What is the real job of the Lumbricals?

Crowley, J. S., Meunier, M., Lieber, R. L., & Abrams, R. A. (2020). The Lumbricals Are Not the Workhorse of Digital Extension and Do Not Relax Their Own Antagonist. The Journal of Hand Surgery.

The Skinny:

What do the lumbricals do?

There is a long-standing belief that the lumbricals act as a counterforce to the digital flexors enough to relax the digital flexors and act as primary digital extensors. This article works to update our understanding of the role of the lumbrical based on anatomy and innervation. They build off the work of Wang et al. to develop a better understanding of what the lumbricals function actually is in digit extension and balance of forces.

In The Weeds:

To review the anatomical line of the lumbricals, check out the picture below. Also, the lumbricals have several unique properties. They have the highest spindle fiber density of any UE muscle (a property of muscles intended for refined graded control, not power).  They also have the highest fiber length-to-muscle length ratio.  This means that the lumbricals are best at providing a low amount of force over a wide range of motion.  These two aspects refute the idea that the lumbricals are strong enough to overpower counterbalance the much larger, conversely designed, FDP & FDS.

lumbricals function
Crowley, J. S., Meunier, M., Lieber, R. L., & Abrams, R. A. (2020). The Lumbricals Are Not the Workhorse of Digital Extension and Do Not Relax Their Own Antagonist. The Journal of Hand Surgery.

Bringing It Home:

So what? What does that mean? Rather than the lumbricals being some powerhouse of IP extension, this article builds on the Wang articles proposal.  Want et al. propose that the lumbricals are a tension monitoring device that allows for refined motor coordination to aid in coordinating the stronger extrinsic muscles.  Crowley et al. add to that and suggest that the lumbricals also serve as a spring in a closed loop that helps to balance the 2nd and 3rd phalanx over the 1st, since the 1st phalanx has no tendon insertions to maintain its position. That makes the 1st phalanx an intercalated segment that needs delicately maintained tension to preserve its position. This is a significant shift in the idea that the lumbricals are a strong force in digital extension, counteracting the FDP & FDS.

Rating: 5/5

While not a research study, this article adds a lot of understanding and clarification of the anatomy and role of the lumbricals.  Looking at muscle size, length, fiber type, and anatomical attachment can improve our awareness of a unique muscle like this is actually doing in the hand. It is always good to stay open-minded to alternative views of our sometimes dogmatically held opinions and beliefs. There are many more significant pieces of info in this article, and I highly recommend reading it if you have a chance.

Wang K, McGlinn EP, Chung KC. A biomechanical and evolutionary perspective on the function of the lumbrical muscle. J Hand Surg Am. 2014;39(1):149e155.

1 Comment

  1. Maureen kline on February 16, 2021 at 4:47 am

    Great explanation and illustration

Leave a Comment

More To Read

Phantom Limb Pain, Residual Limb Pain, & Phantom Limb Sensation: Which is Which?

November 15, 2020

Written by Melissa Miller Introduction After amputation, the majority of individuals will experience phantom limb pain (PLP), residual limb pain (RLP), and/or phantom limb sensation (PLS). Experiencing these pains or sensations can greatly disrupt an individual’s quality of life. It is important to know what each of these are as each can impact the client…

Read More

Use of Proprioception in Rotator Cuff Repair

August 2, 2020

Article Review By Brittany Day Upper Limb Active Joint Repositioning During a Multijoint Task in Participants with and without Rotator Cuff Tendinopathy and Effect of a Rehabilitation Program Pairot de Fontenay, Benoit, Mercier, Catherine, Bouyer, Laurent, Savoie, Alexandre, & Roy, Jean-S√©bastien. (2019). Upper limb active joint repositioning during a multijoint task in participants with and…

Read More

6 of our Favorite Adaptive Equipment Tools for CMC Osteoarthritis

October 20, 2019

Individuals struggling with osteoarthritis of the 1st CMC joint usually have difficulty with daily activities and it can become very frustrating. Everyday tasks such as cutting food, opening containers, and donning a button up shirt can become painful and slow. The largest contributor to the overall function of our hand is the thumb. If the…

Read More

Sign-up to Get Updates Straight to Your Inbox!

Sign up with us and we will send you regular blog posts on everything hand therapy, notices every time we upload new videos and tutorials, along with handout, protocols, and other useful information.