380 8.6  Rigid-Body and Semirigid-Body Biomechanics

encapsulates similar themes, but with the inclusion of finite compliance in the mechanical

components of the system.

8.6.1  ANIMAL LOCOMOTION

At the high end of the length scale, this includes analysis of whole animal locomotion,

be it on land, in sea, or in air. Here, the actions of muscles in moving bones are typic­

ally modeled as levers on pivots with the addition of simple linear springs (to model

the action of muscles at key locations) and dampers (to model the action of friction).

Several methods using these approaches for understanding human locomotion have

been developed to assist in understanding of human diseases, but also much work in

this area has been catalyzed by the computer video-​gaming industry to develop realistic

models of human motion.

Comparative biomechanics is the application of biomechanics to nonhuman animals,

often used to gain insights into human biomechanics in physical anthropology or to simply

study these animals as an end in itself. Animal locomotion includes behaviors such as

running/​jumping, swimming, and flying, all activities requiring an external energy to accel­

erate the animal’s inertial mass and to oppose various combinations of opposing forces

including gravity and friction. An emergent area of biophysical engineering research that

utilizes the results of human models of biomechanics, in particular, is in developing artifi­

cial biological materials or biomimetics (see Chapter 9). This crosses over into the field of

biotribology, which is the study of friction/​wear, lubrication, and contact mechanics in bio­

logical systems, particularly in large joints in the human body. For example, joint implants in

knees and hips rub against each other during normal human locomotion, and all lubricated

by naturally produced synovial fluid, and biotribology analysis can be useful in modeling

candidate artificial joint designs and/​or engineered cartilage replacement material that can

mimic the shock-​absorbing properties in the joints of natural cartilage that has been eroded/​

hardened through disease/​calcification effects.

8.6.2  PLANT BIOMECHANICS

Plant biomechanics is also an emerging area of research. The biological structures in plants

that generate internal forces and withstand external forces are ostensibly fundamentally

different from those in animals. For example, there are no plant muscles as such and no

equivalent nervous system to enervate these nonexistent muscles anyway. However, there are

similarities in the network of filament-​based systems in plant cells. These are more based on

the fibrous polysaccharide cellulose but have analogies to the cytoskeletal network of animal

cells (see Chapter 2).

Also, although there is no established nervous system to control internal forces, there are

methods of chemical-​ and mechanical-​based signal transduction to enable complex regula­

tion of plant forces. In addition, at a molecular level, there are several molecular motors in

plants that act along similar lines to those in animal cells (see in the following text). Plant root

mechanics is also a particularly emergent area of research in terms of advanced biophysical

techniques, for example, using light-​sheet microscopy to explore the time-​resolved features

of root development (see Chapter 4). In terms of analytical models, much of these have been

more of the level of computational FEA (see in the following text).

8.6.3  TISSUE AND CELLULAR BIOMECHANICS

In terms of computational biomechanics approaches, the focus has remained on the length

scale of tissues and cells. For tissue-​level simulations, these involve coarse-​graining the tissue

into cellular units, which each obeys relatively simple mechanical rules, and treating these