Climbing Notes
Energy management

Critical force: the line between sustainable and not

Critical force is the highest grip you can keep repeating without running down. Below it you can keep going for a long time; above it a limited reserve drains, and when it's empty you let go.

4 min read 5 sections 2 interactive

In plain words

Imagine your forearm has a line drawn across it. If you grip below the line, the forearm keeps up and you could carry on for a long time. If you grip above it, you start spending a reserve, a bit like a phone battery. The harder you grip above the line, the faster the battery drains. Easier sections of a route, below the line, let the battery charge again. That line is called critical force, and the size of the battery is called W′ ("W prime").

Memory hook

Under the line you cruise, over the line you spend.

The line and the reserve

Two numbers describe your forearm endurance

Critical force (CF)

The highest repeated grip, as a share of your maximum, that you can sustain. In the studies it was measured with 7 s grips and 3 s pauses.

W′ (the reserve)

How much work you can do above CF before you have to let go. Bigger W′ means longer hard sections.

Researchers measured climbers' critical force at about 41 % of their maximum grip on average. In a group of 129 climbers, critical force relative to body mass explained most of the differences in sport-climbing grades. For bouldering, the reserve mattered more.

The time-to-failure rule

Above CF, the time you can last follows a simple rule: reserve ÷ (your grip − CF). Double the amount you're over the line, and you last half as long. That's why a hard section that's only slightly over your line can be climbed for a long time, while one far above it is over in seconds.

See it

Raise the grip force above the dashed line and watch the reserve drain. Add hard and easy sections to see it refill.

The evidence

Numbers from adults only.

  • Measured Mean critical force was 41.0 % of maximum voluntary contraction (SD 6.2), or 425.7 N (SD 82.8), with W′ of 30,882 N·s (SD 11,820). Giles et al. 2019: 11 adult male climbers, advanced to elite (red-point French 7b–8b+), intermittent grips at 45, 60 and 80 % of maximum. Int J Sports Physiol Perform 14(7):972–979.
  • Measured Critical force as a share of body mass explained 61 % of sport-climbing ability and 26 % of bouldering ability. W′ per kilogram explained 7 % and 34 %; both together explained 66 % and 44 %. Giles et al. 2021: 129 adult climbers (61 women, 68 men), intermediate to higher elite, self-reported grades, adjusted for sex. Int J Sports Physiol Perform 16(7):942–949.
  • Measured Forearm blood flow is cut off at roughly 45–75 % of maximum grip (Barnes 1980, cited by MacLeod et al. 2007), so CF sits just below the point where flow starts to stop.
  • Model The chart's reserve refills at half the speed it drains, and the default W′ (3,000 %·s) is a rough conversion of the Giles 2019 mean for illustration.

Model assumptions

  • Grips follow a 7 s on, 3 s off pattern. The pauses are part of how CF is defined for that pattern, so only gripping time moves the reserve.
  • Above CF the reserve drains at the amount over CF, per second, in units of % of maximum. Below CF it refills at half the gap. The refill rate is an illustrative choice; it was not measured here.
  • Hard and easy sections alternate every 60 s by the ± amount you choose.
  • The default reserve, 3,000 %·s, comes from dividing the measured mean W′ by the mean maximum grip implied by CF and its share of maximum. It's an order-of-magnitude guide, not a personal value.
  • Explained variance (R²) describes how much of the spread in grades lines up with a measure. It doesn't prove the measure causes the grade.

Try it

Scenario

Pumped below the crux.

Drill

Rest-hold refill. Grip lightly enough on rests to stay under the line.

Next read

Body size and the pump, then Pacing a timed competition route.

Common mistakes

Thinking of endurance as one number

two things matter, the line (CF) and the reserve above it (W′).

Over-gripping easy holds

easy sections only refill the reserve if you actually grip below the line.

Copying test protocols as training

this site doesn't give fingerboard or hang programmes. Ask a qualified coach, and growing climbers should not do this kind of training (see growth plates).

Reading 61 % as "61 % of performance is CF"

it means CF lined up with 61 % of the differences in grade within that group.