Critical Force Testing for Climbers: What the New Research Says About the 4-Minute 7:3 Test
I've used critical force style testing (before it was called critical force) with climbers for over a decade now. Instead of testing how long someone can hang at a percentage of their max, which is hard to specify with climbing variability, the goal is to identify a post-fatigue sustainable threshold.
The original data on the 4-minute (7 seconds on, 3 seconds off x 25 reps) critical-force test interpreted the end test force as an athlete’s physiological threshold. However, the follow-up validation studies have since questioned it’s validity.
I still think the test can provide useful information about a climber's finger-flexor fatigue profile. But a repeatable test result and a validated physiological threshold are not necessarily the same thing.
That distinction matters if we're going to use the number to prescribe training.
What is critical force?
Critical force comes from the broader critical power concept in exercise physiology.
In endurance sports, critical power represents a boundary between exercise that can reach a relatively stable physiological state and exercise above that boundary, where fatigue-related disturbances continue accumulating until the athlete eventually has to stop.
The same basic model has been applied to intermittent isometric muscle contractions.
For the finger flexors, the idea is that we can describe performance with two components:
Critical force (CF): the force that can theoretically be sustained for a prolonged period.
W′: a finite amount of work—or in these isometric tests, more accurately force-time impulse—that can be produced above CF before exhaustion.
W′ is sometimes described as an "anaerobic energy store," but I would be careful with that interpretation. It is a mathematical performance parameter, not a direct measurement of stored anaerobic energy.
The original climbing critical-force research
Before the 4-minute all-out test, Giles and colleagues published a 2019 study using several separate finger-endurance trials.
Climbers performed intermittent 7:3 contractions at 45%, 60% and 80% MVC until exhaustion.
From the relationship between force and time to failure, the researchers calculated a mean critical force of approximately 41% MVC, although there was meaningful variation between athletes. PubMed
This was important.
It suggested that prescribing every climber's endurance work at something like 40% MVC did not necessarily mean every athlete was training at exactly the same physiological intensity.
The appeal of CF was that we might be able to identify an athlete-specific threshold instead.
The problem with the original method was practicality. Multiple exhausting tests were required.
So the researchers developed a much simpler single-session test.
The 4-minute all-out 7:3 test
In 2021, Giles and colleagues tested 129 climbers using repeated maximal finger-flexor contractions.
The climber performed:
7 seconds of maximal force
3 seconds of recovery
repeated for approximately 4 minutes.
Force falls quickly at first and then begins to level off.
The original protocol defined critical force as the mean force produced during the final six contractions, while W′ represented the accumulated force-time impulse above that end-test force. Most climbers reached the beginning of a plateau after roughly 157 seconds. PubMed
Eight lower-level climbers—about 6% of the sample—didn't develop a stable force-decay pattern and were excluded. The authors therefore suggested reserving the test for more experienced climbers. ResearchGate
The original results were impressive
Relative critical force had a strong relationship with climbing ability.
After adjusting for sex:
CF relative to body mass explained 61% of sport-climbing performance and 26% of bouldering performance.
W′ relative to body mass explained:
7% of sport-climbing performance and 34% of bouldering performance.
Together, CF and W′ explained approximately:
66% of sport-climbing performance
44% of bouldering performance. PubMed
That created an intuitive performance model.
Sport climbers appeared to benefit more from a high sustainable force.
Boulderers appeared to benefit relatively more from the finite capacity above that force.
And from a coaching standpoint, it was tempting to say:
CF = finger aerobic capacity
W′ = finger anaerobic capacity
That's probably too simple.
Reliability was the first warning
A useful test has to be repeatable before we worry about what it actually measures.
A 2023 study had 13 trained climbers repeat the 4-minute test on different days.
Relative reliability looked reasonably good:
CF/end-test force ICC = 0.85
W′ ICC = 0.82
peak force ICC = 0.94
But the within-athlete variation told a less reassuring story.
The coefficient of variation was approximately:
21% for end-test force
13% for W′
6% for peak force.
The limits of agreement were also quite wide. TopScholar
That's important.
A test can rank athletes reasonably consistently while still having enough day-to-day noise that interpreting a small change in one athlete becomes difficult.
Peak force was much more stable than the fatigue-related outcomes.
So even before questioning the physiology, there was reason to be cautious about saying a small improvement in CF represented a meaningful adaptation.
Then researchers tested whether “critical force” was actually sustainable
This is where the story gets much more interesting.
In 2024, Baláš and colleagues directly tested whether CF derived from the 4-minute all-out test actually represented the highest sustainable finger-flexor force.
Twelve climbers first completed the all-out test.
Their calculated CF averaged:
20.1 ± 5.7 kg.
They were then asked on separate visits to perform repeated contractions at:
CF
CF −2 kg
CF −4 kg
CF −6 kg
for as long as 12 minutes. PubMed
If the calculated CF truly represented the boundary of sustainable exercise, the athletes should theoretically have been able to maintain something close to it for a prolonged period.
They couldn't.
At the calculated CF, the average time to failure was only about:
440 ± 140 seconds
and just one participant completed the full 12 minutes.
As force was progressively reduced below CF, more climbers completed the test:
CF −2 kg: 38%
CF −4 kg: 69%
CF −6 kg: 92%. PubMed
That's difficult to reconcile with calling the original value the highest sustainable force.
The way we calculate CF may be part of the problem
During each 7-second contraction, force doesn't remain constant.
It declines.
So averaging the entire final contractions may overestimate what the climber can actually sustain.
Baláš and colleagues therefore also examined the lowest force reached at the end of the final contractions.
That value averaged approximately:
16.8 ± 5.2 kg
compared with:
20.1 ± 5.7 kg
using the traditional mean-force calculation. PubMed
That lower end-force showed a stronger relationship with sustainable exercise.
This raises an important methodological question:
Maybe the force plateau at the end of an all-out fatigue test is useful—but the way we've been averaging those contractions doesn't actually identify the physiological threshold we think it does.
A 2026 study pushed this even further
Boklaschuk, MacDougall and MacInnis published another direct validation study in 2026.
Twenty-six participants completed:
two separate 4-minute all-out tests
and
four constant-force tests to exhaustion.
This allowed the researchers to compare critical force estimated from the convenient all-out test with critical force derived from multiple traditional constant-force trials. DOI
The all-out test produced a considerably higher CF:
122 N
versus:
98 N
from the constant-force model.
Interestingly, the relationship between the two measurements was very consistent, and the repeated all-out tests demonstrated excellent test-retest reliability:
ICC = 0.97 for CF
ICC = 0.97 for W′. DOI
So the test can apparently be quite repeatable under the right conditions.
But it was systematically giving a different answer.
And that demonstrates an important testing principle:
Reliability does not equal validity.
You can measure the wrong thing very consistently.
What did the 2026 authors conclude?
Their conclusion was fairly direct.
The all-out test may be useful for tracking changes in forearm-flexor performance, but it did not accurately identify the highest sustainable force and was therefore probably not appropriate for directly prescribing exercise intensity. DOI
That's a meaningful change from how the test was originally discussed.
The 2021 paper anticipated CF becoming useful for both understanding exercise tolerance and determining training prescription. Eprints Gloucestershire
Five years later, the validation data support the first idea considerably better than the second.
So should we stop testing critical force?
I don't think so.
But I would change the language.
If someone performs a standardized 4-minute 7:3 test and their final force improves substantially after a training block, that's useful information.
They are maintaining more force after four minutes of repeated maximal contractions.
That's a legitimate performance change.
I'm just less convinced that we should automatically say:
“Your physiological critical force increased from 24 kg to 27 kg.”
A safer description might simply be:
4-minute end-test force
or
7:3 fatigue-resistant force.
That sounds less impressive, but it more accurately describes what we actually measured.
The test may still be sensitive to training
Another reason not to throw the test away is that it appears responsive to training.
A 2026 randomized controlled trial compared five weeks of low-load blood-flow-restriction finger training with higher-load finger training in advanced-to-elite climbers.
The researchers reported improvements in their critical-force measure of approximately:
+22% with low-load BFR
and
+24% with high-load training,
with no meaningful improvement in the control group. PubMed
That suggests the fatigue-test outcome can detect physiological or performance changes produced by training.
But again:
Sensitivity to training does not prove that the measurement represents a true metabolic threshold.
Those are different questions.
What about using %MVC for endurance testing?
My original article was probably too aggressive here.
I wouldn't say we should abandon %MVC-based testing.
The criticism is still valid: 40%, 50% or 60% MVC does not necessarily place every climber in an identical metabolic intensity domain.
But that doesn't make the test useless.
It just means we need to understand what we're measuring.
A large 2025 study involving 307 climbers found that maximal finger strength and climbing-specific endurance tests both had meaningful relationships with climbing ability.
Maximal strength explained the largest proportion of performance differences, but intermittent endurance still accounted for approximately:
28% of climbing ability in men
34% in women. Taylor & Francis Online
So a standardized test such as:
60% MVC → repeated contractions → time to failure
can still be useful.
It doesn't have to be a perfect physiological threshold test to provide a useful measure of finger-flexor endurance.
This is where I think testing often gets unnecessarily complicated.
Sometimes a simple, standardized performance test is more useful than a sophisticated metric whose physiology isn't completely validated.
What about W′?
I would also be more cautious with W′ than I was previously.
The original relationship with climbing performance was interesting: W′ was more strongly associated with bouldering than sport climbing. PubMed
But that does not necessarily mean W′ is a clean measurement of a climber's "anaerobic capacity."
The 2023 reliability study found considerably more intra-athlete variability in W′ than peak force, and the 2026 validation study found only moderate absolute agreement between W′ calculated from the all-out and constant-force methods. TopScholar
So I wouldn't currently use W′ as a highly precise target like:
“This boulderer needs to increase W′ by 10%.”
I'd treat it as another descriptive variable from the fatigue test.
Interesting.
Potentially useful longitudinally.
But not something I'd build an entire training program around yet.
What I would measure now
For most climbers, I still think a testing battery can be fairly simple.
Maximal force tells me how high the athlete's force ceiling is.
A 30-second all-out test gives me information about how much high force they can maintain over a shorter, highly glycolytic effort.
A submaximal continuous or intermittent test to failure gives me a repeatable measure of local endurance.
And a 4-minute 7:3 all-out test can provide a fatigue curve and an end-test force if I need a more detailed profile.
Those tests answer slightly different questions.
I don't need to pretend that any single one represents the climber's entire "energy system."
Standardization matters more than the name
Whatever test you choose, keep the protocol the same.
That means controlling:
edge depth,
grip position,
shoulder and elbow position,
hand tested,
warm-up,
rest before testing,
work-to-rest ratio,
instructions,
force calculation,
and how recovery is handled between contractions.
That last one matters more than people realize.
Different CF studies have allowed different recovery behaviors—keeping the arm overhead versus lowering or shaking the hand—and that can alter forearm perfusion and recovery. PubMed Central (PMC)
If you're using a Tindeq or similar force sensor, the device isn't really the test.
The protocol is the test.
Where I land on critical force now
Critical force remains an interesting concept.
And the original climbing research did something valuable: it moved us beyond simply talking about finger endurance as “how long can you hang?”
The 4-minute all-out test also gives us much more information about the way force decays than a single maximal-strength number.
But the newer literature changes how I would interpret it.
I would no longer say:
The 4-minute test gives us the force a climber can sustainably maintain.
The current evidence doesn't support that strongly enough.
I would say:
The 4-minute 7:3 test gives us a standardized measure of how much force a climber can still produce after repeated maximal finger contractions.
That outcome is associated with climbing performance, can be repeatable, and appears responsive to training.
That's useful.
It just isn't necessarily the physiological threshold we originally hoped it was.
Practical takeaway
For coaches and climbers, I think the distinction is fairly straightforward.
Keep using the test if it gives you useful longitudinal information.
But I would not automatically prescribe endurance training directly from the CF number.
I also wouldn't abandon simpler %MVC endurance testing under the assumption that critical force is physiologically superior.
And I would be cautious about treating W′ as a direct measure of a climber's anaerobic energy reserve.
The bigger lesson from the last five years of research is probably this:
A performance test does not have to perfectly represent a physiological threshold to be useful.
It needs to be repeatable enough, sensitive enough to meaningful changes, and interpreted according to what it actually measures.
For the 4-minute critical-force test, we're getting much closer to understanding that.
Research
Giles D, Chidley JB, Taylor N, et al. The Determination of Finger-Flexor Critical Force in Rock Climbers. International Journal of Sports Physiology and Performance. 2019;14(7):972–979. PMID: 30676817. PubMed
Giles D, Hartley C, Maslen H, et al. An All-Out Test to Determine Finger Flexor Critical Force in Rock Climbers. International Journal of Sports Physiology and Performance. 2021;16(7):942–949. PMID: 33647876. PubMed
McClean ZJ, MacDougall KB, Fletcher JR, Aboodarda SJ, MacIntosh BR. Test-Retest Reliability of a 4-Minute All-Out Critical Force Test in Rock Climbers. International Journal of Exercise Science. 2023;16(4):912–923. PMID: 37637240. PubMed
Baláš J, Gajdošík J, Javorský T, Berta P, Feldmann A. Measuring Critical Force in Sport Climbers: A Validation Study of the 4 Min All-Out Test on Finger Flexors. European Journal of Applied Physiology. 2024;124:2787–2798. PMID: 38668851. PubMed
Berta P, Michailov M, Kaško D, et al. Validity and Normative Scores of Finger Flexor Strength and Endurance Tests Estimated From a Large Sample of Female and Male Climbers. Journal of Sports Sciences. 2025;43(3):245–255. PMID: 39754511. PubMed
Boklaschuk P, MacDougall K, MacInnis M. The Validity and Utility of the All-Out Test for Forearm Flexor Critical Force. European Journal of Applied Physiology. 2026;126:4847–4857. DOI
Perrin TP, et al. Low-Load Blood-Flow Restricted Resistance Training Improves Finger Flexor Muscle Function in Experienced Climbers: A Randomized Controlled Trial. Medicine & Science in Sports & Exercise. 2026;58(8):1637–1650. PMID: 41882800.