Critical Force in Climbing: A Better Way to Test Finger Fatigue and Endurance

Testing Critical Force in Climbers: Rethinking How We Measure Finger Fatigue Resistance in Climbers

Fatigue resistance of the finger flexors is often cited as one of the primary physiological determinants of climbing performance. However, how we measure it—and more importantly, how we prescribe training based on it—has been limited primarily to assumptions around maximum voluntary contractions (MVCs). This paper by Giles et al. (2021) proposes a more physiologically accurate and climbing-specific method to assess fatigue resistance using critical force (CF).

The Problem with Using %MVC

A standard method in climbing research has been to test endurance using submaximal contractions at a fixed %MVC, often 40–60%. The issue? MVC does not reliably map onto a metabolic intensity domain, and using %MVC doesn’t guarantee that different athletes are training or testing within the same physiological threshold. For one athlete, 40% MVC might fall below their critical force. For another, it might exceed it. That mismatch leads to inconsistent training effects and poor research comparisons.

What is Critical Force (CF)?

Critical force is the highest intermittent isometric force output that still results in a metabolic steady state, where oxygen consumption, phosphates, and pH reach a plateau. Anything above that threshold taps into anaerobic stores and becomes non-sustainable. This same concept has been validated across cycling (as critical power), running (critical speed), and isometric contractions of larger muscle groups (critical torque performed with overcoming style isometrics).

This study researched the first (although people, including myself, have been doing this before) climbing-specific all-out test to determine finger flexor critical force (ff-CF) and the "energy store" available above it (W′).

Study Design

Subjects: 129 intermediate-to-elite climbers (61 F, 68 M)

Testing device: A half-crimp position on a 20 mm rung connected to a force sensor

Protocol:

Peak force tested using 3x 5s MVCs

ff-CF tested using a rhythmic intermittent protocol: 7s all-out contraction: 3s rest, for 4–5 minutes

Test ends when force output plateaus (last 6 contractions averaged = CF, around 159 seconds)

W′ = total impulse above CF throughout the test

All testing was done unilaterally in a standardized body position with the arm extended above the head, shoulder engaged, and thumb off the hold. They also discussed the need to test critical force with more constrained positions (locking the body down and isolating the finger flexors).

Main Findings

1. The Test Works—But Only for Trained Climbers

The force decay curve stabilized nearly all participants, mimicking the expected response in validated CP and CT models. However, 6% of climbers (RP grades < f6c) did not reach a force plateau and were excluded. This suggests that novice climbers may not generate sufficient motor unit recruitment or a pacing strategy to perform this test reliably.

Recommendation: ff-CF testing should only be used with climbers redpointing ≥ f7a or bouldering ≥ F6C.

2. ff-CF Predicts Climbing Ability Better Than Peak Force

After adjusting for sex, the ff-CF expressed as % body mass explained:

  • 61% of the variance in sport climbing grade

  • 26% of the variance in bouldering grade

In contrast, W′ per kg body mass explained:

  • 7% of sport performance

  • 34% of bouldering performance

Combined, these variables explained:

  • 66% of sport climbing performance

  • 44% of bouldering performance

➡️ Sport climbing is more strongly predicted by CF (fatigue resistance)
➡️ Bouldering is more influenced by W′ (anaerobic capacity above CF)

This is intuitive given the endurance demands of sport climbing vs. the higher-intensity, power-endurance focus of bouldering.

3. MVC is Poorly Aligned with Metabolic Intensity Domains

Another significant finding was the mismatch between MVC and CF. The commonly used 40% MVC endurance testing level did not predictably line up with CF. Some athletes performed below CF (sustainable), while others worked above it (unsustainable).

Figure 4C in the study shows this scatter, highlighting why %MVC should be abandoned for endurance testing or training prescriptions.

Why This Matters for Coaches and Practitioners

ff-CF provides a more individualized endurance threshold, similar to the lactate threshold or ventilatory threshold in endurance sports

W′ gives insight into the "anaerobic reserve" and could help differentiate between endurance and power-endurance climbers.

Using ff-CF to set training intensities (instead of %MVC) may improve the precision of finger training protocols and reduce training variability.

Testing only takes 4 minutes, requires one all-out bout, and is more repeatable than multiple-day CP tests.

Limitations and Considerations

The test also loads the shoulder and upper arm musculature due to the extended arm position—so grip force might not be the only limiting factor

The 7:3s work-to-rest ratio is arbitrary—shorter or longer work phases may require revalidation of the protocol

The test is maximal and fatiguing—not appropriate for all athletes or for frequent re-testing

Future studies need to confirm the trainability of ff-CF and W′ over time and under different interventions

Takeaways

  • It’s less effective to test %MVC to prescribe endurance work.

  • Start using ff-CF testing to find the force threshold where fatigue resistance fails.

  • For boulderers, track improvements in W′ over time.

  • For route climbers, focus on increasing CF as a % of bodyweight.

Paper citation:  

Giles, D., Hartley, C., Maslen, H., Hadley, J., Taylor, N., Torr, O., Chidley, J., Randall, T., & Fryer, S. M. (2021). An all-out test to determine finger flexor critical force in rock climbers. International Journal of Sports Physiology and Performance, 16(7), 942–949.

 



Tyler Nelson