Biomedical Engineering Reference
In-Depth Information
Table 5.1
Uses of Biomechanical Analysis
Area
Application
Archeological surveys, human growth, populational variation, nutritional supple-
mentation, design of tools, furniture, and equipment used in a workplace
Anthropometry
Identification of underlying causes for walking abnormalities in patients with
neuromuscular problems (cerebral palsy, stroke, head injury), designing prosthetic
components, improving athletic performance and rehabilitation engineering, and
tools for persons with physical disabilities
Gait analysis
Prosthesis devices (heart valves and artificial heart), injury criterion for various tis-
sues and organs, load distribution, anthropometry data, biomaterial selection, and
rehabilitation engineering
Stress analysis
Development of safety devices, aircraft ejection seats, underwater rescue proce-
dures, land mine detonation, pedestrian impacts, infant carriers, seat belt systems,
air bag systems,
child safety seats, and helmets and other protective gear in sports
Impact
biomechanics
5.2
Equations of Motion
5.2.1 Center of Mass
The mass of an object is a measure of the amount of matter in an object. The center
of mass is a unique point at which the average of the mass factored by their dis-
tances is considered to be concentrated. The center of mass and center of gravity
(CG) are used synonymously in a uniform gravitational field to represent the unique
point in an object or system which can be used to describe the system's response to
external forces. An object's CG is not always located physically inside of the object.
In the human population, each person has a different weight and height. Locating
the CG is of interest because, mechanically, a body behaves as though all of its mass
were concentrated at the CG. Every time the body changes posture, its weight dis-
tribution and CG locations change.
The CG for the human body can be located either using a reaction board or
segmental method. A reaction board consists of a long rigid board supported on
pointed structures [Figure 5.1(a)]. Under each end of the board, there is a weighing
scale. The CG location is measured using the conservation of momentum principle
(see Section 5.2.2).
The segmental method is a more popular procedure for determining center of
mass of the total body, as it can be used under dynamic situations to predict the
body's response during walking, the joint moment of force, and the design of the
workspace interface (crash test dummies). In the segmental method, the center of
mass of individual segments is determined. Based on bony prominent landmarks,
Dempster [1] grouped the body into 15 segments consisting of the foot, shank
(leg), thigh, pelvis, thorax, hand, forearm, upper arm, and head [Figure 5.1(b)].
Based on the assumption that body segments are considered to be rigid bodies with
frictionless hinges for the purposes of describing the motion of the body, body seg-
ments can be represented by geometric solids (cones, cylinders, spheres), and mass
is uniformly distributed in a given segment. Although no joint in the human body is
a simple single-axis hinge joint, the method can be individualized by capturing im-
