Evolutionary anatomy of the plantar aponeurosis in primates, including humans

Abstract

The plantar aponeurosis in the human foot has been extensively studied and thoroughly described, in part, because of the incidence of plantar fasciitis in humans. It is commonly assumed that the human plantar aponeurosis is a unique adaptation to bipedalism that evolved in concert with the longitudinal arch. However, the comparative anatomy of the plantar aponeurosis is poorly known in most mammals, even among non‐human primates, hindering efforts to understand its function. Here, we review previous anatomical descriptions of 40 primate species and use phylogenetic comparative methods to reconstruct the evolution of the plantar aponeurosis and its relationship to the plantaris muscle in primates. Ancestral state reconstructions suggest that the overall organization of the human plantar aponeurosis is shared with chimpanzees and that a similar anatomical configuration evolved independently in different primate clades as an adaptation to terrestrial locomotion. The presence of a plantar aponeurosis with clearly developed lateral and central bands in the African apes suggests that this structure is not prohibitive to suspensory locomotion and that these species possess versatile feet adapted for both terrestrial and arboreal locomotion. This plantar aponeurosis configuration would have been advantageous in enhancing foot stiffness for bipedal locomotion in the earliest hominins, prior to the evolution of a longitudinal arch. Hominins may have subsequently evolved thicker and stiffer plantar aponeuroses alongside the arch to enable a windlass mechanism and elastic energy storage for bipedal walking and running, although this idea requires further testing.

Effect of the upward curvature of toe springs on walking biomechanics in humans

“…Although recent research shows that transverse splaying of the metatarsal heads helps stiffen the midfoot via the transverse tarsal arch 11 , it has long been argued that dorsiflexion at the MTP joints also helps stiffen the foot through a windlass mechanism 12 . During this action, dorsiflexion of the toes tightens the plantar aponeurosis, a broad sheet of highly fibrous tissue whose collagen fibers span the plantar aspect of the foot from the heel to the toes (for review see 13 ). The increased tension on the plantar aponeurosis pulls the calcaneus and metatarsal heads towards each other, creating an upward force that elevates the longitudinal arch, counters compressive forces from above, and stiffens the foot as a whole (Fig.…”

An ossifying bridge – on the structural continuity between the Achilles tendon and the plantar fascia

“…Using larger sample sizes the here stated site-dependent biomechanical mapping of the human AT-calcaneus-PF complex can be applied to investigate the biomechanical properties across different species. This might provide further insights into the evolutionary anatomy of these structures 8 . This given study provides evidence for a structural continuity of the AT and the PF via the posterior calcaneal trabeculae.…”

“…This given study provides evidence for a structural continuity of the AT and the PF via the posterior calcaneal trabeculae. It should be noted that the plantaris longus muscle has been shown to be de-coupled from the PF in humans and, therefore, lost its potential to strengthen the former 8 . This has been achieved by a firm attachment of the plantaris muscle tendon to the calcaneal periosteum 8 .…”

“…It should be noted that the plantaris longus muscle has been shown to be de-coupled from the PF in humans and, therefore, lost its potential to strengthen the former 8 . This has been achieved by a firm attachment of the plantaris muscle tendon to the calcaneal periosteum 8 . The former concept of immobilizing the spatially closely related plantaris muscle tendon appears to be similar for the AT-calcaneus-PF complex discussed in this given study.…”

“…Supporting the latter, it was shown that AT and PF are connected by thick collagen fibres in neonates, which quantitatively diminished during life and could not be detected in late(r) adulthood as they were likely integrated into the bony calcaneus 7 . Integration of the formerly continuous tendon into the posterior calcaneus, which then further subdivides into the distal PF and the proximal AT likely allows for a more efficient energy use during push-off movements of the foot and shows an adaption to bipedalism 8 . The calcaneal periosteum, which receives fibres from both AT and PF is only vaguely accepted as a continuous morphological connection between the two structures 7 , 9 – 11 , certainly with insignificant load-bearing characteristics.…”