I recently finished the classroom portion of my Doctor of Physical Therapy education, and I am now moving into my final clinical rotation down in San Diego. With a short break in between school and clinic, I took a trip with my girlfriend to Ten Sleep Canyon in Wyoming. The limestone there is incredible! I had never climbed on such pristine, consistent limestone before, and it certainly wet my whistle for trips to famous limestone crags like Céüse in France (Céüse on Mountain Project.) Like most bulletproof limestone crags, the rock is mottled with pockets galore. Along with this abundant array of tiny finger holds comes a potential for injury if the pockets are not utilized safely. If you’ve climbed on pockets, then I’m sure you’ve tested your luck with putting two fingers in a pocket, or even tried out a daring one-finger pocket (mono). In this post I’d like to touch on the common techniques for pocket climbing, and educate you on the proper method to avoid injury. My trip to Ten Sleep Canyon inspired this blog post, so I hope you can take away a helpful message, and climb strong and healthy on your own trip to a pocketed limestone crag!


Muscle to Tendon Ratio

You may remember from my Pulley Injury article that you have two main muscles in your forearm that flex your fingers. These muscles are the Flexor Digitorum Superficialis and Profundus (FDS and FDP respectively). The unique situation posed here relates to the muscle to tendon ratio. Each of these muscles have 4 tendons, each going to one respective finger (index, middle, ring and pinky fingers). What this means for you is that when you want to flex some fingers but extend others (like when climbing in pockets), this introduces an opposing strain for the muscle. I’ll explain the safer alternative in another section.

 Tendon-Tendon Connectivity

Another anatomical consideration I’d like to point out is related to the more specific tendon-tendon connectivity that often gets overlooked. At the level of the carpal tunnel, our FDP tendons are significantly interconnected.1 Leijnse et al.2 pointed out the “fibrous connections between the FDP tendons at the wrist level, which consist of strong tendinous or fascia-like structures.” This adherence affects the mobility of each individual finger flexor tendon with respect to each other. In more simple terms, when you want to flex or extend one individual finger, this affects the other fingers and vice versa. Because of this interconnectedness, you need to respect the anatomy and use safe pulling methods. Refer to this image to help wrap your head around the content.

Lumbricals (Intrinsic Hand Muscles)

The final anatomy note I want to point out is regarding the lumbrical muscles in your hand. These muscles work to flex your fingers at the hand (at the metacarpophalangeal, MCP, joint), and also work to extend your fingers by pulling on the extensor hoods. The unique aspect of these muscles is that they originate from the FDP tendons, and not bone like most other muscles. With muscles attaching to moveable tendons, this may lead to complications when motion is in opposition (flexion vs. extension) as mentioned above. More on this shortly.

The Quadriga Effect:

The first two anatomical concepts explained above lead to what has been described as the quadriga phenomenon.1,3 Dr. Verdan first described it in 1960 and Ton Schreuders, a physical therapist from the Netherlands, elaborated on the phenomenon in his own article more recently. A quadriga is a Roman chariot pulled by four horses, and the four reins of the horse-chariot system are described to resemble the finger flexor tendons (see image below.) In Dr. Verdan’s words, the quadriga syndrome is “a condition in which the flexor tendon excursion is reduced in an unaffected finger when the excursion of the flexor digitorum profundus (FDP) tendon of the adjacent finger is altered by stiffness, injury, or adhesion.” Because of the interconnectedness of the FDP tendons, restriction in one finger will affect the others. Similarly, when you climb in a mono pocket, your one finger is extended in relation to the other fingers and these strained connections may result in injury. Even though you have individually moveable fingers, they are still connected and influence each other’s movement.

To help drive the point home, try out this exercise demonstrating the restrictions caused by our finger tendon connections.

Commonly Seen Pocket Climbing Technique:

Very commonly, climbers use an opposing flexion/extension finger posture when pulling on pockets. The holding finger’s proximal phalanx is in relative extension compared to the adjacent fingers’ phalanges, which are in relative flexion (see images below.) There is a reason for this. The quadriga effect explained above3,4 helps to increase your pulling power by roughly 48% as described by Schweizer in his 2001 article.5 But just because you are stronger using this posture doesn’t mean that you should always use it, especially if it adds a potential for injury. In the next section are some potential injuries resulting from this common pocket finger posture (oppositely flexed/extended fingers), and in a later section I’ll suggest a safer way to hold pockets.

Where Potential Injury May Arise:

When injury occurs during pocket climbing, we generally see three areas affected:

  1. At the muscle belly where the individual tendons differentiate into their respective muscle fibers.
  2. At the lumbrical muscles in the hand, known as Lumbrical Shift Syndrome.
  3. At the tendons themselves.

Forearm Muscle Strain:

Remember that the FDS and FDP muscles each have 4 tendons that course through to each finger from one muscle belly. Different muscle fibers within the muscle belly control respective finger tendons but still perform as one muscle. With this configuration, your muscle may be prone to strain when one finger or some fingers are extended and others are flexed. This opposition can cause conflicting force internally at the muscle, and lead to a muscle strain. Think of it like a leather glove, where the body of the glove is the muscle belly, and the fingers are the tendons. If you pull on one of the fingers (representing a finger in a mono pocket) and pull down on the opposite corner of the glove (representing the muscle fibers dedicated to another finger contracting and opposing the pull of the mono pocket), you can see tension built up in the leather. This represents the internal conflicting strain in the muscle belly, and could lead to a muscle strain injury.

Lumbrical Shift Syndrome:

In extreme cases, usually when pulling on a one-finger pocket, an injury to the lumbricals may occur. Because the lumbrical muscles attach to the FDP tendons, a shearing force presents itself when you oppositely extend and flex adjacent fingers. When the finger/s are holding the pocket, they are extended and this tendon has a relative upward pull due to tension from the mono pocket. If you are flexing your adjacent fingers, these tendons have a downward force, opposing the pocketed finger/s. Due to the muscle attachments, this applies a shear force to the lumbrical, and may lead to a strain or tear. Look at these images to better understand the shear forces described.

Tendon Injury:

This is a more straightforward injury to explain because it’s directly related to the inherent strength of a tendon. If you are pulling on a mono pocket, you only have one finger loaded. If the force you’re pulling with exceeds the strength of the tendon itself, this could result in a sprain or rupture of that tendon. These injuries are not as common, but the potential is there. Usually the injury occurs as a muscle strain in the FDP, FDS, or lumbricals.

Recommended Hand/Finger Posture for Pocket Climbing:

So how should you safely position your fingers when climbing in pocketed limestone? Let’s ask our friendly neighborhood Spider-Man (see image below.) Think about his hand/finger posture when he’s shooting his webs. The fingers that are not actively squeezing his web shooters (analogous to the climber’s fingers that are not in the pocket) are neutral/extended and NOT flexed. Now, I will admit that Spider-Man gets a little carried away with his other fingers (the ones that would be in the pocket), and he is not a perfect analogy for pocket climbing. But the main idea is to avoid oppositely flexing/extending your proximal phalanges. Ideally when climbing, all your proximal phalanges should be in line with each other. Using what you learned above, think about how this new posture will reduce your risk of injury. There is no opposing flexion/extension force at the hand or finger. This eliminates the shear forces at the lumbricals and reduces the strain of the flexor muscles.

My reasoning for bringing up Spider-Man is to leave you with an image you can remember when thinking about how to safely pull on pockets. Not exactly his hand posture, but all proximal phalanges in line. When in doubt, shoot your web! Refer to the images here explaining the proper technique.

Main Take-Away:

When climbing in pockets, protect yourself from injury by utilizing proper finger/hand posture. Yes we may be slightly stronger when we flex the adjacent fingers, but only at the risk of injury. Keep all of your proximal phalanges in line with each other to avoid unnecessary shear forces. Strengthen your arms, hands, and fingers in this new posture to ensure you’ll have the strength necessary to send your project while protecting your body from injury.


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  1. Schreuders TAR. JHS(E) The quadriga phenomenon: A review and clinical relevance. J Hand Surg Am. (0):1-10. doi:10.1.
  2. Leijnse JN, Bonte JE, Landsmeer JM, Kalker JJ, Van der Meulen JC, Snijders CJ. Biomechanics of the finger with anatomical restrictions–the significance for the exercising hand of the musician. J Biomech. 1992;25(11):1253-1264.
  3. Verdan C. Syndrome of the quadriga. Surg Clin North Am. 1960;40:425-426.
  4. Verdan C, Poulenas I. [Anatomic and functional relations between the tendons of the long palmar muscle and the long flexor muscle of the thumb at their crossing in the carpus]. Ann Chir Plast. 1975;20(2):191-196.
  5. Schweizer A. Biomechanical properties of the crimp grip position in rock climbers. J Biomech. 2001;34:217-223.
  6. MacLeod D (Mountaineer). Make or Break : Don’t Let Climbing Injuries Dictate Your Success.