Biomechanics
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2008-07-07T16:08:16Z
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'''Biomechanics''' is the application of mechanical principles on living organisms. This includes research and analysis of the [[mechanics]] of living [[organism]]s and the application of engineering principles to and from biological systems. This research and analysis can be carried forth on multiple levels, from the [[Molecule|molecular]], wherein [[biomaterials]] such as [[collagen]] and elastin are considered, all the way up to the tissue and organ level. Some simple applications of [[Classical mechanics|Newtonian mechanics]] can supply correct approximations on each level, but precise details demand the use of [[continuum mechanics]].
[[Image:penguinu.jpg|thumb|right|Chinstrap Penguin]]
[[Giovanni Alfonso Borelli]] wrote the first book on biomechanics, ''De Motu Animalium'', or [[On the Movement of Animals]]. He not only saw animals' bodies as mechanical systems, but pursued questions such as the physiological difference between imagining performing an action and actually doing it. Some simple examples of biomechanics research include the investigation of the forces that act on limbs, the [[aerodynamics]] of [[Bird flight|bird]] and [[insect]] [[flight]], the [[hydrodynamics]] of [[swimming]] in [[fish]], the anchorage and mechanical support provided by tree roots, and [[locomotion]] in general across all forms of life, from individual [[cell (biology)|cell]]s to whole [[organism]]s. The biomechanics of [[human]] beings is a core part of [[kinesiology]].
Applied mechanics, most notably [[thermodynamics]] and [[continuum mechanics]], and [[mechanical engineering]] disciplines such as [[fluid mechanics]] and [[solid mechanics]], play prominent roles in the study of biomechanics. By applying the laws and concepts of physics, biomechanical mechanisms and structures can be simulated and studied.
It has been shown that applied [[load]]s and [[deformation]]s can affect the properties of living tissue. There is much research in the field of growth and remodeling as a response to applied loads. For example, the effects of elevated [[blood pressure]] on the mechanics of the [[artery|arterial]] wall, the behavior of [[cardiac muscle|cardiomyocytes]] within a heart with a cardiac [[infarct]], and [[bone]] growth in response to exercise, and the acclimative growth of plants in response to wind movement, have been widely regarded as instances in which living tissue is remodelled as a direct consequence of applied loads.
Relevant mathematical tools include [[linear algebra]], [[differential equation]]s, [[vector calculus|vector]] and [[tensor calculus]], numerics and computational techniques such as the [[finite element method]].
The study of [[biomaterial]]s is of crucial importance to biomechanics. For example, the various tissues within the body, such as skin, bone, and arteries each possess unique material properties. The passive mechanical response of a particular tissue can be attributed to characteristics of the various [[protein]]s, such as [[elastin]] and [[collagen]], living cells, ground substances such as [[proteoglycan]]s, and the orientations of fibers within the tissue. For example, if human [[skin]] were largely composed of a protein other than [[collagen]], many of its mechanical properties, such as its [[Young's modulus|elastic modulus]], would be different.
[[Chemistry]], [[molecular biology]], and [[cell biology]] have much to offer in the way of explaining the active and passive properties of living tissues. For example, in [[muscle contraction]]s, the binding of [[myosin]] to [[actin]] is based on a [[Biochemistry|biochemical]] reaction involving calcium ions and [[Adenosine triphosphate|ATP]].
==Applications==
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The study of biomechanics ranges from the inner workings of a [[Cell (biology)|cell]] to the movement and development of [[Limb (anatomy)|limb]]s, to the mechanical properties of [[soft tissue]], and [[bone]]s. As we develop a greater understanding of the physiological behavior of living tissues, researchers are able to advance the field of [[tissue engineering]], as well as develop improved treatments for a wide array of [[pathology|pathologies]].
Biomechanics as a [[sports science]], kinesiology, applies the laws of mechanics and physics to human performance in order to gain a greater understanding of performance in athletic events through modeling, simulation, and measurement.
==Continuum mechanics==
It is often appropriate to model living tissues as [[continuous media]]. For example, at the tissue level, the arterial wall can be modeled as a [[continuum]]. This assumption breaks down when the [[length scale]]s of interest approach the order of the micro structural details of the material. The basic postulates of continuum mechanics are conservation of [[linear momentum|linear]] and [[conservation of angular momentum|angular momentum]], [[conservation of mass]], [[conservation of energy]], and the [[entropy]] inequality. Solids are usually modeled using "reference" or "[[Lagrangian coordinates|Lagrangian]]" coordinates, whereas fluids are often modeled using "spatial" or "[[Eulerian coordinates|Eulerian]]" coordinates. Using these postulates and some assumptions regarding the particular problem at hand, a set of equilibrium equations can be established. The [[kinematics]] and [[constitutive relation]]s are also needed to model a continuum
Second and fourth order tensors are crucial in representing many quantities in electromechanical. In practice, however, the full tensor form of a fourth-order constitutive matrix is rarely used. Instead, simplifications such as [[isotropy]], [[transverse isotropy]], and [[incompressibility]] reduce the number of independent components. Commonly-used second-order tensors include the [[Cauchy stress tensor]], the second [[Piola-Kirchhoff stress tensor]], the [[deformation gradient tensor]], and the [[Green strain tensor]]. A reader of the mechanic's literature would be well-advised to note precisely the definitions of the various tensors which are being used in a particular work.
==Circulation==
[[Image:Redbloodcells.jpg|right|thumb|100px|[[Red blood cell]]s]]
Under most circumstances, [[blood]] flow can be modeled by the [[Navier-Stokes equations]]. Whole blood can often be assumed to be an incompressible [[Newtonian fluid]]. However, this assumption fails when considering flows within arterioles. At this scale, the effects of individual red blood cells becomes significant, and whole blood can no longer be modeled as a continuum. When the diameter of the blood vessel is slightly larger than the diameter of the red blood cell the [[Fahraeus–Lindquist effect]] occurs and there is a decrease in wall shear stress. However, as the diameter of the blood vessel decreases further, the red blood cells have to squeeze through the vessel and often can only pass in single file. In this case, the inverse Fahraeus–Lindquist effect occurs and the wall shear stress increases.
==Bones==
Bones are [[anisotropic]] but are approximately [[transverse isotropy|transversely isotropic]]. In other words, bones are stronger along one axis than across that axis, and are approximately the same strength no matter how they are rotated around that axis.
The stress-strain relations of bones can be modeled using [[Hooke's law]], in which they are related by [[elastic moduli]], e.g. [[Young's modulus]], [[Poisson's ratio]] or the [[Lamé parameters]]. The constitutive matrix, a fourth order [[tensor]], depends on the isotropy of the bone.
:<math>\sigma_{ij}=C_{ijkl}\epsilon_{kl}</math>
==Muscle==
There are three main types of muscles:
*[[Skeletal muscle]] (striated): Unlike cardiac muscle, skeletal muscle can develop a sustained condition known as [[tetany (action potential summation)|tetiny]] through high frequency stimulation, resulting in overlapping twitches and a phenomenon known as wave summation. At a sufficiently high frequency, tetany occurs, and the contracticle force appears constant through time. This allows skeletal muscle to develop a wide variety of forces. This muscle type can be voluntary controlled. [[Hill's Model]] is the most popular model used to study muscle.
*[[Cardiac muscle]] (striated): Cardiomyocytes are a highly specialized cell type. These involuntarily contracted cells are located in the heart wall and operate in concert to develop synchronized beats. This is attributable to a refractory period between twitches.
*[[Smooth muscle]] (smooth - lacking striations): The stomach, vasculature, and most of the digestive tract are largely composed of smooth muscle. This muscle type is involuntary and is controlled by the enteric nervous system.
==Soft tissues==
Soft [[Biological tissue|tissue]]s such as [[tendon]], [[ligament]] and [[cartilage]] are combinations of matrix proteins and fluid. In each of these tissues the main strength bearing element is collagen, although the amount and type of collagen varies according to the function each tissue must perform. Elastin is also a major load-bearing constituent within skin, the vasculature, and connective tissues.
The function of tendons is to connect muscle with bone and is subjected to tensile loads. Tendons must be strong to facilitate movement of the body while at the same time remaining compliant to prevent damage to the muscle tissues. Ligaments connect bone to bone and therefore are stiffer than tendons but are relatively close in their tensile strength. Cartilage, on the other hand, is primarily loaded in compression and acts as a cushion in the joints to distribute loads between bones. The compressive strength of collagen is derived mainly from collagen as in tendons and ligaments, however because collagen is comparable to a "wet noodle" it must be supported by cross-links of glycosaminoglycans that also attract water and create a nearly incompressible tissue capable of supporting compressive loads.
Recently, research is growing on the biomechanics of other types of soft tissues such as skin and internal organs.
This interest is spurred by the need for realism in the development of medical [[simulation]].
==Viscoelasticity==
[[Viscoelasticity]] is readily evident in many soft tissues, where there is energy dissipation, or hysteresis, between the loading and unloading of the tissue during mechanical tests. Some soft tissues can be [[preconditioning|preconditioned]] by repetitive cyclic loading to the extent where the [[stress-strain curve]]s for the loading and unloading portions of the tests nearly overlap. The most commonly used model for viscoelasticity is the Quasilinear Viscoelasticity theory (QLV).
In addition, soft tissues exhibit other viscoelastic properties, including creep, stress relaxation, and preconditioning.
==Nonlinear theories==
[[Hooke's law]] is linear, but many, if not most problems in biomechanics, involve highly nonlinear behavior, particularly for soft tissues. Proteins such as collagen and elastin, for example, exhibit such a behavior. Some common material models include the Neo-Hookean behavior, often used for modeling elastin, and the famous Fung-elastic exponential model. Non linear phenomena in the biomechanics of soft tissue arise not only from the material properties but also from the very large strains (100% and more) that are characteristic of many problems in soft tissues.
==See also==
*[[Anatomy]]
*[[Biomineralization]]
*[[E. Lloyd Du Brul]]
*[[List of publications in biology#Biomechanics|Important publications in biomechanics]]
*[[Mechanics]]
*[[Exoskeleton|Orthosis]]
*[[Physiology]]
*[http://www.coe.neu.edu/~vgohil Biomechanics and FEA of Bone]
==References==
* Dudley, R. 2000. ''The Biomechanics of Insect Flight: Form, Function, Evolution.'' Princeton: Princeton University Press.
* Wainwright, S. A., Biggs, J., Curry, S. and Gosline, J. (1978). "Mechanical Design in Organisms." University Press.
* Fung, Y. C. ''Biomechanics: Mechanical Properties of Living Tissue.'' (2nd ed.). New York: Springer. ISBN 0-387-97947-6.
* Gans, C. 1974. ''Biomechanics: An Approach to Vertebrate Biology.'' Philadelphia: J. B. Lippincott. ISBN-10: 0472080164, ISBN-13: 978-0472080168.
* Humphrey, J. D. "Cardiovascular Solid Mechanics: Cells, Tissues, and Organs." New York: Springer. ISBN 0-387-95168-7.
* Vogel, S. 2003. ''Comparative Biomechanics: Life's Physical World.'' Princeton: Princeton University Press. ISBN 0-691-11297-5
* [[Yoshito Ikada|Ikada, Yoshito]]. ''Bio Materials: An Approach to Artificial Organs'' (バイオマテリアル: 人工臓器へのアプローチ)
* [http://www.coe.neu.edu/~vgohil Biomechanics of Bone]
==External links==
{{wikibooks}}
{{WVD}}
* [http://www.asbweb.org American Society of Biomechanics]
* [http://www.isbweb.org International Society of Biomechanics]
* [http://www.biomch-l.org Biomch-L] - an internet discussion forum for biomechanics and human/animal movement science
* [http://www.thebiomechanicslab.com The Biomechanics Lab] - a medium for connection between individuals in the biomechanics field.
* [http://www.biomechanik.de/index.html Biomechanics Laboratory - Charité Berlin, Germany: Basic research with instrumented orthopaedic implants]
* [http://biomechanics.bio.uci.edu/_html/nh_biomech/running/running.htm ''Natural History'' columns on biomechanics]
* [http://www.biomechanik.fh-aachen.de Biomechanics Laboratory of the Aachen University of Applied Sciences, Germany ]
* [http://www.esm.vt.edu/php/twocol.php?link=underg&content=ugChkLsts&sb=15 The biomechanics undergraduate degree from the Virginia Tech Department of Engineering Science and Mechanics]
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