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125_201+Chapter_2_Biomech-1

# 125_201+Chapter_2_Biomech-1 - CHAPTER 2 Static...

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CHAPTER 2: Static B IOMECHANICAL A NALYSIS In Chapter 1, we briefly illustrated how “traditional” static analyses in mechanics can be applied to biomechanics. In this chapter, we will more thoroughly compare elements of the musculoskeletal system to mechanical analogs in order to allow us to complete more thorough analyses of biomechanical systems. We will also discuss how the natural variation of these biomechanical elements among individuals can influence the analyses and, thus our criteria and choices when designing biomedical devices. 2.1 ‘Mechanics’ vs. ‘Biomechanics’ - Degrees of Freedom and Reactions An essential step in any analysis is the identification of possible movements of bodies in a mechanical system, i.e. identifying the ‘degrees of freedom’. There are 6 possible degrees of freedom for each point in a body: translation in the x, y, and/or z directions, and rotation about the x-, y-, and/or z-axes (or in the y-z, x-z, and x-y planes, respectively). These also represent potential reactions at a point – forces and/or moments required to restrict movement in a particular direction or rotation about a particular axis. In designing a mechanical system, we can build in components that allow or restrict specific movements. We have control over the degrees of freedom. In analyzing the biomechanics of the human body, we must recognize and understand the allowable movement of each ‘component’ and what restricts these movements. This is also critical when designing biomedical devices for replacing functions, for example, in joint replacement. If we consider the tibia (shin bone), on its own it has zero degrees of freedom no point in the tibia can translate or rotate with any other point in the tibia . It can move with respect to the bones in your ankle or knee, but not with any other place in the tibia. But what if you fracture your tibia? Now, unfortunately, points in your tibia can translate and rotate with respect to each other, if they are on opposite sides of the fracture. How do we fix the fracture? We ‘artificially’ restrict the degrees of freedom of the bone with a cast, a bone plate, bone screws, etc. This prevents the relative bone movements and allows the fracture to heal, once again eliminating the degree of freedom. Clinically, we have recognized how controlling the degrees of freedom of muscles, ligaments, tendons, and bones can promote healing by restoring or augmenting the natural ‘constraints’ of the human body. Figure 2-1 shows a variety of common mechanical constraints and their allowable degrees of freedom. Most of these should be well-known and can be found around your home. Hinges, like those found on a door, have one degree of freedom – rotation within a single plane, or about a single axis. If you try to move the hinge in or about any other axis, the hinge prevents the movement and provides a reaction force or moment to balance the force or moment you apply.

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125_201+Chapter_2_Biomech-1 - CHAPTER 2 Static...

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