6 Muscles that not everyone has

Geschatte leestijd: 5 minuten Evolution is never finished, we are a work in progress. Some muscles seem to be remnants from older times. They appear to have no significant function anymore. More importantly; not everyone has them.

Rudimentary organs and ‘useless’ muscles

Although it may not be the time to emphasize the differences between people, today I wanted to write about the so-called ‘rudimentary’ muscles. Rudimentary organs are organs that have a reduced or non-existent function because they have been surpassed by evolution. This effect can, for example, be compared to the function of such organs in evolutionary ancestors or related mammals. The most well-known example is probably the worm-shaped appendix that is removed in cases of appendicitis. Some of these rudimentary organs are muscles. You should be cautious when labeling organs as rudimentary. Rudimentary can mean that an organ no longer serves a function, but also that we do not yet know that function. Many organs previously considered useless now have known functions. The fact, however, that some muscles are completely absent in quite a few people supports the idea that they have become obsolete due to changes in time and our environment.

1. Plantaris

The plantaris is a good example. This muscle in the calves has an attachment that continues into the Achilles tendon. This attachment is the longest in the body (30cm to 45cm), if you happen to have a plantaris and its corresponding attachment. However, it is absent in 8 to 12 percent of the population. In terms of strength, the plantaris adds little to the other calf muscles, the soleus and gastrocnemius. This isn’t surprising since the mass of the plantaris is only 3% of that of the gastrocnemius [8]. The plantaris is even called the ‘freshman’s nerve’, because medical students often mistake the thin muscle for a nerve. You can imagine that evolutionary scientists quickly saw the plantaris as rudimentary. Especially since they can refer to animals in which the plantaris is as large as the gastrocnemius (the American brown bear). Others point to primates where the tendon of the plantaris does not end in the Achilles tendon, but continues further under the foot to the connective tissue layer on the sole of the foot. Some think that in our ancestors, this muscle enabled us to pick things up with our toes. However, as we started to walk more upright and use our feet less to grasp things, the function slowly but surely disappeared or changed. However, the plantaris muscle seems to play a more important role in conveying information about the position of the foot relative to the lower leg [7]. Even this, however, does not seem essential given that the attachment of the plantaris, like that of number two on this list, is often surgically removed to be better utilized elsewhere in the body.

2. Palmaris longus

A thin muscle in the forearm whose attachment may be visible on the underside of the wrist. About 16% of people do not have this muscle and attachment[1]. Due to its small influence on grip strength, the attachment/tendon of this muscle is sometimes removed when it can be better utilized elsewhere in the body. To see if you have the palmaris longus: Hold your arm out in front of you with the palm of your hand facing up. ‘Close’ the hand by bringing the thumb and pinky together. If you have the palmaris longus, you should see it raise up on the wrist.

3. Occipitalis Minor

These flat muscles at the bottom of the skull allow for a bit of movement of the scalp relative to the skull[2]. Think of pulling your ears back. Try to imagine a situation in which you could save your life with this movement or when it would impress during a date. Probably never. It does not add anything to survival or reproduction. However, in our evolutionary ancestors, it was useful if you could adjust your ears slightly to the sound of creeping predators wanting to pounce from behind. Nowadays, most predators come from the front, from a monitor or screen of a mobile phone. Thus, evolutionarily speaking, the usefulness of this muscle has disappeared. The muscle is connected via a connective tissue (aponeurosis) that runs over the skull to muscles on the forehead, which among other things raise the eyebrows. When you pull your ears back, you also tend to have a surprised look.

1. Thompson, N. W., B. J. Mockford, and G. W. Cran. “Absence of the Palmaris Longus Muscle: A Population Study.” The Ulster Medical Journal 70.1 (2001): 22–24. Print.
2.  Drake, Richard L.; Vogl, A. Wayne; Mitchell, Adam W. M. (2010). Gray´s Anatomy for Students (2nd ed.). p. 857. ISBN 978-0-443-06952-9.
3. Encyclopedia Britannica. Retrieved 2017-11-09.
4. Snosek, Michael; Tubbs, R. Shane; Loukas, Marios (2014-09-01). “Sternalis muscle, what every anatomist and clinician should know”. Clinical Anatomy. 27 (6): 866–884. doi:10.1002/ca.22361. ISSN 1098-2353.
5. Ge, Zufeng; Tong, Yunlong; Zhu, Shiqiang; Fang, Xiong; Zhuo, Lang; Gong, Xiangyang (2014-04-01). “Prevalence and variance of the sternalis muscle: a study in the Chinese population using multi-detector CT”. Surgical and radiologic anatomy: SRA. 36 (3): 219–224.
6. Raikos, Athanasios; Paraskevas, George K.; Yusuf, Faisal; Kordali, Panagiota; Ioannidis, Orestis; Brand-Saberi, Beate (2011-12-01). “Sternalis muscle: a new crossed subtype, classification, and surgical applications”. Annals of Plastic Surgery. 67 (6): 646–648.
7. Moore, Keith L; & Dalley Arthur R (2008). Clinically Oriented Anatomy (6th ed.). Lippincott Williams and Wilkins.
8. Voss, H., Tabelle der absoluten und relativen Muskelspindelzahlen der menschlichen Skelettmuskulatur, Anat. Anz. 129:562–572, 1971.
9. Mcg Farias (1 Jan. 2012). Morphological and morphometric analysis of Psoas Minor Muscle in cadavers.

Therefore, various peoples no longer have these muscles. In Europe, about a third of people lack these muscles, and in Japan, about half do.

4. Pyramidalis

A triangular muscle in the abdomen of which most people have two, while 20% of people do not have these muscles[3]. The muscle is located at the bottom of the abdomen near the pubic region. Its function is to tighten the linea alba, the beautiful line in the middle of the abdomen that separates the left abs from the right abs. Not that this really adds anything to the function of the linea alba, which is formed from connective tissue that holds the abdominal muscles together. Because the pyramidalis actually does very little, you cannot test if you have it. Only an MRI can determine that. After, for example, you, as a skater, decide to high five the rail with your balls while grinding and end up in the emergency room.

5. Sternalis

The most remarkable variation/mutation, in my opinion, is the sternalis. The sternalis can still have a significant influence, even though it is one of the ‘freely useless’ muscles not everyone has, especially if you are a bodybuilder, for example. The sternalis runs over the pectoralis, over the chest muscles. However, while the chest muscles run from the inside out, the sternalis runs from bottom to top as an extension of the abdominal muscles that continue over the chest muscle. In the drawing below, you can see the sternalis, although it may be a bit unclear due to the different layers of the drawing. The muscle occurs in various forms, ranging from a single muscle belly ending in a single head to multiple muscle bellies ending in multiple muscle heads [5]. You can probably imagine, however, that this extra muscle on the inside of the chest can change its appearance. Due to the location of the sternalis, the separation between both sides of the chest muscles can look very different. This can make the inside of the chest appear much more developed than normal because this part protrudes further forward from the sternum. After all, there is an extra muscle in front. Although it may look ‘better developed’, it also looks a bit freaky. It is questionable to what extent you can train the muscle given the unclear function [6]. Fun, though, if you ever have a discussion about whether or not you can train the inside of the chest and want to create more confusion. The sternalis occurs in almost 8% of the world’s population, making it ‘the most exclusive’ muscle in this list [4].

6. Psoas Minor

The last in the list is certainly not the least, given that this muscle was the reason for this article. This morning, I wrote an article about low back pain when running. New research identified weak deep core muscles as the cause. One of the mentioned deep core muscles was the psoas major. The psoas major runs from the lower vertebrae of the spine downwards and sideways to the pelvis. Some people also have a psoas minor in front of the psoas major. The psoas major plays a role in hip flexion while the psoas minor plays a role in flexing the bottom of the spine. However, the influence of the psoas minor is very small. The psoas minor is also quite exclusive, as the muscle is present in only 27% of the population [9].

References

1. Thompson, N. W., B. J. Mockford, and G. W. Cran. “Absence of the Palmaris Longus Muscle: A Population Study.” The Ulster Medical Journal 70.1 (2001): 22–24. Print.
2.  Drake, Richard L.; Vogl, A. Wayne; Mitchell, Adam W. M. (2010). Gray´s Anatomy for Students (2nd ed.). p. 857. ISBN 978-0-443-06952-9.
3. Encyclopedia Britannica. Retrieved 2017-11-09.
4. Snosek, Michael; Tubbs, R. Shane; Loukas, Marios (2014-09-01). “Sternalis muscle, what every anatomist and clinician should know”. Clinical Anatomy. 27 (6): 866–884. doi:10.1002/ca.22361. ISSN 1098-2353.
5. Ge, Zufeng; Tong, Yunlong; Zhu, Shiqiang; Fang, Xiong; Zhuo, Lang; Gong, Xiangyang (2014-04-01). “Prevalence and variance of the sternalis muscle: a study in the Chinese population using multi-detector CT”. Surgical and radiologic anatomy: SRA. 36 (3): 219–224.
6. Raikos, Athanasios; Paraskevas, George K.; Yusuf, Faisal; Kordali, Panagiota; Ioannidis, Orestis; Brand-Saberi, Beate (2011-12-01). “Sternalis muscle: a new crossed subtype, classification, and surgical applications”. Annals of Plastic Surgery. 67 (6): 646–648.
7. Moore, Keith L; & Dalley Arthur R (2008). Clinically Oriented Anatomy (6th ed.). Lippincott Williams and Wilkins.
8. Voss, H., Tabelle der absoluten und relativen Muskelspindelzahlen der menschlichen Skelettmuskulatur, Anat. Anz. 129:562–572, 1971.
9. Mcg Farias (1 Jan. 2012). Morphological and morphometric analysis of Psoas Minor Muscle in cadavers.