Clinical Biomechanics: Basic Factors of Biodynamics and Joint Stability
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The following is Chapter 3 from RC’s best-selling:
Second Edition ~ Wiliams & Wilkins
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Chapter 3: Basic Factors of Biodynamics and Joint Stability
The techniques used for analyzing static positions of the body are only approximate inasmuch as forces accompanying movement incorporate such dynamic factors as acceleration, momentum, friction, the changing positions of rotational axes, and the resistance and support offered by tissues other than muscles. This chapter discusses the basic concepts and terms of biodynamics, biomechanical stress, and the biomechanical aspects of articular cartilage pertinent to the clinical setting.
The study of dynamics is concerned with loads and the motions of bodies (kinematics) and the action of forces in producing or changing their motion (kinetics). Kinematics lets us describe the characteristics of motion position, acceleration, and velocity such as in gait or scoliotic displacements. Here we are concerned with the position of the center of mass of the body and its segments, the segmental range of motion, and the velocity and direction of their movements. In kinetics, we become concerned with the forces that cause or restrict motion such as muscle contraction, gravity, and friction. A complete biomechanical analysis of human motion or motion of a part would include both kinematic and kinetic data.
Motion can be defined as an object’s relative change of place or position in space within a time frame and with respect to some other object in space. Thus, motion may be determined and illustrated by knowing and showing its position before and after an interval of time. While linear motion is readily demonstrated in the body as a whole as it moves in a straight line, most joint motions are combinations of translatory and angular movements that are more often than not diagonal rather than parallel to the cardinal planes. In addition to muscle force, joint motion is governed by factors of movement freedom, axes of movement, and range of motion.
Degrees of Freedom
As previously discussed, the body is composed of numerous uniaxial, biaxial, and multiaxial joints. Joints with one axis have one degree of freedom to move in one plane such as pivot and hinge joints, joints with two axes have two degrees of freedom to move in two different planes, and joints with three axes have three degrees of freedom to move in all three planes, eg, the ball-and-socket joints. Thus, that motion in which an object may translate to and fro along a straight course or rotate one way or another about a particular axis equals one degree of freedom.
In Chapter 1, joint classification was given under the major divisions of synarthrodial, amphiarthrodial, and diarthrodial joints. This is the classic anatomic classification. However, from a purely biomechanical viewpoint, joint motion can be reduced to just two types: (1) ovoid, which permits motion in one plane, X; and (2) sellar, which permits motion in two planes, Y and Z (Fig. 3.1).
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