BIOMECHANICS AS AN INTERDISCIPLINE - Biomechanics and Motor Control of Human Movement - page 10

their frequency content, digitize them, analog or digitally filter them, and
correlate or average their waveforms. Based on their signal characteristics,
we can make decisions as to sampling rate, minimum length of data files,
and filter cutoff frequencies. Also, there are correlation and covariance tech-
niques that allow us to explore more complex total limb and total body motor
patterns.
1.3.2 Kinematics
Kinematic variables are involved in the description of the movement, inde-
pendent of forces that cause that movement. They include linear and angular
displacements, velocities, and accelerations. The displacement data are taken
from any anatomical landmark: center of gravity of body segments, centers of
rotation of joints, extremes of limb segments, or key anatomical prominances.
The spatial reference system can be either relative or absolute. The former
requires that all coordinates be reported relative to an anatomical coordinate
system that changes from segment to segment. An absolute system means that
the coordinates are referred to an external spatial reference system. The same
applies to angular data. Relative angles mean joint angles; absolute angles are
referred to the external spatial reference. For example, in a two-dimensional
(2D) system, horizontal to the right is 0 ◦ , and counterclockwise is a positive
angular displacement.
The basic kinematic concepts are taught on a 2D basis in one plane. All
kinematic displacement and rotational variables are vectors. However, in any
given direction or rotation, they are considered scalar signals and can be
processed and analyzed as such. In three-dimensional (3D) analysis, we add
an additional vector direction, but we now have three planes to analyze. Each
segment in 3D analyses has its own axis system; thus, the 3D orientation of
the planes for one segment is not necessarily the same as those for the adjacent
segments.
1.3.3 Kinetics
The general term given to the forces that cause the movement is kinetics .Both
internal and external forces are included. Internal forces come from muscle
activity, ligaments, or the friction in the muscles and joints. External forces
come from the ground or from external loads, from active bodies (e.g., those
forces exerted by a tackler in football), or from passive sources (e.g., wind
resistance). A wide variety of kinetic analyses can be done. The moments of
force produced by muscles crossing a joint, the mechanical power flowing to
or from those same muscles, and the energy changes of the body that result
from this power flow are all considered part of kinetics. It is here that a major
focus of the topic is made, because it is in the kinetics that we can really
get at the cause of the movement and, therefore, get some insight into the