Which wrist flexor is the most phenotypically variable?

Question

Though a short question, there are many clues to extract.

1. Wrist flexors: narrow your anatomy to the muscles at the forearm and specifically those that cross the wrist joint as this question involves wrist flexion
2. Genetics: the phenotype refers to what is physically present regardless of the genotype, or genetic makeup. Sure, we may have a gene for different hair or eye color, but the phenotype is the actual color that is expressed.
3. Variability: there are many muscles in the forearm, but only a few that cross the wrist joint to flex the wrist. Of the wrist flexors, which do you expect to be anatomically consistent versus which do you believe may have more anatomical variance?

Answer 1

The answer is: Palmaris longus wrist muscle

Some people do not have the palmaris longus muscle. This muscle is absent in around 10-30% of the population, either bilaterally or unilaterally.

Despite its absence, there is no functional impact, as other wrist flexors compensate for its role. Therefore, it is considered a vestigial muscle and less crucial for modern humans.

There is also variability in its structure by way of differences in size, shape and insertion points. Some people have accessory (extra) palmaris longus, while others have differences in tendon length.

Answer 2

The answer: palmaris longus.

The palmaris longus is a wrist flexor as its tendon crosses the wrist joint allow its flexion to decrease the wrist angle. The palmaris longus is a small muscle and is usually discussed with the caveat that it has a high anatomical variance where it may be completely absent in some individuals which is the most common variant, or may have an extra muscle belly.

To help narrow down anatomy questions involving muscles, it may be most helpful to group muscles based on their function, such as wrist flexors in this case, to help quickly narrow down possible answers.