Human Modeling: Kinematics - Human Motion Simulation: Predictive Dynamics - page 7

CHAPTER
2

Human Modeling:
Kinematics

The optimist proclaims that we live in the best of all possible worlds, and
the pessimist fears this is true.
James Branch Cabell

2.1 Introduction
The objective of this chapter is to establish a systematic method for representing
human anatomy and to develop mathematical methods for kinematic analysis as
the human body undergoes motion. Kinematic analysis in this context means the
study of motion characterized by the position, velocity, and acceleration of human
segmental links.
Throughout this chapter, we shall consider various segments of the body as
individual rigid bodies that are connected via joints. Human modeling techniques
have rapidly evolved in recent years, driven by the need for safety, security, and
better ergonomics, as well as the need for avatars to perform tasks that could not
be performed in the real world. Perhaps the most influential force behind this fast
pace is the gaming industry where avatars are extensively used to interact and
respond in real time. Similarly, in the movie industry, digital characters are used
to replace actors where it has now become difficult in some cases to differentiate
the real from the virtual. In general, human models have been represented as stick
figures, skeletons, mesh surfaces, profiles, and mannequins ( Figure 2.1 ).
Consider the motion of a person's arm from one position to another, where
only the elbow joint is changed. As a result of this simple motion, the hand is
also moved in space to a final configuration defined by a position and an orienta-
tion (also called a pose). In order to characterize the motion of these segmental
links and their associated joints, it is necessary to establish a systematic approach
for specifying coordinate systems defined by xyz on each link, and establish a
method for relating any two such coordinate systems.
Let us assume for a moment that we are able to specify values of the joint
variables for the shoulder complex, the elbow, and the wrist, and we wanted to
know the final position of the hand ( Figure 2.2 ). In this case, we specify a vector
x that describes the position of the hand with respect to another coordinate system