It describes the forearm’s ability to sustain submaximal force, restore capacity between contractions, and support repeated hard efforts. This module explains that process through Critical Force and recovery, and why the traditional “energy systems” model can be misleading when applied too literally to climbing.
Why the old “energy systems” model breaks down in climbing
In the previous section, we showed that endurance in climbing is not about suffering or fighting pump — but about how efficiently your forearms handle repeated submaximal work.
Yet most discussions of endurance in climbing still rely on the classic “energy systems” model — aerobic, anaerobic, lactic, alactic — to interpret fatigue and performance.
What is wrong with the old model?
The classic model suggests that effort starts “anaerobic” and only becomes “aerobic” later, as intensity drops.
In practice, climbers often use this idea to justify endurance training built around discomfort:
long circuits, heavy pump, and conditioning sessions built around fatigue tolerance.
But this interpretation misses what actually happens in the forearms:
local blood flow is restricted early, not late
oxygen delivery matters even during high-force contractions
recovery between contractions often determines success or failure
fatigue is driven by local conditions, not by a clean sequence of “systems”
The outcome is predictable — and consistently disappointing.
Many climbers train endurance by chasing fatigue —
and then wonder why repeatability, recovery, and session-to-session consistency don’t improve.
A better model starts with a different question: How expensive is each move — and how fast can you recover between them?
A better model: cost, recovery, sustainability
In climbing, performance is rarely limited by a lack of effort.
It is limited by how quickly the forearms drift from sustainable to unsustainable output.
Most of what climbers call “endurance” can be explained by three interacting variables.
Cost per move
The physiological stress created by each contraction.Recovery speed
How quickly oxygenation is restored and by-products are cleared during rests.Sustainable ceiling
The highest force you can repeat without rapid decay.
Improve these variables, and the effects become visible quickly in real climbing.
more consistent attempts
more quality burns per session
less drop-off late in a route, boulder circuit, or board session
This is why aerobic capacity matters — even for boulderers.
What the data actually shows
Up to this point, we’ve described a model that explains endurance in terms of cost, recovery, and sustainability.
At the level of experience, this already matches what most climbers feel on the wall.
But models become useful only when they can be checked against measurements.
When you look at climbing-specific force–time data and forearm oxygenation, several things become clear very quickly:
climbers with similar strength can show very different sustainability
rapid force decay often appears before subjective “pump”
recovery capacity, not peak force, predicts repeatability
aerobic limitations show up as unstable output, not just fatigue
These differences are not subtle when you know where to look.
In the next section, we’ll move from description to evidence.
You’ll see real examples from climbing-specific tests:
CF curves recorded under controlled conditions
synchronized force and oxygenation data from the same efforts
The goal is to show how the variables introduced above — cost, recovery, sustainability — appear in real measurements, and how to interpret them reliably.
What a CF curve actually tells you
In the following examples, we use a Critical Force (CF) test — a standardized, climbing-specific protocol designed to probe sustainable force output under repeated submaximal contractions.
A Critical Force (CF) test produces a force–time curve under tightly controlled, repeatable conditions.
At first glance, many climbers focus on single summary values — peak force or average force.
Those numbers matter, but they are rarely the most informative part of the test.
What distinguishes athletes with similar CF values is how force output evolves over time when contractions are repeated. The curve itself carries information that no single number can capture.
In particular, a CF curve reveals:
how quickly force output decays,
whether output stabilizes or continues to drift,
and how repeatable force production remains beyond the initial phase.
The examples below illustrate two CF curves with comparable CF values, but very different time-dependent behavior under repeated load.
Figure 1A: Stable force output with low variability and gradual decay. Output settles near the CF line, indicating good sustainability under repeated load.
Figure 1B: Rapid early force decay with increasing variability and no stabilization. Despite similar CF, sustainability is lost quickly under repeated load.
Two climbers can have the same Critical Force — and completely different ability to sustain it.