It is usually one of a few things
A run that comes apart in a triathlon almost always traces back to one of these, and they leave different fingerprints in the data.
- You overbiked. The bike sat above what you could hold, and the run started fine then decayed as the legs gave out. This is the most common one.
- You under-fuelled. The wall arrives suddenly, often at a specific point, and pace collapses out of proportion to how the effort felt earlier.
- Heat. Heart rate drifts up across the day while power and pace fall, and the drop is bigger than your fitness or your bike alone would explain.
- You went out too hard on the run itself. The first kilometre or two is quick, then a long slow fade. This one is on the run, not the bike.
Why you cannot always tell which from memory
By the time you are walking, every one of these feels the same. Legs gone, pace gone, morale gone. The difference between them is in the shape of the data, not in how it felt. Whether power faded in the back half of the bike, whether heart rate climbed while power did not, whether the run decayed steadily or fell off a cliff. That is what tells you which story is true.
Getting this right matters because the fix is different for each. Training more run volume will not help a run that fell apart because the bike was too hard. Fixing your pacing will not help a fuelling problem.
Read it from the file
Upload your race file and The Debrief reads the bike and the run together and tells you which pattern your data actually shows. In plain language, the way a coach would walk you through it, and honest about what the file can and cannot prove. If two causes are tangled together it will say that too, rather than picking one to sound confident.