Triathlon Is a Complex System. We Keep Treating It Like It Isn't.
What complexity science actually means for how you train
Most athletes and coaches think about training as a relatively simple equation with a stimulus / response framework. Apply the right stimulus, get the right adaptation. Manage the inputs carefully enough, and the results will follow. If something isn’t working, find the variable that’s wrong and fix it.
This is a reasonable way to think. It is also, at a fundamental level, wrong.
Triathlon is a complex system. Once you accept it, a lot of what follows in training and coaching becomes clearer.
What is a complex system?
A complex system is one where multiple variables interact with each other in non-linear ways. Small changes can have large effects. Large changes can have no effect at all. The relationship between cause and effect is not predictable in the way we’d like it to be.
Bill Gurley, venture capitalist and board member of the Santa Fe Institute (one of the world’s leading centres for complexity research), put it this way on The Knowledge Project podcast: “Multivariable nonlinear systems are very hard to predict. They can behave one way for a long time, and then one variable can switch and they can behave another way: the weather, stock markets, all these things. There are consequences that can be first, second, or third-order. You can’t just think with a linear model or just think about one variable because things can go way off the path. You need to be aware that if you make a change here, it could change something here, which could change something there, and it has to be the whole system.”
Weather. Stock markets. Ecosystems. And training for triathlon.
Manuel Sola Arjona describes it well: the body is not a puzzle we’re slowly solving. It’s a Rubik’s cube with infinite faces: every time you move one piece, all the others shift.
How triathlon is a complex system
In triathlon, the interactions multiply quickly. Three disciplines that affect each other. Training load, life stress, sleep, nutrition, psychology, environment: all interacting simultaneously, all influencing each other in ways that are rarely linear and almost never predictable from a single data point.
The swimmer who adds bike volume and finds their run improves. The taper that worked once and never seemed to work the same way again. These are the system behaving as complex systems do.
The second and third-order effects are where coaching decisions most often go wrong. You add heat training to prepare for a hot race. The physiology supports it. There is real evidence it helps. But added on top of an already loaded training block, the systemic cost exceeds what the athlete can absorb. The performance dips. The preparation unravels. You changed one variable in a system, and the system responded to the whole.
The single metric can mislead you badly here. VO2max, FTP, lactate threshold, HRV score: these are real measurements of real things, but they are snapshots of a dynamic system at one moment. As Sola Arjona puts it: “Identical lactate levels can reflect completely different physiological contexts and stress or performance levels.” A single reading tells you about one part, at one moment. The system keeps moving.
What it means practically
The most important implication of thinking in complex systems is this: performance is emergent. It arises from the interaction between the athlete and their environment: the culture of the training group, the quality of the relationships, the accumulated work over months and years, the daily decisions made well or badly. The right training group, the right daily structure, the right culture around intensity management and recovery: these sit at the centre of the performance process, not alongside it. They are the conditions from which performance emerges.
One distinction worth making here is between complexity and complication. They are not the same thing. Complication is adding more protocols, more metrics, more tech, more structure. An athlete who stacks heat training on top of altitude on top of a new strength programme on top of a recovery device has made their training complicated. They have not necessarily engaged with the complexity of the system at all. As Sola Arjona puts it, breaking the organism down into parts and treating them separately “is the typical mistake of confusing complex with complicated.” True complexity thinking asks how everything interacts, not how many things you are doing. (For a deeper read on this distinction, Sola Arjona’s writing at The Nature of Training is worth your time.)
In practical terms, this means being very cautious about single-variable thinking. As Gurley puts it: “You’ve got to be really conscious of the consequence and not get too deterministic about a single metric or a single variable.” A common version of this is the athlete who wants to improve their run and adds training load to address it. The run may respond. But that additional load lands on top of the swim and the bike, recovery is compromised across all three, and the athlete ends up worse overall than before they started. One variable changed, the whole system responded. Adding something new to a training programme changes everything around it. Removing something does too. The question is never just will this work? but what does this do to everything else? What are the unintended consequences?
Reading the system at the right level also matters. A single session tells you very little. A single biomarker tells you even less. The pattern across weeks is where the real information lives: whether the athlete is backing up day after day, whether their movement quality is holding, whether they’re communicating well or going quiet, whether small niggles are appearing and disappearing or starting to accumulate. These are the signals that tell you the state of the whole system. They are harder to put in a spreadsheet than a watt or a heart rate number, but they are more honest. The macro signal is usually more reliable than the micro data point.
Understanding this is what makes consistency make sense. The body adapts to a changing environment in real time, which means the most reliable path to good outcomes is creating the right conditions, repeatedly and patiently. Performance follows from that. You cannot shortcut the process by which it emerges.
The system has its own language. The athlete and coach’s job is to learn to read it, and to understand what it tells you.
This piece draws on Manuel Sola Arjona's writing on complexity and physiology at The Nature of Training, and on 30 years of watching complex systems behave in ways training plans didn't predict.