KINEMATICS - Biomechanics and Motor Control of Human Movement - page 46

small fraction of the available kinematic variables. An assessment of a run-
ning broad jump, for example, may require only the velocity and height of
the body's center of mass. On the other hand, a mechanical power analysis
of an amputee's gait may require almost all the kinematic variables that are
available.

3.1 KINEMATIC CONVENTIONS

In order to keep track of all the kinematic variables, it is important to estab-
lish a convention system. In the anatomical literature, a definite convention
has been established, and we can completely describe a movement using
terms such as proximal, flexion , and anterior. It should be noted that these
terms are all relative, that is, they describe the position of one limb rela-
tive to another. They do not give us any idea as to where we are in space.
Thus, if we wish to analyze movement relative to the ground or the direc-
tion of gravity, we must establish an absolute spatial reference system. Such
conventions are mandatory when imaging devices are used to record the
movement. However, when instruments are attached to the body, the data
become relative, and we lose information about gravity and the direction of
movement.

3.1.1 Absolute Spatial Reference System
Several spatial reference systems have been proposed. The one utilized
throughout the text is the one often used for human gait. The vertical
direction is Y , the direction of progression (anterior - posterior) is X , and the
sideways direction (medial - lateral) is Z . Figure 3.1 depicts this convention.
The positive direction is as shown. Angles must also have a zero reference
and a positive direction. Angles in the XY plane are measured from 0 ◦ in
the X direction, with positive angles being counterclockwise. Similarly, in
the YZ plane, angles start at 0 ◦ in the Y direction and increase positively
counterclockwise. The convention for velocities and accelerations follows
correctly if we maintain the spatial coordinate convention:

x =

velocity in the X direction, positive when X is increasing

y = velocity in the Y direction, positive when Y is increasing

z ˙ = velocity in the Z direction, positive when Z is increasing

x ¨ = acceleration in the X direction, positive when x is increasing ˙

y ¨ = acceleration in the Y direction, positive when y is increasing ˙

z ¨ = acceleration in the Z direction, positive when z is increasing ˙

The same applies to angular velocities and angular accelerations. A coun-
terclockwise angular increase is a positive angular velocity, ω . When ω is
increasing, we calculate a positive angular acceleration, α .