Midtarsal break variation in modern humans: Functional causes, skeletal correlates, and paleontological implications

Abstract

The midtarsal break was once treated as a dichotomous, non-overlapping trait present in the foot of non-human primates and absent in humans. Recent work indicates that there is considerable variation in human midfoot dorsiflexion, with some overlap with the ape foot. These findings have called into question the uniqueness of the human lateral midfoot, and the use of osteological features in fossil hominins to characterize the midfoot of our extinct ancestors. Here, we present data on plantar pressure and pedal mechanics in a large sample of adults and children (n = 671) to test functional hypotheses concerning variation in midfoot flexibility. Lateral midfoot peak plantar pressure correlates with both sagittal plane flexion at the lateral tarsometatarsal joint, and dorsiflexion at the hallucal metatarsophalangeal joint. The latter finding suggests that midfoot laxity may compromise hallucal propulsion. Multiple regression statistics indicate that a low arch and pronation of the foot explain 40% of variation in midfoot peak plantar pressure, independent of age and BMI. MRI scans on a small subset of study participants (n = 19) reveals that curvature of the base of the 4th metatarsal correlates with lateral midfoot plantar pressure and that specific anatomies of foot bones do indeed reflect relative midfoot flexibility. However, while the shape of the base of the 4th metatarsal may reliably reflect midfoot mobility in individual hominins, given the wide range of overlapping variation in midfoot flexibility in both apes and humans, we caution against generalizing foot function in extinct hominin species until larger fossils samples are available.

Reevaluating the functional implications of Australopithecus afarensis navicular morphology

“…The fossilized footprints at Laetoli, Tanzania, display modern human-like characteristics, such as a relatively adducted hallux compared to chimpanzees, a deep impression associated with heel-strike, and a medial longitudinal arch (Leakey and Hay, 1979;Day and Wickens, 1980;White, 1980;White and Suwa, 1987;Bennett et al, 2009;Raichlen et al, 2010;Crompton et al, 2012;Bennett et al, 2016). However, more recent quantitative studies suggest the Laetoli footprints show less medial weight transfer than would be found in modern humans (Bennett et al, 2009(Bennett et al, , 2016Crompton et al, 2012;see also;DeSilva and Gill, 2013;DeSilva et al, 2015) and a longitudinal arch that was probably not as well developed as the modern human arch (Bennett et al, 2009(Bennett et al, , 2016). The reduction in medial weight transfer Figure 6.…”

“…The ape foot lacks these features of the tarsometatarsal, calcaneocuboid, and talonavicular joints resulting in midtarsal mobility during midstance, a kinematic pattern that has been termed the 'midtarsal break' (Elftman and Manter, 1935;Vereecke et al, 2003;DeSilva, 2010;Thompson et al, 2014). It must be noted that modern humans with an arched foot can still display some midfoot mobility (Lundgren et al, 2008;Bates et al, 2013;DeSilva and Gill, 2013;DeSilva et al, 2015) and fossil hominins with a flat foot can have a rigid midfoot (Prang, 2015b).…”

The association of waist circumference with walking difficulty among adults with or at risk of knee osteoarthritis: the Osteoarthritis Initiative

“…Most biomechanical research on gait and obesity finds that obesity is associated with impaired gait and subsequent limitation in function 2–4,7,41,42 . Specifically, compared to normal weight adults without knee OA, adults with obese BMI walk more slowly with decreased walking velocities, expend more energy and relative effort when walking 7 , and use more mechanical external work 12 .…”

Functional aspects of metatarsal head shape in humans, apes, and Old World monkeys

“…What remains clear, however, is that modern human feet are well adapted for obligate bipedal locomotion. In combination, several derived features (e.g., an adducted, robust hallux; a midfoot stabilized by various soft tissues) enable the human foot to act efficiently as a stiff lever during the propulsive phases of bipedal gait (Elftman and Manter, 1935;Hicks, 1954;Morton, 1964;Lewis, 1980;Gomberg, 1985;Sarrafian, 1987;Harcourt-Smith and Aiello, 2004; but see Bates et al, 2013;DeSilva and Gill, 2013;DeSilva et al, 2015). These derived features presumably evolved as a consequence of selective pressures acting on the hominin foot during its transition from a primitive grasping organ used in arboreal locomotion (Stern and Susman, 1983;Susman et al, 1984;Stern, 2000;Lovejoy et al, 2009; but see Ward, 2002) to one used during habitual terrestrial bipedalism (Susman and Stern, 1982;Spoor et al, 1994;Ruff, 2008Ruff, , 2009Pontzer et al, 2010).…”