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

Abstract

Human bipedal locomotion is characterized by a habitual heel-strike (HS) plantigrade gait, yet the significance of walking foot-posture is not well understood. To date, researchers have not fully investigated the costs of non-heel-strike (NHS) walking. Therefore, we examined walking speed, walk-to-run transition speed, estimated locomotor costs (lower limb muscle volume activated during walking), impact transient (rapid increase in ground force at touchdown) and effective limb length (ELL) in subjects (n=14) who walked at self-selected speeds using HS and NHS gaits. HS walking increases ELL compared with NHS walking since the center of pressure translates anteriorly from heel touchdown to toe-off. NHS gaits led to decreased absolute walking speeds (P=0.012) and walk-to-run transition speeds (P=0.0025), and increased estimated locomotor energy costs (P<0.0001) compared with HS gaits. These differences lost significance after using the dynamic similarity hypothesis to account for the effects of foot landing posture on ELL. Thus, reduced locomotor costs and increased maximum walking speeds in HS gaits are linked to the increased ELL compared with NHS gaits. However, HS walking significantly increases impact transient values at all speeds (P<0.0001). These trade-offs may be key to understanding the functional benefits of HS walking. Given the current debate over the locomotor mechanics of early hominins and the range of foot landing postures used by nonhuman apes, we suggest the consistent use of HS gaits provides key locomotor advantages to striding bipeds and may have appeared early in hominin evolution.

Rethinking the evolution of the human foot: insights from experimental research

“…This strategy distinguishes humans from other terrestrial bipeds (Usherwood et al, 2012), and may partly account for the relatively low metabolic cost of walking in humans (Biewener et al, 2004;Cunningham et al, 2010). Additionally, Webber and Raichlen (2016) demonstrated that use of a heel strike at the beginning of stance during plantigrade walking allows the center of pressure to roll forward under the foot during stance phase, effectively increasing stride length and improving human walking economy. Because great apes use plantigrade foot postures, we hypothesize that they also benefit from the advantages described above during bipedalism.…”

“…The foot also displays a fully adducted hallux, but its first metatarsal is relatively short, and its cuboid and navicular bones are shaped similarly to those of African apes, suggesting a longitudinal arch that was only weakly developed or absent (Jungers et al, 2009b). These features do not preclude effective bipedal walking, as a long foot should reduce metabolic costs assuming the use of heel-strike plantigrady (Cunningham et al, 2010;Webber and Raichlen, 2016), and alternative mechanisms could have been utilized to stiffen the foot even in the absence of a longitudinal arch (Bates et al, 2013;Venkadesan et al, 2017 preprint). However, lacking a human-like longitudinal arch, H. floresiensis would have been unable to take advantage of significant elastic energy savings in its foot (Stearne et al, 2016), and its long toes would have required high muscle force production during running (Rolian et al, 2009).…”

“…Recently, Webber and Raichlen (2016) demonstrated that humans can reduce or avoid impact forces during walking by first contacting the ground with the forefoot. Additionally, by using digitigrade postures during walking humans could increase the length of the lower limb from hip to point of ground contact, which would theoretically increase the distance traveled per stride.…”

“…However, because great apes use a variety of foot strike postures (Schmitt and Larson, 1995;Vereecke et al, 2003), we would predict that they have the longest effective stride lengths and lowest collisional costs when landing with a heel strike. Whether great ape heel strikes generate higher impact forces than other foot strike postures, as in humans (Webber and Raichlen, 2016), remains to be tested.…”

“…Assuming A. ramidus used plantigrade walking postures, an elongated midfoot would have increased the overall length of the foot, and therefore the distance traveled by the center of pressure under the foot over stance phase. This would have effectively reduced collisional energy loss (Adamczyk and Kuo, 2013) and increased the effective length of lower limb to improve walking economy (Webber and Raichlen, 2016). Furthermore, dorsal doming of the third metatarsal head suggests that the lateral (non-hallux) metatarsophalangeal joints were loaded in highly dorsiflexed joint postures during bipedal walking (Lovejoy et al, 2009;Fernández et al, 2018).…”