Tendon Fatigue in Climbers: What Some In-vivo Research Tells Us About Finger Flexor Failure

Tendon Fatigue in Climbers: What Some In-vivo Research Tells Us About Finger Flexor Failure

Understanding how tendons fail isn’t just academic—it has real consequences for how climbers train, recover, and manage injury risk over time. And while there's no shortage of anecdotal advice out there, much of it ignores the physiology of tendon fatigue and strain accumulation.

A 2025 study by Firminger et al. finally gives us tendon-specific fatigue data on the actual tissues climbers use most: the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons. This study tested these tendons directly, rather than relying on data from lower-limb tendons like the Achilles or patellar. The goal was to understand how these tendons fatigue under cyclic loading, exactly the kind of loading rock climbers experience during repeated hangs, lifts, and climbing cycles.

Here's a breakdown of the findings, why they matter, and what climbers can do with this information.

Tendon Fatigue = Cumulative Load, Not Just Peak Load

The researchers took cadaveric FDP and FDS tendons and applied either:

  • A quasi-static pull to failure (to determine ultimate tensile strength), or

  • Cyclic loading at 40%, 60%, or 80% of ultimate strength until the tendon failed

They found a logarithmic relationship between applied stress and the number of cycles to failure. That means:

  • Tendons exposed to higher loads fail faster

  • Tendons exposed to lower loads last longer, but still fail eventually with repetitions

This reinforces a fundamental but often overlooked idea: tendon failure in climbing is not just about doing something heavy once—it's about doing something moderate hundreds or thousands of times without enough recovery.

Tendon Strain at Failure ≈ 19%

One of the most important findings is that these tendons consistently failed at around 19% strain, regardless of the absolute force applied.

That tells us failure is strain-controlled, not force-controlled.

In climbing terms:

  • You can load the tendon at relatively low forces, but if the joint angles or grip mechanics stretch the tendon too far—especially under fatigue—it can still fail.

  • Dynamic movement, passive hangs, deep lock-offs, and uncontrolled swings (think Kilterbaord-style climbing) can all increase strain on the tendon even if the force isn't high.

Takeaway: Monitor mechanics under fatigue. If your form degrades—fingers hyperextend, shoulders sag, elbows flare—you’re likely increasing strain, not just effort.


Creep and Damage Rate Are Early Warning Signs

The study found that creep rate (the extent to which the tendon elongates with repeated loading) and damage rate (the rate at which stiffness is lost) were better predictors of failure than force or the number of repetitions.

This is critical for climbers.

Before there's pain, inflammation, or even noticeable performance loss, your tendon might already be degrading. You might feel:

  • Sluggish recruitment

  • Delayed force transfer

  • A lack of finger “snap” during power moves

Takeaway: If your contact strength feels off or your Tindeq metrics start drifting, don’t ignore it. It's not always a neural issue—it could be a mechanical decline in tendon stiffness.

Let’s Talk Numbers: How Much Load Does It Take to Fail a Tendon?

The average ultimate tensile strength (UTS) for a single tendon slip in this study was about 41.2 MPa. That's a unit of stress—megapascals—so we need to convert that into something climbers can visualize.

What is a "tendon slip"?

Each FDP and FDS muscle sends four separate tendon branches ("slips") to your fingers. So when this study says “one FDP or FDS tendon,” it means one slip to a single finger—not all of the tendons or muscle.

Let’s estimate how much load would rupture one slip:

  • Assume a tendon cross-sectional area (CSA) of 5 mm²

  • 41.2 MPa=41.2×106 N/m241.2 \, \text{MPa} = 41.2 \times 10^6 \, \text{N/m}^241.2MPa=41.2×106N/m2

  • Multiply by area: 41.2×106×5×10−6=206 N41.2 \times 10^6 \times 5 \times 10^{-6} = 206 \, \text{N}41.2×106×5×10−6=206N

  • That’s about 21 kg, or 46lbs of force

So: A single tendon slip could rupture at a direct load of ~21 kg

In real climbing:

  • Load is distributed across multiple tendon slips and fingers

  • But certain positions (e.g., mono pockets, two-finger pockets, one-arm hangs) focus the load onto fewer slips.

  • Failure often occurs well before rupture due to repetitive submaximal loading

Takeaway:
You don't need to rupture a tendon to injure it. Tendon damage builds below the threshold of rupture, especially if your training is high volume or you’re stacking fatigue across sessions.

Weibull Probability Model: When the Risk Spikes

The researchers used a Weibull distribution to model failure probability over time. What they found:

  • Tendons loaded at 40% of UTS failed inconsistently—some early, some late ("infant mortality")

  • Tendons loaded at 60–80% of UTS had predictable, accelerating failure.

For climbers, this means there's often a deceptive window where things seem fine—until they’re not. Once you're in a fatigue-loaded zone, even small changes in load or technique can dramatically increase the risk.

Takeaway: Plan deloads. For every 3–4 weeks of high-volume or high-load training, program a lower-load week. Especially during trip prep, projecting cycles, or campus-heavy training blocks.

Morrow Model: More Work = Faster Failure

The paper also applied the Morrow energy model, which links mechanical work per repetition to fatigue life.

Translation: the more energy each rep demands, the fewer reps you can safely perform before the tissue fails.

This directly applies to:

  • Campus board

  • Weighted hangs/lifts

  • Board-style climbing

Takeaway: These sessions have value—but they also burn through tendon capacity quickly. Cap your volume. Keep high-RFD training early in the session. Avoid layering these loads on top of accumulated fatigue.

Practical Applications for Climbers

Here's how to use this information in your own training:

Fatigue life is limited. Balance load and volume. Avoid stacking max hangs on top of hard climbing.

Strain is the key variable. Use controlled joint angles. Don’t let the form break down under fatigue.

Tendon damage is subtle. Watch for reduced finger snap or sluggish contact strength as an early warning.

Failure risk increases over time. Insert deload weeks. Don’t go all-in for months without tapering.

High-energy reps = fewer safe reps. Limit campus and velocity hangs. Spread power sessions throughout the week.

Final Thoughts

This paper finally gives us specific data on the actual tendons climbers use. It shows that FDP and FDS tendons are mechanically weaker than previously studied tendons, such as the patellar or Achilles, and are more vulnerable to cyclic fatigue than most models assume.

As a climber or coach, this information should shift your thinking:

  • From “Can I pull harder?” to “How often, and under what conditions?”

  • From “Is it painful?” to “Is it degrading?”

If you train like tendons are indestructible, they’ll eventually remind you they’re not. But if you train with a better understanding of fatigue mechanics, you can extend the life of your tissues, climb harder for longer, and avoid unnecessary injury downtime.

Reference:

Firminger, C. R., Smith, N. C., Edwards, W. B., & Gallagher, S. (2025). In vitro fatigue of human flexor digitorum tendons. Journal of the Mechanical Behavior of Biomedical Materials, 163, 106842. https://doi.org/10.1016/j.jmbbm.2024.106842


Tyler Nelson