Tibial torsion and pressures in the feet during walking: Implications for patterns of metatarsal robusticity.

OBJECTIVES: The Dmanisi Homo fossils include a tibia with a low degree of torsion and metatarsals with a pattern of robusticity differing from modern humans. It has been proposed that low tibial torsion would cause a low foot progression angle (FPA) in walking, and consequently increased force applied to the medial rays. This could explain the more robust MT III and IV from Dmanisi. Here we experimentally tested these hypothesized biomechanical relationships in living human subjects. MATERIALS AND METHODS: We measured transmalleolar axis (TMA, a proxy for tibial torsion), FPA, and plantar pressure distributions during walking in young men (n = 40). TMA was measured externally using a newly developed method. A pressure mat recorded FPA and pressure under the metatarsal heads (MT I vs. MT II-IV vs. MT V). RESULTS: TMA is positively correlated with FPA, but only in the right foot. Plantar pressure under MT II-IV does increase with lower TMA, as predicted, but FPA does not affect pressure. Body mass index also influenced plantar pressure distribution. DISCUSSION: Lower tibial torsion in humans is associated with slightly increased pressures along the middle rays of the foot during walking, but not because of changes in FPA. Therefore, it is possible that the low degree of torsion in the Dmanisi Homo tibia is related to the unusual pattern of robusticity in the associated metatarsals, but the mechanism behind this relationship is unclear. Future work will explore TMA, FPA, and plantar pressures during running.

Pelvic Breadth and Locomotor Kinematics in Human Evolution.

A broad pelvis is characteristic of most, if not all, pre-modern hominins. In at least some early australopithecines, most notably the female Australopithecus afarensis specimen known as "Lucy," it is very broad and coupled with very short lower limbs. In 1991, Rak suggested that Lucy's pelvic anatomy improved locomotor efficiency by increasing stride length through rotation of the wide pelvis in the axial plane. Compared to lengthening strides by increasing flexion and extension at the hips, this mechanism could avoid potentially costly excessive vertical oscillations of the body's center of mass (COM). Here, we test this hypothesis. We examined 3D kinematics of walking at various speeds in 26 adult subjects to address the following questions: Do individuals with wider pelves take longer strides, and do they use a smaller degree of hip flexion and extension? Is pelvic rotation greater in individuals with shorter legs, and those with narrower pelves? Our results support Rak's hypothesis. Subjects with wider pelves do take longer strides for a given velocity, and for a given stride length they flex and extend their hips less, suggesting a smoother pathway of the COM. Individuals with shorter legs do use more pelvic rotation when walking, but pelvic breadth was not related to pelvic rotation. These results suggest that a broad pelvis could benefit any bipedal hominin, but especially a short-legged australopithecine such as Lucy, by improving locomotor efficiency, particularly when carrying an infant or traveling in a foraging group with individuals of varying sizes. Anat Rec, 300:739-751, 2017. © 2017 Wiley Periodicals, Inc.

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation.

The fossil record of the human pelvis reveals the selective priorities acting on hominin anatomy at different points in our evolutionary history, during which mechanical requirements for locomotion, childbirth and thermoregulation often conflicted. In our earliest upright ancestors, fundamental alterations of the pelvis compared with non-human primates facilitated bipedal walking. Further changes early in hominin evolution produced a platypelloid birth canal in a pelvis that was wide overall, with flaring ilia. This pelvic form was maintained over 3-4 Myr with only moderate changes in response to greater habitat diversity, changes in locomotor behaviour and increases in brain size. It was not until Homo sapiens evolved in Africa and the Middle East 200 000 years ago that the narrow anatomically modern pelvis with a more circular birth canal emerged. This major change appears to reflect selective pressures for further increases in neonatal brain size and for a narrow body shape associated with heat dissipation in warm environments. The advent of the modern birth canal, the shape and alignment of which require fetal rotation during birth, allowed the earliest members of our species to deal obstetrically with increases in encephalization while maintaining a narrow body to meet thermoregulatory demands and enhance locomotor performance.

Limb length and locomotor biomechanics in the genus Homo: an experimental study.

The striking variation in limb proportions within the genus Homo during the Pleistocene has important implications for understanding biomechanics in the later evolution of human bipedalism, because longer limbs and limb segments may increase bending moments about bones and joints. This research tested the hypothesis that long lower limbs and tibiae bring about increases in A-P bending forces on the lower limb during the stance phase of human walking. High-speed 3-D video data, force plates, and motion analysis software were used to analyze the walking gait of 27 modern human subjects. Limb length, as well as absolute and relative tibia length, were tested for associations with a number of kinetic and kinematic variables. Results show that individuals with longer limbs do incur greater bending moments along the lower limb during the first half of stance phase. During the second half of stance, individuals moderate bending moments through a complex of compensatory mechanisms, including keeping the knee in a more extended position. Neither absolute nor relative tibia length had any effect on the kinetic or kinematic variables tested. If these patterns apply to fossil Homo, groups with relatively long limbs (e.g. H. ergaster or early H. sapiens) may have experienced elevated bending forces along the lower limb during walking compared to those with relatively shorter limbs (e.g. the Neandertals). These increased forces could have led to greater reinforcement of joints and diaphyses. These results must be considered when formulating explanations for variation in limb morphology among Pleistocene hominins.