Energy Systems in Climbing: What the Pump Is, and Why Climbing to Failure Isn't Endurance Training

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You may end every gym session the same way: pick a moderate route and lap it until your forearms are too hard to hold on, then lower off. You think of it as "endurance training."

This post explains why that does not work well. If the diagnosis post concluded that fitness is your limit, this post sorts out what kinds of fitness there are, so that the hangboard, strength, and endurance posts make sense. There is no program here; programs are in posts 2–4.

Three systems working together, not taking turns

Where muscle energy comes from

Every time your fingers grab a hold, the muscle spends a molecule called ATP. Muscles store very little ATP, so the body has three pathways that replenish it on demand, usually grouped as aerobic and anaerobic:

Lattice Training's energy systems guide corrects a common diagram that shows the systems "switching" over time: this one for the first seconds, that one after tens of seconds. In reality all three work at once. The higher the intensity, the larger the anaerobic share; the lower the intensity, the more the aerobic system can cover.

What researchers measured

Bertuzzi and colleagues (2007) measured where the energy came from while climbers climbed indoor routes. For recreational climbers on an easy route, the aerobic system supplied about 40%, and the two anaerobic systems supplied the rest. The authors concluded that indoor climbing relies mainly on the aerobic and anaerobic alactic systems, and that climbing economy (how much effort the same route costs you) matters more than energy metabolism.

In other words, strong climbers spend far less energy on the same route. That is technique at work, which is why the Climbing Movement series remains a prerequisite for fitness training.

Something you can do tonight: climb a route you know well twice. The first time, climb as usual. The second time, deliberately hold every handhold only just tight enough not to fall off. Compare how your forearms feel afterward.

What the pump is

The moment you let go matters most

Climbing is intermittent: grab, pull, let go, move, grab the next hold. Lattice's guide explains that the moment you let go, blood flows fully into the forearm muscles, bringing in oxygen and carrying away the metabolites from anaerobic energy production, and the aerobic system uses that window to refill the anaerobic "stores." The longer and harder you grip, the harder it is for blood to get in; metabolites build up faster than they are cleared, and that is the pump.

Research supports how much the intermittency matters. Giles and colleagues' 2006 review found that in repeated intermittent grip tests climbers lasted longer than sedentary people, but in sustained grips to exhaustion there was no difference. MacLeod and colleagues (2007) found that how well the forearm re-oxygenated during rest phases explained about 40% of the variation in finger endurance.

A common saying: "the pump is lactic acid"

This idea is everywhere, but Lattice's guide points out that lactate used to be blamed for muscle fatigue and the view has changed: the loss of force is now thought to be linked mainly to the build-up of inorganic phosphate. TUK's energy systems explainer (in Chinese) also stresses that lactate is not the main cause of soreness and in fact helps you keep going at high intensity. In practice, shaking out and finding rests let blood back in; you are not "flushing lactic acid."

The water tank analogy

Lattice uses a water tank:

• The water level is your anaerobic capacity: how much work you can do above your aerobic level.
• Water flowing in is your aerobic capacity: the better your aerobic system, the faster it flows in.
• Water flowing out is the force you are producing: the harder the climbing, the faster it drains.

When outflow equals inflow, you can climb for a long time without getting pumped. When outflow exceeds inflow, the level drops until you fall off. Only at a good rest, where outflow drops below inflow, does the level come back up. Climbers with better aerobic capacity recover faster at rests, and can recover even at poor ones.

Something you can do tonight: find a jug on a route, hang on it with straight arms, and alternate shaking out each hand. Count how many shakes it takes until your forearm stops feeling full. Next time you climb the same route, see whether that number drops.

Why climbing to failure trains neither system

Lattice's article on endurance training puts it bluntly: the aerobic system is developed by volume, the anaerobic system by intensity. Climbing until you are pumped out hits the middle ground between the two and trains neither system effectively; it also piles up fatigue, so you take longer to recover.

Lattice also reminds you that specific training does not need to look or feel like the route you are training for. What needs to be specific is the adaptation you want. To train the aerobic system, keep the intensity low enough that you barely get pumped and climb long enough. To train the anaerobic system, push the intensity up.

These methods are coaching consensus. Langer and colleagues' 2024 randomized trial says so in its introduction: local muscular endurance training of the forearms and upper limbs is frequently recommended, but there is not yet enough evidence that it improves climbing performance.

Sorting out the names

In his UKClimbing column, Lattice founder Tom Randall notes that the same thing goes by three names: Lattice calls it aerobic power, textbooks call it strength endurance, and the internet mostly says power endurance. Lattice then splits it in two: the more aerobic "aerobic power" and the more anaerobic "anaerobic capacity." When you read a training article, check how the author defines the term before relying on the name.

Something you can do tonight: the next time you want to "train endurance," decide which kind first. Pick a wall that is not too steep, choose routes three or four grades below your limit, and climb continuously for ten minutes without getting pumped. Note afterward whether you managed it.

Critical force: an invisible line

Below the line you can keep going; above it, the clock starts

Giles and colleagues (2021) brought the "critical power" concept from general exercise science to the fingers and called it critical force (CF). Climbers performed repeated all-out grip contractions; force declines and then levels off at a plateau, and that plateau is the critical force.

Below critical force, your fingers can keep producing force for a long time. Above it, the extra comes out of a finite reserve that researchers call W′. When W′ runs out, you fall off.

Among 129 climbers, critical force relative to body mass explained 61% of the variation in sport climbing grade, and much less for bouldering; W′ was more clearly related to bouldering grade.

Baláš and colleagues (2022) did the same on a motorized treadwall with adjustable angle and estimated each climber's "critical angle." Two degrees less steep than their critical angle, every climber tested lasted the full 20 minutes; two degrees steeper, most failed before the 20 minutes were up. A few degrees separate "can keep going" from "the clock is running."

What it means for you

You cannot measure your critical force at home, but the idea is useful in two ways:

• Routes and boulders ask for different things: long routes lean on critical force (the aerobic side); boulders lean on W′ and maximal strength (the anaerobic side).
• Endurance hinges on rest: the higher your critical force, the more moves on a given route fall "below the line," and the easier it is to refill W′ at rests. That is why resting and route reading is both technique and fitness.

Something you can do tonight: find a bouldering wall you can traverse continuously. Traverse on very easy holds, going two holds farther each time, until you feel the pump starting to build. Record that distance as how far you can go at this angle without getting pumped, and compare later on the same wall.

Which system do you want to train?

Match the diagnosis post with this one:

The series covers hangboards before endurance because maximal strength is a foundation: Lattice's guide calls energy system training and finger strength development the backbone of forearm physiology. So the series covers hangboard and strength work aimed at maximal strength first, then endurance. Hangboards can also train the aerobic side with low-intensity repeaters, and post 2 deals with that separately.

Something you can do tonight: pick the row in the table that sounds most like you and write down "the system I want to train is ___." When you read posts 2–4, start with that one.

Practice: four scenarios

The answer is below each one. Think first, then open it.

1. A friend ends every session lapping the same route until his hands can't hold on at all. He says it's endurance training.

2. Mid-route on lead, you shake out on a jug for a long time, but your forearms stay full. What does that tell you?

3. Someone says: "The pump is lactic acid, so stretch afterward to flush it out."

4. You mostly boulder and want to train "endurance." Should you do the same long continuous climbing as route climbers?

Next

Once you know maximal strength is a foundation, the next questions are when you can start hangboarding, and how to choose between MaxHangs and repeaters. Read hangboard training next.

References

• Training Energy Systems: The Climbers Guide (Lattice Training)
• What is endurance training in climbing? (Lattice Training)
• Lattice Training Series with Tom Randall - Part 2: Power Endurance (UKClimbing)
• Understanding energy systems (TUK, in Chinese)
• Bertuzzi RC et al. Energy system contributions in indoor rock climbing. Eur J Appl Physiol 2007
• Giles LV et al. The physiology of rock climbing. Sports Med 2006
• MacLeod D et al. Physiological determinants of climbing-specific finger endurance and sport rock climbing performance. J Sports Sci 2007
• Giles D et al. An All-Out Test to Determine Finger Flexor Critical Force in Rock Climbers. Int J Sports Physiol Perform 2021
• Baláš J et al. The Estimation of Critical Angle in Climbing as a Measure of Maximal Metabolic Steady State. Front Physiol 2022
• Baláš J et al. Isolated finger flexor vs. exhaustive whole-body climbing tests? Eur J Appl Physiol 2021
• Langer K et al. The effects of five weeks of climbing training, on and off the wall… in female climbers. PLoS One 2024