Stiffness of the human foot and evolution of the transverse arch

The stiff human foot enables an efficient push-off when walking or running, and was critical for the evolution of bipedalism(1-6). The uniquely arched morphology of the human midfoot is thought to stiffen it(5-9), whereas other primates have flat feet that bend severely in the midfoot(7,10,11). However, the relationship between midfoot geometry and stiffness remains debated in foot biomechanics(12,13), podiatry(14,15) and palaeontology(4-6). These debates centre on the medial longitudinal arch(5,6) and have not considered whether stiffness is affected by the second, transverse tarsal arch of the human foot(16). Here we show that the transverse tarsal arch, acting through the inter-metatarsal tissues, is responsible for more than 40% of the longitudinal stiffness of the foot. The underlying principle resembles a floppy currency note that stiffens considerably when it curls transversally. We derive a dimensionless curvature parameter that governs the stiffness contribution of the transverse tarsal arch, demonstrate its predictive power using mechanical models of the foot and find its skeletal correlate in hominin feet. In the foot, the material properties of the inter-metatarsal tissues and the mobility of the metatarsals may additionally influence the longitudinal stiffness of the foot and thus the curvature-stiffness relationship of the transverse tarsal arch. By analysing fossils, we track the evolution of the curvature parameter among extinct hominins and show that a human-like transverse arch was a key step in the evolution of human bipedalism that predates the genus Homo by at least 1.5 million years. This renewed understanding of the foot may improve the clinical treatment of flatfoot disorders, the design of robotic feet and the study of foot function in locomotion

Defining Polyamory: A Thematic Analysis of Lay People’s Definitions

This study aimed to analyze laypeople’s definitions of polyamory and compare definitions presented by people who are not willing to engage in consensual non-monogamy (CNM) and those who are or are willing to be in a CNM relationship. This exploratory qualitative study used data collected from a convenience sample through a web survey, where people answered the question “What does polyamory mean?” We conducted thematic analysis to examine patterns in meaning and used demographic data to compare themes among groups. The final sample comprised 463 participants aged 18–66 years (M = 32.19, SD = 10.02), mostly heterosexual (60%). Of the total sample, 54% were in a monogamous relationship, followed by 21% not in a relationship, and 13% in a non-monogamous relationship. Analysis showed that people define polyamory mostly as a set of behaviors in a relationship, followed by the potential of multiple relationships or feelings for multiple people. Definitions also include emotional, sexual, and ethical aspects. People in CNM relationships are more likely to define polyamory as constituting a potential form of relating, focus more on interpersonal feelings and ethics, and include consent in their definitions than those unwilling to engage in CNM. People in CNM relationships also focus particularly on the non-central role of sex within these relationships, which might challenge assumptions about sexuality in these relationships in clinical and research settings