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I'm wondering:

there is the musculus extensor digitorum, that extends finger 2 to 5 up.

There is an extra muscle for the 2. and the 5. finger (m. extensor digiti minimi and m. extensor indicis).

But why can I than lift my middle finger (3. finger) up without lifting my ring finger (4. finger) up?

I mean, when the musculus extensor digitorum is stimulated (by the nervus radialis), shouldn't it than extend EVERY finger (2 to 5), not only one of them?

My guess was that the antagonist of the musculus extensor digitorum is pulling the other fingers back, but the musculi flexor digitorum superficialis and profundus are also just one big muscle, so this doesn't seem to be the right explanation.

Another idea was that I can activate only parts of the nervus radialis and with that only activate parts of the musculus extensor digitorum. But that doesn't seem to be very likely.

So, what is the reason behind that?

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  • Might you be looking at finger muscles on the wrong level… as, for instance, comparing wrist to hand or hand to fingers? Feb 8 at 23:01

1 Answer 1

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Muscles aren't singular entities, they're made up of multiple muscle fibers. In the case of the extensor digitorum, the muscle fibers are attached to one of various tendons that manipulate the fingers, see e.g. this image:

Extensor_digitorum_muscle

Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain.

Multiple motoneurons innervate the muscle, and triggering different fibers moves different fingers through the attached tendons. Some sharing of tendons can make it more difficult to move individual fingers, but for most peoples' anatomy this is most difficult for the ring finger and not the others. Hu X, Suresh NL, Xue C, Rymer WZ. Extracting extensor digitorum communis activation patterns using high-density surface electromyography. Front Physiol. 2015 Oct 6;6:279. doi: 10.3389/fphys.2015.00279. PMID: 26500558; PMCID: PMC4593961. refers to both the motoneuron mapping and individual variability:

The extensor digitorum communis muscle plays an important role in hand dexterity during object manipulations. This multi-tendinous muscle is believed to be controlled through separate motoneuron pools, thereby forming different compartments that control individual digits. However, due to the complex anatomical variations across individuals and the flexibility of neural control strategies, the spatial activation patterns of the extensor digitorum communis compartments during individual finger extension have not been fully tracked under different task conditions.

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    Never encountered that spelling "motoneuron". Is that a US thing or just less frequently in pop culture I wonder. Feb 5 at 14:55
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    @JiminyCricket. I'm familiar with them as "motor neurons", two words, or "motoneurons", one word; "motor neurons" are various neurons in the nervous system involved in motor control. It's common to separate them into "upper" and "lower" motor neurons. "Motoneuron" in my experience is used almost exclusively for the neurons specifically innervating muscle.
    – Bryan Krause
    Feb 5 at 15:04
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    @JanusBahsJacquet I believe it's hardest for most to move the ring finger independently because it also moves the pinky; for some it may also or instead move the middle. But usually the middle is easy to move by itself, I think... Maybe I need a more robust source on that. I can move all of mine independently in both directions which I assume is from learning to type at a young age.
    – Bryan Krause
    Feb 6 at 13:38
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    @JanusBahsJacquet I edited to just say "ring" finger; it seems from a quick Google scan that for most people the issue is with moving their ring finger; there are differences in whether the problem is that the pinky moves too or the middle moves too, but it seems like most can move the middle and pinky independently, and actually this fits very well with the diagram in my answer if you follow the tendons, since there isn't any connection to the ring finger that isn't at least partially shared.
    – Bryan Krause
    Feb 6 at 15:26
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    @JanusBahsJacquet - I'd say your joints are somewhat hypermobile, maybe at the extreme of within normal limits. I've seen people hyperextend their fingers so that they almost touch the backs of their hands, which makes me very uncomfortable, because I know what that would do to normal joints, and it isn't pretty (think torture devices). Most people who can do that have an underlying connective tissue disorder that allows that elasticity. It has its pros and cons. In terms of teaching your fingers to do things... Feb 6 at 23:13

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