The three energy systems of a climb
Every move is paid for by three energy systems at once. Their share changes with the route and the climber, and on indoor routes measured in a lab, the aerobic and phosphocreatine systems carried most of the load.
In plain words
Muscles run on a fuel called ATP and have three ways to keep it topped up. The phosphocreatine system is instant but tiny: a few seconds of hard effort, then it needs a pause to recharge. The glycolytic system is fast and bigger, but it can't keep going for long and it is the one people blame for the "burn". The aerobic system uses oxygen. It is slow to start but never really runs out. On a route all three work together, and a lab study found the aerobic and phosphocreatine systems doing most of the work.
Memory hookSpark, burn, breath: phosphocreatine for the move, glycolysis for the push, oxygen for the whole route.
How the three systems share the work
Ready instantly, empties within seconds of hard effort, recharges in the pauses between moves. Pays for the burst of each move.
Takes over when effort stays above what oxygen can cover. Powerful, but the reserve is limited and recovery is slow.
Rises over the first minute and then covers steady work, including recharging phosphocreatine. Never really runs out on a route.
Why the systems take turns within every move
A move is a short burst followed by a few seconds of holding still. The burst is paid for mostly by phosphocreatine. The aerobic system can't speed up that fast, so it fills the gap more slowly and recharges phosphocreatine while you hold still. If the bursts come too fast, or the route stays hard for too long, glycolysis covers more of the gap. That's what lets you climb hard, and it's also what makes you fail if it goes on too long.
See it
Change the difficulty and the pace, then press play. Save one setting as strategy A and compare it with another.
The evidence
Numbers from adults only.
- Measured On indoor routes, elite climbers' energy came 41.5 % aerobic, 41.1 % phosphocreatine and 17.4 % glycolytic on an easy route; 45.8 / 34.6 / 21.9 % on a moderate route; 41.9 / 35.8 / 22.3 % on a difficult route. Recreational climbers on the easy route: 39.7 / 34.0 / 26.3 %. Bertuzzi et al. 2007: 6 elite and 7 recreational adult climbers. Eur J Appl Physiol 101(3):293–300.
- Measured In the same study the easy route cost the elite climbers about a third less energy than the recreational climbers (40.3 vs 60.1 kJ). The authors concluded that climbing economy, how little energy you waste, seemed to matter more than having a bigger engine.
- Model The route simulator splits each hold into a one-second move and a holding phase. Its parameters were tuned so a typical route lands near the measured shares above.
Model assumptions
Power is measured in units of the climber's aerobic maximum, and stores in unit-seconds. Difficulty raises both the cost of the move and the cost of holding still; crux holds cost 40 % more.
moves toward the current demand with a 20 s time constant, capped at 1.
covers whatever aerobic supply can't, up to 4 units, from a store of 14 unit-seconds. It recharges with a 20 s half-time whenever demand is low.
covers the rest. Its reserve (60 unit-seconds) clears with a 60 s half-time when demand is below aerobic maximum. When the reserve is used up, the climber falls.
uses the same rules as the forearm model, with the 47 s half-time measured in intermediate climbers. Grip rises with difficulty, and each hand works half the time. If forearm oxygen reaches zero, the climber falls.
technique, fear, chalking, clipping, and differences between people. The numbers are illustrations, not predictions.
Try it
Running out of time. Pace against the clock.
Critical force, the forearm's own sustainable line.
Common mistakes
on the measured routes glycolysis supplied about a fifth of the energy. Aerobic supply and phosphocreatine did most of the work.
fast, hard moves lean harder on glycolysis, which is the system that runs out.
rests are when phosphocreatine recharges and the glycolytic reserve clears.
the measured elite advantage on an easy route was spending less energy, not producing more.