Cancellous bone and theropod dinosaur locomotion. Part III—Inferring posture and locomotor biomechanics in extinct theropods, and its evolution on the line to birds

This paper is the last of a three-part series that investigates the architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is highly sensitive to its prevailing mechanical environment, and may therefore help further understanding of locomotor biomechanics in extinct tetrapod vertebrates such as dinosaurs. Here in Part III, the biomechanical modelling approach derived previously was applied to two species of extinct, non-avian theropods, Daspletosaurus torosus and Troodon formosus. Observed cancellous bone architectural patterns were linked with quasi-static, three-dimensional musculoskeletal and finite element models of the hindlimb of both species, and used to derive characteristic postures that best aligned continuum-level principal stresses with cancellous bone fabric. The posture identified for Daspletosaurus was largely upright, with a subvertical femoral orientation, whilst that identified for Troodon was more crouched, but not to the degree observed in extant birds. In addition to providing new insight on posture and limb articulation, this study also tested previous hypotheses of limb bone loading mechanics and muscular control strategies in non-avian theropods, and how these aspects evolved on the line to birds. The results support the hypothesis that an upright femoral posture is correlated with bending-dominant bone loading and abduction-based muscular support of the hip, whereas a crouched femoral posture is correlated with torsion-dominant bone loading and long-axis rotation-based muscular support. Moreover, the results of this study also support the inference that hindlimb posture, bone loading mechanics and muscular support strategies evolved in a gradual fashion along the line to extant birds

A Guideline for Humanoid Leg Design with Oblique Axes for Bipedal Locomotion

The kinematics of humanoid robots are strongly inspired by the human archetype. A close analysis of the kinematics of the human musculoskeletal system reveals that the human joint axes are oriented within certain inclinations. This is in contrast to the most popular humanoid design with a configuration based on perpendicular joint axes. This paper reviews the oblique joint axes of the mainly involved joints for locomotion of the human musculoskeletal system. We elaborate on how the oblique axes affect the performance of walking and running. The mechanisms are put into perspective for the locomotion types of walking and running. In particular, walking robots can highly benefit from using oblique joint axes. For running, the primary goal is to align the axis of motion to the mainly active sagittal plane. The results of this analysis can serve as a guideline for the kinematic design of a humanoid robot and a prior for optimization-based approaches

A comparative analysis of primate first metatarsals : implications for Ardipithecus ramidus

Advisors: Daniel Gebo.Committee members: Leila Porter; Karen Samonds.Ardipithecus ramidus is a controversial fossil in terms of its phyletic position relative to the hominid lineage. Lovejoy and White argue that Ar. ramidus is a stem hominid while others, like Wood, Harrison and Sarmiento do not agree and propose alternative interpretations. These later authors argue that the proposed "human-like" characteristics used by Lovejoy and White to support their stem hominid hypothesis can also be attributed to other lineages, like fossil apes, and they further believe that Ar. ramidus might not even be a hominid at all. Given these alternative interpretations concerning Ardipithecus, the first metatarsal of Ar. ramidus was examined relative to early fossil humans such as O.H. 8 (Homo habilis), A.L. 333-54 (Australopithcus afarensis), A.L. 333-115 (A afarensis), the great apes, gibbons, and finally to modern humans to help inform the debate on the inferred locomotive strategies and phyletic placement of Ar. ramidus . A comparative anatomical approach was utilized to assess the morphological ratios of these taxa relative to each other. The eleven measurements of the first metatarsal included aspects of the shaft, the distal articular surface, and the proximal articular surface. The results show that Ar. ramidus has more features in common with non-human primates than to modern humans and does not exhibit any of the unique first metatarsal characteristics linked to modern humans and bipedality. Ar. ramidus shows a mosaic of first metatarsal characteristics in comparison to the ape species examined here. Human-like bipedality was unlikely to have been the main form of locomotion of Ar. ramidus and this study suggests that Ar. ramidus is not the best representation for the last common ancestor of chimpanzees and modern humans.M.A. (Master of Arts

Investigating the Performance of Soft Robotic Adaptive Feet with Longitudinal and Transverse Arches

Biped robots usually adopt feet with a rigid structure that simplifies walking on flat grounds and yet hinders ground adaptation in unstructured environments, thus jeopardizing stability. We recently explored in the SoftFoot the idea of adapting a robotic foot to ground irregularities along the sagittal plane. Building on the previous results, we propose in this paper a novel robotic foot able to adapt both in the sagittal and frontal planes, similarly to the human foot. It features five parallel modules with intrinsic longitudinal adaptability that can be combined in many possible designs through optional rigid or elastic connections. By following a methodological design approach, we narrow down the design space to five candidate foot designs and implement them on a modular system. Prototypes are tested experimentally via controlled application of force, through a robotic arm, onto a sensorized plate endowed with different obstacles. Their performance is compared, using also a rigid foot and the previous SoftFoot as a baseline. Analysis of footprint stability shows that the introduction of the transverse arch, by elastically connecting the five parallel modules, is advantageous for obstacle negotiation, especially when obstacles are located under the forefoot. In addition to biped robots' locomotion, this finding might also benefit lower-limb prostheses design.Comment: Submitted to Frontiers in Robotics and A

3D hindlimb joint mobility of the stem-archosaur Euparkeria capensis with implications for postural evolution within Archosauria.

Triassic archosaurs and stem-archosaurs show a remarkable disparity in their ankle and pelvis morphologies. However, the implications of these different morphologies for specific functions are still poorly understood. Here, we present the first quantitative analysis into the locomotor abilities of a stem-archosaur applying 3D modelling techniques. μCT scans of multiple specimens of Euparkeria capensis enabled the reconstruction and three-dimensional articulation of the hindlimb. The joint mobility of the hindlimb was quantified in 3D to address previous qualitative hypotheses regarding the stance of Euparkeria. Our range of motion analysis implies the potential for an erect posture, consistent with the hip morphology, allowing the femur to be fully adducted to position the feet beneath the body. A fully sprawling pose appears unlikely but a wide range of hip abduction remained feasible-the hip appears quite mobile. The oblique mesotarsal ankle joint in Euparkeria implies, however, a more abducted hindlimb. This is consistent with a mosaic of ancestral and derived osteological characters in the hindlimb, and might suggest a moderately adducted posture for Euparkeria. Our results support a single origin of a pillar-erect hip morphology, ancestral to Eucrocopoda that preceded later development of a hinge-like ankle joint and a more erect hindlimb posture

Analytic and Learned Footstep Control for Robust Bipedal Walking

Bipedal walking is a complex, balance-critical whole-body motion with inherently unstable inverted pendulum-like dynamics. Strong disturbances must be quickly responded to by altering the walking motion and placing the next step in the right place at the right time. Unfortunately, the high number of degrees of freedom of the humanoid body makes the fast computation of well-placed steps a particularly challenging task. Sensor noise, imprecise actuation, and latency in the sensomotoric feedback loop impose further challenges when controlling real hardware. This dissertation addresses these challenges and describes a method of generating a robust walking motion for bipedal robots. Fast modification of footstep placement and timing allows agile control of the walking velocity and the absorption of strong disturbances. In a divide and conquer manner, the concepts of motion and balance are solved separately from each other, and consolidated in a way that a low-dimensional balance controller controls the timing and the footstep locations of a high-dimensional motion generator. Central pattern generated oscillatory motion signals are used for the synthesis of an open-loop stable walk on flat ground, which lacks the ability to respond to disturbances due to the absence of feedback. The Central Pattern Generator exhibits a low-dimensional parameter set to influence the timing and the landing coordinates of the swing foot. For balance control, a simple inverted pendulum-based physical model is used to represent the principal dynamics of walking. The model is robust to disturbances in a way that it returns to an ideal trajectory from a wide range of initial conditions by employing a combination of Zero Moment Point control, step timing, and foot placement strategies. The simulation of the model and its controller output are computed efficiently in closed form, supporting high-frequency balance control at the cost of an insignificant computational load. Additionally, the sagittal step size produced by the controller can be trained online during walking with a novel, gradient descent-based machine learning method. While the analytic controller forms the core of reliable walking, the trained sagittal step size complements the analytic controller in order to improve the overall walking performance. The balanced whole-body walking motion arises by using the footstep coordinates and the step timing predicted by the low-dimensional model as control input for the Central Pattern Generator. Real robot experiments are presented as evidence for disturbance-resistant, omnidirectional gait control, with arguably the strongest push-recovery capabilities to date

How are human gait and energetics modified when walking over substrates of varying compliance?

Locomotion in the real-world requires humans to negotiate a variety of surfaces that have different material and mechanical properties and thus, require gait adjustments to maintain stability and efficiency. However, our current understanding of human gait and energetics is dominated by studies on hard, level surfaces in a laboratory environment. Previous research has shown that when walking on more irregular terrains such as loose rock surfaces, uneven surfaces and compliant substrates such as snow, grass and sand, there is an increase in energy expenditure. However, the primary mechanistic causes of this increase in energy costs is unclear. Previous studies suggest various biomechanical mechanisms including disruption to pendular energy recovery, increased muscle work, decreased muscle-tendon efficiency and increased gait variability. Yet, comparisons between studies is hindered by the measurement of different variables across studies and variation in substrates used. In this thesis, I focus on human walking over compliant substrates. This thesis aims to improve our understanding of the relationship between energetic costs, substrate properties, gait biomechanics and muscle activities. This is done by presenting a large experimental data set of human walking on both artificial (foam) and natural (sand) compliant substrates. The studies showed that compliant substrates had a considerable effect on gait biomechanics, muscle activation and energetics. On foam, there was greater energetic expenditure on more compliant substrates. On all compliant substrates, participants displayed greater ankle dorsiflexion during stance and greater knee and hip flexion during swing, increased muscle activation and changes to spatiotemporal parameters such as increased cycle time, stance time and swing time and decreased walking speed. The findings of this thesis suggests that overall gross adaptations like sagittal kinematics, spatiotemporal parameters and muscle activation are adopted in response to the depth of depression into a compliant substrate. However, there are specific gait changes due to substrate properties. Further research is required to explore gait adaptations on substrates with different material and mechanical properties. Furthermore, some of our results suggest there is large participant variability even in a relatively homogeneous study population. Therefore, future work should not only look at other demographic groups but also explore individual participant differences such as gender effects and variations in anatomical parameters