Rigid Bones Research Articles

Modal Locomotion: Animating Virtual Characters with Natural Vibrations

AbstractWe present a general method to intuitively create a wide range of locomotion controllers for 3D legged characters. The key of our approach is the assumption that efficient locomotion can exploit the natural vibration modes of the body, where these modes are related to morphological parameters such as the shape, size, mass, and joint stiffness. The vibration modes are computed for a mechanical model of any 3D character with rigid bones, elastic joints, and additional constraints as desired. A small number of vibration modes can be selected with respect to their relevance to locomotion patterns and combined into a compact controller driven by very few parameters. We show that these controllers can be used in dynamic simulations of simple creatures, and for kinematic animations of more complex creatures of a variety of shapes and sizes.

Mechanical influences on suture development and patency.

In addition to their role in skull growth, sutures are sites of flexibility between the more rigid bones. Depending on the suture, predominant loading during life may be either tensile or compressive. Loads are transmitted across sutures via collagenous fibers and a fluid-rich extracellular matrix and can be quasi-static (growth of neighboring tissues) or intermittent (mastication). The mechanical properties of sutures, while always viscoelastic, are therefore quite different for tensile versus compressive loading. The morphology of individual sutures reflects the nature of local loading, evidently by a process of developmental adaptation. In vivo or ex vivo, sutural cells respond to tensile or cyclic loading by expressing markers of proliferation and differentiation, whereas compressive loading appears to favor osteogenesis. Braincase and facial sutures exhibit similar mechanical behavior and reactions despite their different natural environments.

On foundations of discrete element analysis of contact in diarthrodial joints.

Information about the stress distribution on contact surfaces of adjacent bones is indispensable for analysis of arthritis, bone fracture and remodeling. Numerical solution of the contact problem based on the classical approaches of solid mechanics is sophisticated and time-consuming. However, the solution can be essentially simplified on the following physical grounds. The bone contact surfaces are covered with a layer of articular cartilage, which is a soft tissue as compared to the hard bone. The latter allows ignoring the bone compliance in analysis of the contact problem, i.e. rigid bones are considered to interact through a compliant cartilage. Moreover, cartilage shear stresses and strains can be ignored because of the negligible friction between contacting cartilage layers. Thus, the cartilage can be approximated by a set of unilateral compressive springs normal to the bone surface. The forces in the springs can be computed from the equilibrium equations iteratively accounting for the changing contact area. This is the essence of the discrete element analysis (DEA). Despite the success in applications of DEA to various bone contact problems, its classical formulation required experimental validation because the springs approximating the cartilage were assumed linear while the real articular cartilage exhibited non-linear mechanical response in reported tests. Recent experimental results of Ateshian and his co-workers allow for revisiting the classical DEA formulation and establishing the limits of its applicability. In the present work, it is shown that the linear spring model is remarkably valid within a wide range of large deformations of the cartilage. It is also shown how to extend the classical DEA to the case of strong nonlinearity if necessary.

Mechanical Properties of the Tibiotarsus of Broilers and Poults Loaded with Artificial Weight and Fed Various Dietary Protein Levels

Bone strength comparisons were made on tibiotarsi of chicks and poults fed various levels of dietary protein and either loaded or not loaded with artificial weights. Stress and modulus of elasticity were evaluated for their ability to measure strength and rigidity differences in bones while accounting for various bone size. Often comparisons between birds of different ages and body weights are attempted despite various bone size and dimension. These terms identified the weaker, less rigid bones in a comparison between chicks fed a low calcium, low phosphorus diet and a control group matched for age, weight, and bone size. Stress values were not significantly different when ad libitum-fed controls and restriction-fed controls were compared even though body weights were significantly different. This suggests that bone strength comparisons can be made on bones that differ in size.Artificial weight loading of chicks or poults had no consistent effect on stress or modulus of elasticity. This would suggest that this type of exercise had no influence on bone strength. Turkey poults fed 28% dietary protein appeared to have stronger bones than those fed lower protein levels; however, when bone size and dimension are accounted for, no differences in bone strength resulted from the feeding of various protein levels. These findings suggest that stress and modulus of elasticity are better terms to use than force in making bone strength comparisons between groups of birds which may differ in body size and bone dimension.