Body mass estimation from footprint size in hominins.

Although many studies relating stature to foot length have been carried out, the relationship between foot size and body mass remains poorly understood. Here we investigate this relationship in 193 adult and 50 juvenile habitually unshod/minimally shod individuals from five different populations-Machiguenga, Daasanach, Pumé, Hadzabe, and Samoans-varying greatly in body size and shape. Body mass is highly correlated with foot size, and can be predicted from foot area (maximum length × breadth) in the combined sample with an average error of about 10%. However, comparisons among populations indicate that body shape, as represented by the body mass index (BMI), has a significant effect on foot size proportions, with higher BMI samples exhibiting relatively smaller feet. Thus, we also derive equations for estimating body mass from both foot size and BMI, with BMI in footprint samples taken as an average value for a taxon or population, estimated independently from skeletal remains. Techniques are also developed for estimating body mass in juveniles, who have relatively larger feet than adults, and for converting between foot and footprint size. Sample applications are given for five Pliocene through Holocene hominin footprint samples from Laetoli (Australopithecus afarensis), Ileret (probable Homo erectus), Happisburgh (possible Homo antecessor), Le Rozel (archaic Homo sapiens), and Barcin Höyük (H. sapiens). Body mass estimates for Homo footprint samples appear reasonable when compared to skeletal estimates for related samples. However, estimates for the Laetoli footprint sample using the new formulae appear to be too high when compared to skeletal estimates for A. afarensis. Based on the proportions of A.L. 288-1, this is apparently a result of relatively large feet in this taxon. A different method using a ratio between body mass and foot area in A.L. 288-1 provides estimates more concordant with skeletal estimates and should be used for A. afarensis

Variation in foot strike patterns during running among habitually barefoot populations.

Endurance running may have a long evolutionary history in the hominin clade but it was not until very recently that humans ran wearing shoes. Research on modern habitually unshod runners has suggested that they utilize a different biomechanical strategy than runners who wear shoes, namely that barefoot runners typically use a forefoot strike in order to avoid generating the high impact forces that would be experienced if they were to strike the ground with their heels first. This finding suggests that our habitually unshod ancestors may have run in a similar way. However, this research was conducted on a single population and we know little about variation in running form among habitually barefoot people, including the effects of running speed, which has been shown to affect strike patterns in shod runners. Here, we present the results of our investigation into the selection of running foot strike patterns among another modern habitually unshod group, the Daasanach of northern Kenya. Data were collected from 38 consenting adults as they ran along a trackway with a plantar pressure pad placed midway along its length. Subjects ran at self-selected endurance running and sprinting speeds. Our data support the hypothesis that a forefoot strike reduces the magnitude of impact loading, but the majority of subjects instead used a rearfoot strike at endurance running speeds. Their percentages of midfoot and forefoot strikes increased significantly with speed. These results indicate that not all habitually barefoot people prefer running with a forefoot strike, and suggest that other factors such as running speed, training level, substrate mechanical properties, running distance, and running frequency, influence the selection of foot strike patterns

Frequencies of foot strike patterns across running speeds.

<p>Frequencies of foot strike patterns across running speeds.</p

Least-squares regression of impact force by running speed.

<p>A significant relationship was found between the relative magnitude of impact forces and running speed (y = 0.38+0.10×; r² = 0.20, p<0.0001).</p

Mean residuals (and 95% confidence limits) from regression of relative impact force by running speed.

<p>Mean residuals (and 95% confidence limits) from regression of relative impact force by running speed.</p

Foot strike patterns of Daasanach (N = 38) over 133 trials at preferred endurance running speeds.

*<p>average based on data from 18 subjects; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052548#s4" target="_blank">Materials and Methods</a>.</p

Close-up images of subjects using a rearfoot strike (A) and a midfoot strike (B).

<p>Most Daasanach subjects used a rearfoot strike (A) at their self-selected endurance running speeds, rather than a midfoot strike (B) or a forefoot strike (not shown).</p

Frequencies of strike patterns by running speed.

<p>The majority of the Daasanach sample group used a RFS (black circles) when running at Froude speeds less than 2.5. For Froude speeds greater than 2.5, RFS and MFS (white circles) running were observed with equal frequencies. FFS running (black triangles) was most frequently observed at the highest speeds (Froude >3.5), but this pattern was never used by the majority of the Daasanach sample group.</p

Digit clearance patterns in primates vary by limb and substrate reflecting different strategies between arboreal and terrestrial locomotion

During swing phase of walking, animals are faced with the challenge of keeping digits clear from the substrate. On the ground, animals can flex or abduct the limbs, strategies that shorten effective limb length during the swing phase and require muscular effort. On a branch or pole, however, primates can allow their long limbs to swing below the substrate, lengthening the swinging limb and increasing limb protraction, while allowing for flexion and compliance of the supporting limb. This pattern was suggested in several early studies carried out at the SBU Locomotion Lab, but was never tested. We examine digit clearance patterns in 24 primate species, including chimpanzees and gorillas, and10 arboreal and terrestrial non-primate mammal species. On poles, almost all species swing their forelimb slightly to the side of the support and allow the hand to drop alongside or below the level of the pole. However, the swinging hind-limb is relatively more abducted and flexed and the foot remains most often at or above the level of the pole. On the ground, animals often use abducted limbs, especially great apes, and flexed limb joints to provide ample clearance of the hand and foot. Hindlimb similarities between ground and pole suggest that hindlimb motion may be constrained by the risk of interfering with the supporting hand. Shifts in forelimb behavior from ground to pole, along with other changes in forelimb function may represent a key mechanical adaptation for arboreal locomotion, providing a wide swing arc with limited muscular effort