What is time to exhaustion (Tlim)?
Tlim is how long you can hold a given intensity before your glycogen runs out — a function of pace, your stored carbohydrate, and how much you take in along the way.
Time to exhaustion, written Tlim, answers a specific question: at this pace, with this much carbohydrate on board and this much going in per hour, when does the tank hit empty? It is not a measure of willpower or of muscular fatigue — it is a fuel calculation, and for the marathon it is usually the binding constraint.
Three levers move it. Slowing down reduces the carbohydrate burn rate per hour and shifts a larger share of the load onto fat. Starting with more stored glycogen raises the ceiling — that is what carbohydrate loading buys. And taking carbohydrate in during the race adds fuel from outside the tank, which for a 60–90 g/hr intake can extend Tlim by a substantial fraction over a marathon-length effort.
The number to compare it against is your projected finish time. If the model says your glycogen runs out at 3 hours 10 and you plan to finish in 3 hours 30, the plan has a problem — and the fix is one of the three levers, chosen before race day rather than discovered at 32 km.
Key numbers
| Typical unfuelled depletion | 90–120 min | at marathon pace, no intake |
|---|---|---|
| Main levers | pace, stored glycogen, intake rate |
Work it out for yourself
Substrate & time-to-exhaustion calculator — Compute your own Tlim and compare it with your goal finish time.
Frequently asked questions
How long before you hit the wall in a marathon?
Without carbohydrate intake, glycogen depletion at marathon pace typically arrives after roughly 90–120 minutes, which is why unfuelled marathons go badly. With 60–90 g/hr coming in, the depletion point moves out past the finish for most runners at most paces.
Does Tlim mean you have to stop?
Not literally. It marks the point where muscle glycogen can no longer support the current intensity, so you slow dramatically rather than halt. The subjective experience is the classic wall: legs that will not respond, effort that suddenly feels enormous at a pace that was comfortable minutes earlier.
Sources
- Rapoport (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology.
- Jeukendrup (2011). Nutrition for endurance sports. Journal of Sports Sciences.
Related explainers
Glossary: Tlim (Time to Exhaustion), Glycogen