskier proposition. I would actively intervene and pull that athlete back. I apply similar heuristics across the more intensive training domains as well.
• It is also important to remember that most athletes do not sit at either extreme. The distribution is closer to a bell curve, with the majority landing somewhere in the middle. When I use extreme examples to illustrate a concept, it is purely for clarity.
• For most athletes, the training implications of profiling will matter, but probably only to a moderate degree (far less so than for someone sitting right at one end of the spectrum).

Importance of Physiological Profiling 

09:04 -

• The main reason physiological profiling matters is fatigability. An athlete with a more fast-twitch dominant profile will fatigue more easily (both within sessions and between them) and will recover more slowly.
• Fast-twitch fibres can produce much more force than slow-twitch fibres, but they also take considerably longer to recover from that effort. That is the core reason we care about this: it has a direct impact on how much fatigue accumulates and how quickly the body bounces back.
• That said, while fatigability is the biggest factor, it pales in comparison to life stress and general recovery outside of training. A fast-twitch dominant athlete might be physiologically well set up to recover, but if they have a demanding job, high stress, and poor sleep, none of that will help them. Physiological profile is one piece of the puzzle, not the whole picture.
• A practical way of thinking about why profiling matters is to consider two athletes with identical 20-minute power outputs and therefore the same FTP estimate. One of them can push only marginally higher for a five-minute effort, while the other can produce significantly more power at that shorter duration.
• Those are two completely different profiles, and treating them the same from a training prescription standpoint would be a mistake. The athlete with the higher five-minute power almost certainly has a high VLamax, and everything around their training needs — intensity distribution, recovery demands, race-specific preparation — will differ accordingly. This is exactly why a single FTP test is not enough on its own.
• There is also solid research backing the fatigability point, worth looking up for those interested in the science.
• Phil Bellinger's work on muscle fibre typology and its association with overreaching in response to high training loads is particularly relevant and goes into the different fatigue profiles across fibre types in detail.

Methods of Profiling Athletes 

13:10 -

• The first thing I look at is training history. Coming from a middle-distance running background myself is quite telling — both in terms of the training I did and the profile I likely developed during that time. Backgrounds in other explosive sports are equally informative, and not just the training itself but how the athlete felt they performed in those disciplines. It is a crude starting point, but a useful one.
• From there, I move to racing and training data to build a power-duration or speed-duration curve from existing performances — no new testing required at that stage. If five-minute power is high but there is a sharp drop-off to the 20-minute effort, that already suggests threshold and fractional utilisation may be the limiting factor.
• When I want to do formal testing, I prefer to keep it to a single session to minimise disruption to training: a five-minute and a 20-minute effort on the same day with adequate recovery between them.
• From those I can derive critical power and W prime — the capacity above critical power — which gives a clear indication of where the athlete sits on the fast-twitch to slow-twitch spectrum. 
• In the pool I use a 200 and an 800, looking at the drop-off and the D prime. If the athlete has a lactate meter, a step test is also valuable to see the gap between LT1 and LT2, and occasionally a VLamax test using an all-out effort followed by repeated lactate sampling.
• Racing history is the first reference point. An athlete who significantly overperforms at 5K relative to the marathon is already telling you something meaningful about their profile — though it could be training-related rather than profile-related, so it is a starting point rather than a conclusion.
• From there, training data alone can often build a reasonable initial speed or power-duration curve before any formal testing takes place.
• Where my approach differs slightly is that I tend to spread critical power testing across three separate days rather than combining efforts in one session.
• I apply this mainly to cycling and swimming — not running, where the mechanical load makes repeated maximal efforts considerably more demanding. I also tend to reserve this more structured approach for intermediate to advanced athletes, where the detail is more actionable.
• One thing I also pay attention to is how an athlete describes their enjoyment of different session types. If someone tells me they love long threshold intervals — eight or ten minutes at threshold — that already points toward a slow-twitch dominant profile.
• Similarly, an athlete who says they dislike the half-marathon because it feels too intense and would rather do a full marathon or longer is very likely telling me the same thing.
• On the lactate side, peak lactate concentration is a useful data point wherever you can get it. If an athlete is already doing a critical power test with a lactate meter, continuing to sample every two minutes after the effort — for up to ten minutes if needed — will reveal the true maximum, since lactate keeps rising well beyond the end of the effort.
• A low peak lactate value tends to indicate a slow-twitch profile; a high value suggests the opposite. This is not a pure VLamax test, but it adds a meaningful data point without requiring an entirely separate protocol.
• There are other methods coaches use — countermovement jump tests, strength assessments, and how easily an athlete puts on muscle — and some of these have reasonable evidence behind them.
• But my main methods remain the same: performances across short and long events, the shape of the power and speed-duration curve, and critical power testing.
• Without a lactate meter, durability over longer sessions is a practical proxy for VLamax. If an athlete needs to fuel heavily during long aerobic work, or shows a significant drop in output over time, that is usually a sign of a high VLamax that needs to be addressed.
• A 20-second sprint included in the same session as a three-minute max test is another way to get a ballpark range for VLamax without a dedicated protocol.
• I do not use it directly for critical power calculation, but a high 10 to 20-second sprint power already carries information regardless of how you interpret it physiologically — whether through the lens of VLamax, neurological capacity, or something else. The functional outcome of that number matters on its own terms.

VLaMax

21:41 -

• It is also worth distinguishing between VLamax and VO2 max, because many athletes assume that being speed-oriented means both are automatically strengths.
• That is not always the case. Having all the data points — 20-second, five-minute, and 20-minute power — sometimes reveals an athlete with a strong VO2 max but a surprisingly low VLamax, or the reverse. That distinction can easily be missed if you are only looking at the broad fast-twitch versus slow-twitch picture.
• There are some interesting theories around this in cycling — figures like Peter Sagan and similar one-day classics specialists come to mind.
• The suggestion is that an athlete can have a very high phosphocreatine system and a relatively moderate VLamax, which allows them to survive a long classics day without accumulating excessive fatigue, while still producing exceptional power in the final sprint or on a short punchy climb — drawing heavily from VO2 max and the phosphocreatine system rather than from raw glycolytic output. 
• I would not claim there is hard science behind this specific framing, but it illustrates well how these different energy systems interact and how a profile that looks like a weakness in one context can become a distinct advantage in another.

Training Implications from Profiling 

25:42 -

• In the context of amateur triathlon, the key practical question is how physiological profiling should shape periodisation. One of the main adjustments I make is around the length of intensity blocks — a VO2 max block, for example, will run slightly longer for a slow-twitch dominant athlete than for a fast-twitch one.
• That is partly because the slow-twitch athlete may have more to gain from it, but also because they can sustain that type of work for longer without tipping into overreaching.
• A fast-twitch dominant athlete accumulates fatigue more quickly at those intensities, so the risk of going too deep into that block is real.
• That said, it is important to be clear that no intensity should be entirely dropped from training regardless of profile. All athletes should be touching every part of the spectrum at some point throughout the year — not necessarily every week, but across the training year.
• The profile shapes the emphasis and duration of each block, not whether it appears at all.

Adjusting Training for Profiles 

29:39 -

• Volume is a nuanced variable. At the highest professional level, total volume does not necessarily need to change based on profile — what changes is the intensity of the easy work and how much structured intensity sits within that volume. A fast-twitch dominant professional can and should still train at high volume, but the aerobic work needs to be kept at a relatively low intensity compared to what a slow-twitch athlete could comfortably sustain.
• At age group level, where life stress is a significant recovery factor, I find in practice that fast-twitch dominant athletes often end up with slightly less total volume — not because it is physiologically impossible for them to handle more, but because recovering from it alongside normal life demands becomes genuinely difficult.
• If an athlete has a reasonable amount of training time available, I will sometimes simply not use all of it for that type of athlete.
• On the intensity side, fast-twitch dominant athletes need less volume of work at intensities like threshold, sweet spot, and tempo — these are the zones where it is easy to accumulate a lot of fatigue without noticing until it is too late. I also adjust the work-to-rest ratio within sessions.
• My default for threshold work is three-to-one, but for a clearly fast-twitch dominant athlete I move that to two-to-one. At sweet spot it shifts from four-to-one to three-to-one. The principle is consistent across intensities: one step down in work-to-rest ratio relative to what I would give a more slow-twitch athlete.
• Nutrition is another area where fast-twitch dominant athletes need to be more attentive, given the higher carbohydrate demands that come with their profile.
• And pacing and race selection can also come into play — an explosive athlete may be better suited to a different type of terrain than a slow-twitch athlete who would struggle on a punchy, technical course.
• In age group triathlon this is rarely the primary driver of race selection, and rightly so — most athletes choose races for personal reasons — but it is worth having in the back of your mind as one more factor.

The Impact of Volume on Training 

32:40 -

• On the question of volume and the fast-twitch athlete, my preference is to keep volume as high as practically possible rather than defaulting to reducing it.
• The real lever to pull is intensity control, not total load. The RPE at LT1 for a fast-twitch dominant athlete sits considerably lower than it would for a slow-twitch athlete, which means the endurance and aerobic training needs to be kept at a stricter, lower intensity to stay below that threshold and capture the right adaptations.
• If that intensity control is managed well, volume can still be maximised within the realistic weekly constraints most amateurs are working with.
• In practice, for a fast-twitch dominant athlete on the bike I prescribe endurance work at around two to three out of ten RPE, compared to four to five for a slow-twitch athlete.
• I also lean more heavily on heart rate caps rather than power targets, and pull all the paces and powers back across the board. Athletes often resist it because they are not used to moving that slowly for extended periods — but it consistently produces good results.
• The reason this matters goes beyond recovery. Riding at 70 to 75 percent of FTP — which was a very common prescription for a long time — can be genuinely damaging for a fast-twitch dominant athlete.
• It is not just that it makes the harder sessions harder to absorb. It is that the session itself stops serving its intended purpose. The training objective drifts into something else entirely, and if that pattern continues over weeks and months, the athlete ends up accumulating work that is not targeted at anything useful.
• That is a difficult position to be in and not one you want to find yourself in during a build phase.

Limitations of Physiological Profiling 

36:25 -

• One important caveat is that volume and load remain the primary goals in triathlon training regardless of profile.
• Even if the profile points strongly toward a need for more VO2 max work or more glycolytic development, maximising what an athlete can consistently do across a week, month, and year is ultimately what translates into results. Profile shapes how that load is distributed, not whether it is pursued.
• It is also worth remembering that VO2 max is frequently a limiter for amateur athletes, even those with a fast-twitch profile.
• Unlike elite athletes who may have already reached their ceiling after years of structured training, most amateurs still have significant room to develop aerobic capacity.
• Raising VO2 max lifts everything below it, even if fractional utilisation remains low. The profile should inform training decisions, but it should not become something an athlete gets overly fixated on.
• On the broader question of how important physiological profiling is relative to other variables — I would say it matters more than factors like age or sex when it comes to making actual training decisions.
• But it is still secondary to life stress and everything that surrounds recovery. For an athlete dealing with a demanding job, limited sleep, and high general stress, those factors will override almost everything the profile suggests. That is the most important caveat to keep in mind.

Practical Examples of Profiling

39:12 -

• To make this concrete, I want to walk through a real example from testing data that came in recently. This athlete completed a three-day critical power test protocol: day one was a 20-second sprint plus a three-minute max effort, day two a five-minute test, and day three a ten-minute test.
• He is relatively new to structured training — though not to sport — and is in his early sixties, which adds another layer of interest to the results.
• When I first started working with him I used a ramp test to get a rough FTP estimate, which I find useful for athletes new to pacing and structured work.
• Once they have built some familiarity with that, I move to a proper critical power test to get more precise data — and that is what we have here.
• The results: critical power of 256 watts, W prime of 19.8 kilojoules, and a five-minute power of 322 watts. That five-minute number gives an estimated VO2 max of around 56 mL/kg.
• The W prime figure is the one that stands out most. I have said before on this podcast that approaching 20 kJ is a moderate to moderately high W prime for a triathlete — but that reference point assumes a critical power closer to 300 watts.
• At 256 watts, a W prime of 19.8 kJ is actually quite high relative to critical power, and particularly so given the athlete's age. That is not a problem in itself, but it does shape how I approach his training.
• Because he is new to structured training and at this age, we have already been keeping endurance work at a very low relative intensity — and knowing the profile, that is clearly the right call to continue. Fuelling is another area to develop, since athletes newer to structured training often have not given much thought to it.
• On the top-end side, the 322-watt five-minute power is a genuinely strong number given the context, so there is less urgency to develop that further. What matters more at this stage — and I think this applies broadly as athletes get older — is not letting VO2 max deteriorate. It is somewhat use it or lose it.
• The focus is maintenance and building from below rather than chasing higher top-end numbers. Since this athlete is time-rich, there is a real opportunity to build a substantial volume of low-intensity aerobic work, and I think that could produce very good results.

Practical Tips for Testing 

44:10 -

• Without knowing the full swim, bike, and run history and recent training load, it is hard to be too prescriptive, but if life allows moderate volume and there are no injury concerns in the way, that is a solid foundation to build from.
• Progression for an athlete new to structured training can come quickly, which also means retesting sooner than I would with a more experienced athlete — the numbers can become outdated fast and it is important to stay on top of that. Intensity control on the aerobic sessions remains the thing I would stress most.
• One practical note on the testing data itself: the three-minute average power in this case came out at 319 watts, the five-minute at 322 watts. That is an immediate red flag — five-minute power cannot be higher than three-minute power.
• This is more likely to happen with athletes who are newer to pacing time trials, and it is one of the clearest arguments for spreading tests across three separate days rather than one.
• When one result is clearly out of line with the other two, I simply discard it and use the remaining two for the critical power calculation, which is exactly what I did here.
• A useful way to sense-check the numbers is to convert power and time into kilojoules versus time and plot them out. The relationship should form a broadly straight line. If one point sits well off that line, it becomes immediately visible. This is straightforward to do in Excel or Google Sheets and is worth doing as a matter of habit.
• The broader point is that maximal power tests are genuinely difficult to execute well. Doing them on the same day, as I prefer, at least ensures that fatigue, nutrition, and training status are consistent across both efforts.
• The second test will always carry some fatigue from the first, but at least that is a known and constant factor — particularly useful with newer athletes whose numbers can vary considerably from one session to the next.
• For athletes who are not yet comfortable with this type of testing, a ramp test remains a much easier way to get a reliable starting estimate.

Using Profiling in Coaching Decisions

48:17 -

• A recent example that comes to mind is an athlete I have been working with for a few years. After a recent round of testing, the gap between his LT1, LT2, and higher-end power had become very small — the curve had flattened considerably.
• He had done two full Ironman blocks the previous year, which explains it. Even though a VO2 max emphasis had always been part of the plan, I doubled down on it significantly off the back of those test results.
• Beyond that dedicated VO2 max block, I have also continued to weave in VO2 max work on a more regular basis throughout the subsequent training, rather than treating it as a one-off intervention. The profiling gave a clear direction and I followed it.

VO2 Max Session Design by Profile 

49:35 -

• The evidence suggests that micro-intervals are particularly well suited to slow-twitch dominant athletes, but in the context of triathlon I tend to use them across all profiles.
• Managing swim, bike, and run simultaneously means that getting as much time near VO2 max as possible — without generating excessive fatigue — becomes the priority, and micro-intervals are simply the most practical way to do that.
• Formats like 30-15s, 40-20s, and 30-30s all have their place, though my preference leans toward a two-to-one work-to-rest ratio. The shorter recoveries in a 30-15 or 40-20 mean you are driving oxygen uptake continuously without needing to hit very high peak powers, which matters when an athlete is already carrying fatigue from three disciplines.
• The 30-30 format has less of my favour for pure VO2 max development — much of the original research behind it was running-based and conducted some time ago — but it is useful when you want a small VO2 max stimulus combined with a moderate glycolytic component, or for older athletes who struggle to access that intensity range through shorter work intervals.
• For single-discipline athletes the calculus is different, but in triathlon the micro-interval approach holds up well across the board.

Managing Group Training Environments

53:40 -

• Group training presents a real challenge when athletes of different profiles are sharing the same session. The pool is probably the hardest environment to manage, since everyone is sharing a lane and tied to the same turnarounds.
• One practical option for a fast-twitch athlete wanting to limit anaerobic exposure is to drop to the back of the lane and turn it into a more continuous swim.
• Alternatively, moving into a faster lane on a tighter turnaround can achieve the same effect — you end up doing the work aerobically rather than in bursts.
• On the bike and run it is more straightforward: the simplest approach is to follow the session as prescribed but adjust the targets to match what the coach has given individually.
• That said, the most important consideration is always whether attending the group session gets the training done at all. The motivational and technical value of training with others is real, and that often justifies a degree of compromise on optimal profile-based prescription. Alternating between group and individual sessions is one way to balance both.
• The main thing to avoid is not imperfection — it is the kind of consistent mistakes that lead to overreaching. A runner whose club runs two track sessions and a hard long run every week might simply need to drop one of those sessions rather than try to modify all three.
• On the track, adjusting pace targets or the number of intervals for more fatigable athletes is a relatively simple change for a coach to make. In the pool, changing the number of intervals or the work-to-rest ratio is equally manageable, though asynchronous sessions can get logistically awkward.
• The first step is always to have an open conversation with the coach — share your thoughts on your profile and discuss how that can work within the squad structure.
• Beyond that, not overthinking it matters. A local running club that tends toward middle-distance rep sessions is a good example: doing half the session and finishing with a tempo effort is a reasonable way to stay in touch with all intensities without overloading the glycolytic system.
• You still get the best of both worlds without turning every group run into a problem to solve.

Key Takeaways

59:19 -

• My first take-home message is to identify where you sit on the physiological profile spectrum. Start with the more subjective observations — how you perform at different race distances, what types of sessions you enjoy or struggle with — and then move toward direct testing through critical power, critical speed, and lactate. Get a clear picture of where you sit.
• The second message is directed specifically at fast-twitch dominant athletes, because that is where training mistakes tend to be most costly.
• If that is you, pay close attention to keeping endurance work at a genuinely low intensity, and be careful about accumulating too much volume of intensity in general — particularly the kind of sessions where you can easily do a lot of work at threshold or in the heavy domain without noticing the fatigue building up.
• For slow-twitch dominant athletes, the key message is not to get hung up on specificity. Do not panic about doing VO2 max work just because it does not resemble what you will do on race day.
• For most amateur athletes, VO2 max remains a significant limiter and the ceiling has not been reached, so pursuing it is absolutely worthwhile. The broader point for all athletes is that profiling shapes the emphasis of training (it does not eliminate whole intensity ranges).
• Consistent volume and load remain the primary drivers of adaptation and performance.

Final Thoughts on VO2 max

01:01:20 -

• The point about VO2 max is worth expanding on. It is easy for amateur triathletes to look at how professionals train (through YouTube, Strava, or other sources) and see a strong threshold bias, then assume that is the model to follow.
• What that misses is that most professionals have already reached, or come very close to, their genetic ceiling for VO2 max through years of high volume and intensity.
• At that point, the only meaningful gains come from raising threshold as a percentage of VO2 max and improving fractional utilisation at race pace. Threshold work makes sense for them because there is nowhere else to go.
• For amateurs, the situation is fundamentally different. The range of VO2 max across the amateur field is far wider, and most athletes have not come close to tapping their potential there.
• Copying the training distribution of a professional without accounting for that difference is a red herring. The caveat, however, is timing. As racing approaches, particularly for Ironman, the body still needs to be tuned toward its glycolytic tendencies to some degree.
• Profile informs the emphasis, but race-specific preparation still has its place.

A slow-twitch dominant profile has a better chance of becoming an elite short-course triathlete than a fast-twitch dominant profile.

01:04:10 -

• Strongly disagree.
• The way I would frame it is this: turning a miler into a marathoner is easier than the reverse. The VO2 max ceiling is harder to develop than fractional utilisation, and elite short-course triathlon demands a very high VO2 max.
• A fast-twitch dominant athlete tends to come with the mechanical and physiological characteristics — excellent running mechanics, a high glycolytic ceiling — that set the potential for continued VO2 max development.
• The reverse, taking a world-class Ironman athlete and making them competitive over sprint distance, is essentially impossible at the highest level.
• But athletes crossing over from short course to long course happens consistently every year. The direction of travel matters, and it is considerably easier in one direction than the other.

LINKS AND RESOURCES

• Muscle fiber types, recovery and training adaptations, and overreaching with Phil Bellinger, PhD | EP#297

• Estimating Muscle Fiber-Type Composition in Elite Athletes: A Survey on Current Practices and Perceived Merit - Lievens et al. 2024

• Muscle fiber typology is associated with the incidence of overreaching in response to overload training - Bellinger et al. 2020