Metabolic Profiling and Interdisciplinary Fatigue Transfer: Insights from Coach and PhD Parker Spencer

At the cutting edge of triathlon performance, metabolic testing is increasingly shaping how elite athletes train and race. And few coaches are working more deeply in this space than Parker Spencer, head coach of USA Triathlon’s Project Podium, who is also completing a PhD focused on metabolic testing and interdisciplinary fatigue transfer.

As triathletes, we intuitively understand that the swim impacts the bike, with both in turn impacting the run. But how, exactly, does fatigue transfer between disciplines, and what can athletes do to better manage the physiological cost that accumulates across a multi-disciplinary race?

This sits at the centre of Spencer’s work. We sat down with him to discuss the findings emerging from his research, how he is applying them to athletes on the Olympic pathway, and what the broader triathlon community may be able to take away from this evolving understanding of performance.

A Case Study: Two Races, Same Fitness, Completely Different Outcomes

To illustrate how interdisciplinary fatigue transfer plays out in real time, Spencer walked us through data from one of his athletes across two World Cup races held on back-to-back weekends.

Importantly, fitness had not meaningfully changed between the races. Physiologically, the athlete was effectively the same. Yet the outcomes were dramatically different.

In the first race in Vina Del Mar, Chile, he came off the bike with a lactate value of 6.5mmol/L, indicating a heavily taxed physiological state entering the run. The result was a comparatively compromised run performance of 15:12 for 5km and a mid-pack finish.

Contrast this with his performance one week later in San Pedro De La Paz. Here, the same athlete came off the bike with a lactate value of 3.5mmol/L and ran 14:34 for 5km, finishing inside the top ten.

Importantly, the distinction was not as simple as how “hard” or “easy” the athlete rode (i.e., it could not be reduced to a single metric such as average power). Rather, total effort as well as how that effort was distributed – how the athlete managed surges, where he positioned himself within the pack, how he recovered between efforts, etc. – collectively contributed to the overall metabolic cost of the ride and the lactate value at the end of the second discipline.

Without any change in the athlete’s underlying fitness, the execution of the second race resulted in a significantly improved physiological state entering the run, and in turn, a much stronger run performance.

“At the highest level, the differences between athletes are extremely small. What separates performances is often not who is the fittest in a single discipline, but who manages fatigue across all three disciplines the best,” Spencer explained. “This is where interdisciplinary fatigue transfer becomes one of the most important concepts in understanding elite triathlon performance.”

“This is the direction I believe the sport is going,” he continued. “And it is something we are actively building into how we develop athletes and how I am approaching my doctoral research.”

The Metabolic Characteristics Behind Performance

To fully understand what drives performance, Spencer explained that it’s necessary to look “underneath the hood” at an athlete’s metabolic profile.

While VO2 max, broadly representing maximal aerobic capacity, is often viewed as one of the most important physiological qualities in endurance sport, Spencer emphasized that it is far from the only factor shaping triathlon performance.

“When we looked at our example athlete’s metabolic profile and ran simulations to explore how we could further improve his performance, we realized that lowering his VLamax would actually be more beneficial than further increasing his VO2 max,” Spencer said.

VLamax refers to maximal glycolytic capacity – essentially how heavily an athlete relies on anaerobic metabolism to produce high outputs. While a high VLamax can support repeated surges and aggressive race dynamics, it can also come at a cost.

“This particular athlete already had world-class bridging and surging abilities,” Spencer shared. “In that regard, he was already one of the strongest cyclists on the circuit. But having such a high anaerobic contribution was taxing him on the run. By shifting his metabolic profile more strongly toward aerobic metabolism, without losing his bridging and surging ability, we could strengthen his triathlon performance overall. Our modelling showed that these types of training adjustments could theoretically allow him to start the run closer to a 2.0mmol/L lactate value.”

To simplify these concepts, Spencer often uses a bathtub analogy. “If VLamax, the anaerobic contribution, is the faucet adding lactate into the bathtub, then VO2 max is effectively the drain,” he explained. “The larger the drain, the greater the athlete’s ability to clear and recycle lactate. In this case, the athlete was accumulating lactate at a rate greater than he could clear it, so our work was focused on shifting that relationship.”

“It’s a fine balance,” Spencer continued. “And it varies enormously from athlete to athlete. Different race formats, distances, and tactical demands also require different metabolic strengths.”

Another key consideration Spencer highlighted is that lactate clearance is not maximized by simply going as easy as possible. Lactate is now understood to be a metabolically reusable fuel, rather than merely a waste product, and there is an optimal intensity at which athletes clear and reuse it most effectively.

“This is why, tactically, I’ll often have athletes recover third or fourth wheel in a pack rather than sitting right at the back,” Spencer explained. “The output still needs to be high enough to optimize lactate clearance and recycling. Sometimes I’ll even have them ride slightly off to the side of the group to ensure the recovery intensity is metabolically optimized.”

In other words, recovery in elite racing is not simply about minimizing effort. It is about finding the precise physiological balance that allows athletes to recover quickly enough to respond to the next surge while still preserving the metabolic state needed to run at the highest level.

A More Nuanced Understanding of Performance

If all of this feels like a rethink of how triathlon performance is optimized, that’s because it is. And while this work is being applied at the highest levels of the sport, the principles can also help inform how the rest of us approach training and racing, even if we’re not world-class athletes.

Here are a few key takeaways from our conversation with Spencer that challenge, or at least refine, some long-held assumptions:

1. You don’t have a fixed number of “matches to burn.”

The idea that athletes only have a limited number of above-threshold surges they can use in a race is a common one – and directionally, it makes sense. But it also oversimplifies the physiology. The example athlete above highlights Spencer’s work to make bike dynamics less metabolically taxing, facilitating strong run performance even following the surge-and-recover demands of draft-legal racing.

A better way to think about it may be this: there is only so much “metabolic debt” an athlete can accumulate, but the cost of each “match” is not fixed. It can be modified, managed, and deployed more strategically. Especially in short-course racing, this becomes a key training and racing objective.

2. Traditional training zones fail to account for metabolic individuality.

This is not to say that training zones lack value; rather, it highlights a growing limitation in generalized zone-based training models. Two athletes with identical FTPs may produce that output through very different underlying metabolic profiles, with different strengths, weaknesses, recovery characteristics, and physiological costs associated with maintaining that effort.

As a result, training science is increasingly shifting toward identifying and modifying individual metabolic limiters rather than relying solely on standardized percentage-based prescriptions. For athletes competing at the pointy end of age-group racing (or pursuing a pro card), metabolic testing is worth considering, as it offers a level of precision that traditional training models cannot.

3. Race execution matters metabolically, not just tactically.

Race execution has always included tactical considerations, for example, when to push to hold a draft in the swim, when to bridge a gap on the bike, when to make a move up a hill, and so on. But what is now entering the strategic picture is tailoring those decisions to an understanding of an athlete’s unique physiology – that is, knowing the “cost” of each move, not only within the discipline in which it occurs, but in terms of its downstream effect on the rest of the race.

This is exactly where Spencer’s work comes in. He is actively shaping training and race execution strategies for his athletes such that tactical decisions are informed by their individual metabolic profiles, blending an understanding of what is happening “under the hood” with the broader race dynamics at play.

When athletes arrive in T2, their ability to run well depends not only on their pure run speed, but also on how effectively they have managed fatigue and metabolic cost up to that point, which in turn determines their ability to express their run fitness.

This level of awareness and individualization is undoubtedly the future of the sport, and represents an opportunity for marginal gains that become decisive on the world stage.

Metabolism Is Fitness

In closing, if one thing stood out most from our discussion with Parker Spencer, it was the line: “Metabolism is fitness.”

Perhaps it is as simple as this: we cannot improve what we cannot see or measure. And to date, triathlon has largely sought to optimize performance with less-than-ideal insight into the metabolic factors at play within – and especially between – disciplines.

Spencer’s current doctoral work is focused on the bike-to-run interaction, with the swim representing an intended post-PhD focus in pursuit of a more complete understanding of fatigue transfer across all three disciplines.

He also shared that substrate utilization, economy, and durability are integral components of his work, which we’ll explore further in the next installment of our Physiology of Performance series with Parker Spencer.