How to Target the FDS and FDP Tendons in Climbing and Rehab

How to Target the FDS and FDP Tendons in Climbing and Rehab

Understanding the individual contributions of the finger flexor tendons—the flexor digitorum profundus (FDP) and the flexor digitorum superficialis (FDS)—isn’t just for healthcare providers. It’s an essential concept for injury prevention, rehabilitation, and training specificity, particularly in climbing, where repeated isometric loading of finger flexors happens at different joint angles.

A 2016 in vitro (cadaver) study gives us some clarity on this issue. Using a novel setup that applied equal force to both tendons via a mechanical pulley system, researchers were able to isolate and compare how the FDP and FDS contribute across different finger joints during motion. This allowed for measurement of joint angles, tendon excursion, and moment arms simultaneously, something that EMG-based studies can't directly capture.

And for climbers, this data has real utility.

Joint Motion vs. Tendon Contribution: What the Study Found

●      FDP contributes most to DIP and PIP joint motion. It had the highest excursion at the PIP joint and was the primary driver of free joint flexion.

●      FDS contributes more at the MCP joint, particularly early in flexion.

●      During isolated PIP motion, FDP excursion exceeded FDS (slope = 1.34).

●      During isolated MCP motion, FDS excursion exceeded FDP (slope = 0.95).

●      In free flexion, FDP still had greater excursion overall.

So in simple terms: the deeper you flex (especially the DIP), the more FDP you’re recruiting. If your MCP is flexing and your DIP stays extended (as in a large sloper), FDS is doing more of the work.

Grip Position Biomechanics With Climbing

Let’s relate this to three common climbing grip types:

🟢 Open Hand

  • Joint Positions: Slight flexion at DIP, PIP, and MCP (~15–30°)

  • Primary Tendon Used: FDP-dominant

  • Notes: Low joint angles but higher tendon excursion. Good for early FDP rehab or lower strain training.

🟡 Half Crimp

  • Joint Positions: MCP neutral, PIP ~90° flexed, DIP slightly flexed

  • Primary Tendon Used: FDP-dominant

  • Notes: Maximizes FDP excursion. Add MCP flexion (“squeeze the fruit”) to increase total force and engage FDS more.

🔴 Full Crimp

  • Joint Positions: DIP extended, PIP ~80° flexed, MCP flexed

  • Primary Tendon Used: FDS-dominant early phase

  • Notes: DIP extension reduces FDP leverage early; FDS kicks in first, with FDP joining later under higher force.

So while the half-crimp is often used to improve "finger strength," the mechanical setup biases different muscle-tendon contributions. The open hand and half crimp are both FDP-dominant, but full crimp—due to DIP extension—reduces FDP leverage initially. That means FDS kicks in more to generate torque at the MCP and PIP until force ramps up and FDP joins in more. We can also add some MCP joint flexion (squeezing the fruit) with the normal half-crimp to increase FDS activity.

Tendon-Specific Loading: Implications for Rehab and Training

In climbing rehab, we often aim to bias one tendon system over the other—either to promote tendon gliding or to offload a painful structure.

Here’s a breakdown based on the research and traditional tendon gliding concepts:

🔵 Hook Fist

  • Joint Pattern: PIP and DIP flexed, MCP extended

  • Target Tendon: FDP (Flexor Digitorum Profundus)

  • Climbing Equivalent: Open hand on an incut hold

  • Best For: Promoting FDP excursion, tendon gliding, and early-stage FDP rehab

🟣 Tabletop Position

  • Joint Pattern: MCP and PIP flexed, DIP extended

  • Target Tendon: FDS (Flexor Digitorum Superficialis)

  • Climbing Equivalent: Starting phase of a full crimp

  • Best For: Biased loading of FDS, helpful in FDS overload recovery or tendon differentiation

🟠 Straight Fist

  • Joint Pattern: MCP flexed, PIP flexed, DIP extended

  • Target Tendon: FDS

  • Climbing Equivalent: Rare in climbing — hard to replicate

  • Best For: Controlled isolation of FDS in rehab (non-climbing)

⚫ Full Fist

  • Joint Pattern: MCP, PIP, and DIP all flexed

  • Target Tendon: Both FDS and FDP

  • Climbing Equivalent: Closed-hand jam or fist crack

  • Best For: Advanced tendon gliding and full-finger rehab loading

If you’re managing an FDP-related issue (e.g., distal pulley strain, DIP capsulitis, or tenosynovitis), early-stage loading can focus on open hand or hook fist postures, where the FDP has maximum excursion and gliding. If you’re managing an FDS-related issue (e.g., medial elbow pain from FDS overload), modify positions to emphasize DIP extension with MCP/PIP flexion.

Training Considerations

●      Most hangboard protocols target the FDP, especially in half crimp or open hand.

●      Very few protocols challenge the FDS in isolation, despite its role in full crimp initiation and force distribution at the MCP joint.

●      Including MCP joint flexion while using the half-cimp during your training will better engage the FDS and increase total muscle force.

Overuse of one tendon system—especially via repeated half crimping—may lead to imbalances in tendon excursion and gliding dynamics. This is particularly relevant in high-frequency training schedules, where pulley strain or synovitis might develop due to poor tendon load distribution.

Takeaways

●      The FDP dominates most climbing grips, particularly in half crimp and open hand.

●      The FDS is more active when the DIP is extended—seen in early full crimp or rehab postures.

●      Different grip positions can and should be used strategically in both training and tendon rehab.

●      Isolating glides through structured hand movements remains a valuable tool to reduce adhesions and restore function after injury.

Citation:

Yang, T-H., Lu, S-C., Lin, W-J., Zhao, K., Zhao, C., An, K-N., et al. (2016). Assessing Finger Joint Biomechanics by Applying Equal Force to Flexor Tendons In Vitro Using a Novel Simultaneous Approach. PLoS ONE, 11(8), e0160301. https://doi.org/10.1371/journal.pone.0160301

Tyler Nelson