The Sternalis: The Top Contender
For anatomists and researchers, the sternalis muscle represents one of the most enigmatic and, by prevalence, rarest muscles in the human body. Located in the chest, it runs vertically along the side of the sternum, or breastbone, and lies on top of the pectoral muscles.
- Extremely Low Prevalence: Documented in only about 8% of the population, the sternalis muscle is exceptionally uncommon. Because of its rarity and position, it is often only discovered during mammograms, other imaging, or surgery.
- Unknown Function: The precise function of the sternalis remains a mystery. One theory suggests it is a remnant of the panniculus carnosus, a muscle layer found in many animals that allows them to twitch their skin. This vestigial status explains why its absence is not missed and has no functional impact on those without it.
Other Common Vestigial Muscles
While the sternalis is a strong candidate for the rarest, other muscles are frequently cited for their high degree of variability or absence in the population. These are largely considered vestigial, meaning they have lost their original function over the course of human evolution.
- Palmaris Longus: This slender forearm muscle, with its long tendon, is absent in approximately 15% of the population, with prevalence varying significantly by ethnic group. It is often removed for reconstructive surgery and has no noticeable effect on grip strength. It is arguably the "most variable" muscle due to its many possible forms and insertions.
- Pyramidalis: A small, triangular muscle in the lower abdomen that is absent in about 20% of people. Its only known function is to tense the linea alba, the connective tissue that runs down the middle of the abdomen, a task that has minimal impact.
- Plantaris: Located in the calf, this muscle has a short belly and a very long tendon. It is absent in about 7–10% of the population. While once thought to be purely vestigial, recent research suggests it may still play a minor role in sensorimotor feedback, contributing to some lower leg pathologies.
- Anconeus Epitrochlearis: This is not a separate muscle but a rare variation of the anconeus, a small elbow muscle. In approximately one-third of people, the anconeus is a continuation of the triceps. However, a rare variation, the anconeus epitrochlearis, can form and cross the ulnar nerve, sometimes causing pain and numbness known as cubital tunnel syndrome.
Comparison of Rare and Variable Muscles
| Feature | Sternalis | Palmaris Longus | Pyramidalis | Plantaris |
|---|---|---|---|---|
| Location | Chest, on pectoral muscles | Forearm, between wrist flexors | Lower abdomen, on rectus abdominis | Calf, posterior to knee |
| Prevalence (Approx.) | Present in ~8% | Absent in ~15%, but highly variable by ethnicity | Absent in ~20% | Absent in ~7-10% |
| Primary Function | Unclear; likely none in modern humans | Weak wrist flexion and tensing palmar fascia | Tenses the linea alba | Weak knee flexion; potential sensorimotor role |
| Vestigial Status | Highly probable | Yes, functionally redundant | Yes, functionally redundant | Once thought so, now debated |
| Clinical Relevance | Can be mistaken for a tumor or chest anomaly on imaging | Common source for tendon grafts | Often asymptomatic, but variations exist | May cause Achilles tendinopathy; potential donor site |
The Evolutionary Story Behind Anatomical Variations
These muscles tell an interesting story about human evolution. Structures like the palmaris longus and plantaris are often linked to our primate ancestors, who relied on strong grip and climbing abilities. As humans transitioned to bipedalism and lost the need for arboreal locomotion, these muscles became less crucial. Because they no longer provided a significant survival advantage, the selective pressure to maintain them was reduced, leading to their high variability and occasional absence in the population.
Anatomical variations like these are common and usually benign. However, in some cases, a muscular variation can cause problems. For example, a reversed palmaris longus or accessory slips of the plantaris can cause nerve compression or other syndromes. This is why awareness of these potential variants is important for medical professionals during imaging, diagnosis, and surgical procedures.
The Clinical Importance of Missing Muscles
While functionally insignificant, these muscles are far from irrelevant. For surgeons, particularly those specializing in hand and orthopedic procedures, the tendons of the palmaris longus and plantaris are prized resources for grafting. Since their removal has no negative functional consequence, their tendons can be used to repair or reconstruct damaged ligaments and tendons in other parts of the body.
In the case of the palmaris longus, a simple clinical test can determine its presence or absence. By touching the thumb to the little finger and flexing the wrist, an individual can see if the tendon visibly pops out. This simple maneuver is often a fun and easy way for people to engage with their own personal anatomy.
Conclusion
While there is no single, definitively “rarest” muscle that is consistent across all populations, the sternalis muscle stands out with the lowest reported rate of presence. However, other frequently absent or highly variable structures like the palmaris longus and pyramidalis also represent fascinating remnants of our evolutionary past. These vestigial muscles highlight the ongoing process of human evolution and serve important clinical purposes, such as providing donor tissue for reconstructive surgery. Ultimately, understanding these anatomical variations offers a deeper appreciation for the complex and diverse nature of the human body.
For more detailed information on anatomical variations, the Wikipedia article on Human Vestigiality is an excellent resource.
