The Plantaris Muscle Is Not Vestigial: Developmental, Comparative, and Functional Evidence for Its Sensorimotor Role

The functional status of the plantaris muscle (PM) remains controversial and is historically dismissed as vestigial; yet, it is increasingly recognized for its structural and clinical complexity. This narrative review synthesizes current evidence from embryological development, adult morphological studies, comparative mammalian anatomy, and clinical case reports to reassess the role of the PM in humans. Developmental data reveal that the PM is consistently present during fetal life, with tendon morphology and insertion patterns emerging early and resembling adult anatomical variants. Rather than indicating postnatal regression, it suggests a stable polymorphism rooted in prenatal development. Across mammalian species, the PM varies in presence and function, correlating with locomotor specialization from proprioception in primates to propulsion in carnivores, and absence in ungulates. In humans, high proprioceptive fiber density and anatomical variability support the hypothesis that the PM may be undergoing functional repurposing from a contractile to a sensorimotor role. Clinically, its relevance is evident in imaging interpretation, surgical tendon harvesting, and the pathophysiology of Achilles tendinopathy. Recent discoveries, including the identification of the plantaris ligamentous tendon (PLT), further underscore the complexity of this region and support the need to reassess its structural and clinical significance. We conclude that the PM should not be regarded as a regressing remnant but as a dynamically adapting structure with potential neuromechanical function. Future studies involving electromyography and neuroanatomical mapping are essential to elucidate its evolving role.