Walking, running and the evolution of short toes in humans

Abstract

SUMMARYThe phalangeal portion of the forefoot is extremely short relative to body mass in humans. This derived pedal proportion is thought to have evolved in the context of committed bipedalism, but the benefits of shorter toes for walking and/or running have not been tested previously. Here, we propose a biomechanical model of toe function in bipedal locomotion that suggests that shorter pedal phalanges improve locomotor performance by decreasing digital flexor force production and mechanical work, which might ultimately reduce the metabolic cost of flexor force production during bipedal locomotion. We tested this model using kinematic, force and plantar pressure data collected from a human sample representing normal variation in toe length (N=25). The effect of toe length on peak digital flexor forces, impulses and work outputs was evaluated during barefoot walking and running using partial correlations and multiple regression analysis, controlling for the effects of body mass, whole-foot and phalangeal contact times and toe-out angle. Our results suggest that there is no significant increase in digital flexor output associated with longer toes in walking. In running, however, multiple regression analyses based on the sample suggest that increasing average relative toe length by as little as 20% doubles peak digital flexor impulses and mechanical work, probably also increasing the metabolic cost of generating these forces. The increased mechanical cost associated with long toes in running suggests that modern human forefoot proportions might have been selected for in the context of the evolution of endurance running.

Locomotor anatomy and biomechanics of the Dmanisi hominins

“…For the early modern human (EMH) and Neanderthal samples, body masses were taken from Ruff et al (1997) while femur and tibia lengths were taken from Trinkaus (1981). The Neanderthal sample consists of nine individuals (Amud 1, La Chappelle 1, La Ferrassie 2 and 4, Spy 2, Shanidar 1, 5 and 6, and Tabun C1), while the EMH sample consists of 12 (Grotto des Enfants 4 and 5, Predmosti 3, 4, 9, 10 and 14, Skhul 4, 5 and 6, Caviglione 1, Baousso da Torre 1).…”

“…The Neanderthal sample consists of nine individuals (Amud 1, La Chappelle 1, La Ferrassie 2 and 4, Spy 2, Shanidar 1, 5 and 6, and Tabun C1), while the EMH sample consists of 12 (Grotto des Enfants 4 and 5, Predmosti 3, 4, 9, 10 and 14, Skhul 4, 5 and 6, Caviglione 1, Baousso da Torre 1). For the KNM-WT 15,000 skeleton, a body mass of 51 kg was used, following a recent study of body mass estimation in juvenile humans (Ruff, 2007).…”

“…The shorter toes of Homo decrease the digital flexor muscle work required during the second half of the stance phase (Rolian et al, 2009) and should, in principle, decrease the energy needed to swing the leg by decreasing the mass of the foot (Myers and Steudel, 1985;Browning et al, 2007). Other features, such as the fully adducted and more robust first ray, larger articular surfaces in the hind limb, derived anatomy of the gluteus maximus muscle, and enlarged calcaneal tuberosity evident in Homo may also improve walking and running endurance and economy (Bramble and Lieberman, 2004;Lieberman et al, 2006).…”

“…For the Dmanisi sample, linear measurements of the tarsals and metatarsals were taken using standard digital calipers. In the extant sample, metatarsal linear measurements were collected from digital scans taken on a flatbed scanner at a resolution of 300 dpi (MicroTek i320 ScanMaker, Carson, CA, see Rolian et al, 2009 for details). The scanner method has been validated in several morphometric studies (e.g., Hallgrimsson et al, 2002;Young and Hallgrimsson, 2005), and provides several advantages over camera or caliper based methods, including reduced measurement error and parallax.…”

The role of plantigrady and heel-strike in the mechanics and energetics of human walking with implications for the evolution of the human foot

“…While the feet of early bipeds lack derived features seen in the genus Homo that allow for effective endurance running, they may have been well adapted to HS walking (Bramble and Lieberman, 2004;Spoor et al, 1994). For example, the relatively short toes, and therefore shorter feet, seen in modern humans are advantageous to endurance running because they reduce plantar-flexor moments at the metatarso-phalangeal joints (Rolian et al, 2009) and probably evolved with the origins of the Homo genus. Australopithecus, a hominin genus that preceded Homo, had relatively long feet, which Rolian et al (2009) hypothesize would have detracted from running performance.…”