Critical Force in Climbing: Concepts, Measurement, and Practical Applications

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introduction

Critical force (CF) is a key physiological threshold in climbing, representing the highest sustainable force output that the finger flexors can maintain over time. It extends the concept of critical power, widely used in endurance sports, to climbing-specific isometric tasks. CF is often accompanied by W′, a finite energy store that represents the capacity for work performed above CF before exhaustion. Together, CF and W′ provide a framework to quantify fatigue resistance, monitor training adaptations, and prescribe training intensities.

Measurement and Validation of Critical Force

Critical force (CF) in climbing can be measured using two main approaches: the multi-trial test and the single-trial all-out test. Both methods are validated but differ in complexity, equipment, and reliability characteristics.

1. Multi-Trial Intermittent Contraction Test

The multi-trial test estimates CF and W′ from multiple time-to-exhaustion trials at different intensities of maximum finger strength. In the study by Giles et al. (2019) [1], participants performed intermittent isometric contractions (7 s on, 3 s off) at 45%, 60%, and 80% of their maximum finger strength. For each trial, the total work performed (Wlim) and the time to exhaustion (Tlim) were recorded. A hyperbolic relationship between work and time allows CF to be calculated as the slope and W′ as the intercept of the work–time model.

Test Quality

  • A high correlation was found for the parameter estimates from the different CF models, with the work–time model fitting the data best.
  • Single-trial estimates of CF show good agreement with single-trial results, whereas W′ demonstrates lower agreement.
  • The protocol uses equipment readily available in most climbing gyms, making it practical for both research and applied settings.

Testing Protocol

Purpose

Determine CF and W′ using multiple trials at different percentages of maximum finger strength.

Equipment needed

  • Fix installed hangboard for both hands
  • Pulley system
  • Various weight plates or similar loading options
  • Climbing harness

Instructions

  • Warm-up
    • Perform 5–10 min of general climbing warm-up.
    • Include hangs on the target edge to prepare the finger flexors, progressively increasing the load.
  • Finger Maximum Strength Test
    • Test your maximum finger strength by hanging with both arms in a half-crimp position. Keep arms straight and shoulders engaged. A 23 mm edge is commonly used, as it targets the finger flexors equally.
    • Estimate the load you can sustain for 7s. Add weight (via harness and belay loop) or reduce load with a pulley system.
    • Attempt a 7 shang. If you cannot hold for 7s, reduce the load in the next trial. If you can hold easily, increase the load. Repeat until you find the maximum load with which you can just complete 7s. Rest >5 min between attempts and aim to find your maximum within ~4 trials.
  • Multi-Trial Intermittent Contraction Test
    • Schedule 3 separate trials: one at 45%, one at 60%, and one at 80% of maximum finger strength.
    • Space trials either by 20 minutes (for short-term comparison) or >24 hours (preferred for best accuracy).
    • Execution
      • Hang intermittently (7 s on, 3 s off) on the rung at the target % of maximum finger strength until failure. Adjust load using weights and pulley as needed.
      • Record time to exhaustion (Tlim) and calculate total work (Wlim) by multiplying absolute load by Tlim.
    • Calculation
      • Plot Wlim versus Tlim to establish a hyperbolic relationship.
      • Determine CF as the slope of the linear work–time relationship and W′ as the intercept
      • For convenience, you can use the spreadsheet where only measured parameters need to be entered. (Link).
    • Data interpretation
      • CF reflects the highest sustainable finger-flexor force.
      • W′ represents the finite capacity for work above CF.

2. Single-Trial All-Out Test

The single-session all-out approach typically involves a 4-minute intermittent maximal contraction test, consisting of 24 intervals. During each interval, climbers apply maximal force on the rung for 7 s, then rest for 3 s [2][3][4]. CF is determined from the force plateau reached during the final contractions, and W′ is computed as the cumulative work performed above CF.

Test Quality

  • Both CF and W′, normalized to body mass and adjusted for sex differences, correlate strongly with climbing performance: CF is the stronger predictor for sport climbing (r = 0.61), while W′ is more relevant for bouldering (r = 0.34). A combined model explained 66% of sport climbing and 44% of bouldering performance [2].
  • Test–retest reliability is generally good to excellent (CF: ICC = 0.848, W′: ICC = 0.820), though intra-individual variability is higher for CF and W′ [4].
  • Verification trials showed that participants often failed when CF was defined as the mean force of the last 3 contractions, but were more successful when CF was defined as the minimum force of the last 3 contractions (CFmin). CFmin also correlated more strongly with climbing performance [3].
  • Muscle oxygen dynamics measured during the test showed high variability and were less reliable for distinguishing exhaustion, warranting cautious interpretation [3].

Testing Protocol

Purpose

Determine CF and W′ using a single-trial all-out test.

Equipment needed

  • Force measurement device: Climbro hangboard or Tindeq Progressor
    • For the Tindeq Progressor: A portable fingerboard and setup to suspend it securely
  • Climbing harness & weight plates (if you can hang on the testing rung with one hand)

Instructions

  • Warm-up
    • Perform 5–10 min of general climbing warm-up.
    • Include hangs on the target edge, progressively increasing the load.
  • Test Execution
    • Follow the instructions of the app linked to your device (Climbro or Tindeq Progressor).
    • Perform 24 repetitions of 7 s maximal hangs with 3 s rest between each (~4 min total). Pull as hard as possible on every repetition; avoid pacing.
    • Use a standardized edge (typically 20 mm).
    • Optionally, perform verification trials at reduced loads (CF − 2 kg, CF − 4 kg) in a separate session.
  • Data Recording
    • Data are recorded and processed by the respective app.
    • CF is best calculated as the minimum force of the last 3 contractions (CFmin).
    • W′ is the cumulative work performed above CF.
  • Interpretation
    • CF represents fatigue resistance and sustainable finger-flexor strength.
    • W′ indicates the capacity for short, high-intensity efforts.
    • Expect some intra-individual variability; repeat testing may be needed for accuracy.

References

[1] Giles, D., Hartley, C., Maslen, H., Hadley, J., Taylor, N., Torr, O., Chidley, J., Randall, T., & Fryer, S. An All-Out Test to Determine Finger Flexor Critical Force in Rock Climbers.. International journal of sports physiology and performance. 2020 https://doi.org/10.1123/ijspp.2020-0637

[2] Giles, D., Chidley, J., Taylor, N., Torr, O., Hadley, J., Randall, T., & Fryer, S. The Determination of Finger-Flexor Critical Force in Rock Climbers.. International journal of sports physiology and performance. 2019 https://doi.org/10.1123/ijspp.2018-0809

[3] Baláš, J., Gajdošík, J., Javorský, T. et al. Measuring critical force in sport climbers: a validation study of the 4 min all-out test on finger flexors. Eur J Appl Physiol 124, 2787–2798 (2024). https://doi.org/10.1007/s00421-024-05490-7

[4] McClean, Z., MacDougall, K., Fletcher, J., Aboodarda, S., & MacIntosh, B. Test-Retest Reliability of a 4-Minute All-Out Critical Force Test in Rock Climbers.. International journal of exercise science. 2023; 16 4. https://doi.org/10.70252/dbic1991

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