KINEMATICS - Biomechanics and Motor Control of Human Movement - page 48

3. Linear acceleration ( z ) of segment center of mass

4. Angle of segment in two planes, θ xy , θ yz
5. Angular velocity of segment in two planes, ω xy , ω yz
6. Angular acceleration of segment in two planes, α xy , α yz

x , y ,

Note that the third angle data are redundant; any segment's direction
can be completely described in two planes. For a complete description
of the total body (feet + legs + thighs + trunk + head + upper arms +

forearms and hands = 12 segments), movement in three-dimensional (3D)

space required 15 180 data variables. It is no small wonder that

we have yet to describe, let alone analyze, some of the more complex
movements. Certain simplifications can certainly reduce the number of
variables to a manageable number. In symmetrical level walking, for
example, we can assume sagittal plane movement and can normally ignore
the arm movement. The head, arms, and trunk (HAT) are often considered
to be a single segment, and assuming symmetry, we need to collect data
from one lower limb only. The data variables in this case (four segments,
one plane) can be reduced to a more manageable 36.

× 12 =

3.2 DIRECT MEASUREMENT TECHNIQUES

3.2.1 Goniometers ∗
A goniometer is a special name given to the electrical potentiometer that can
be attached to measure a joint angle. One arm of the goniometer is attached
to one limb segment, the other to the adjacent limb segment, and the axis
of the goniometer is aligned to the joint axis. In Figure 3.2, you can see the
fitting of the goniometer to a knee joint along with the equivalent electrical
circuit. A constant voltage E is applied across the outside terminals, and the
wiper arm moves to pick off a fraction of the total voltage. The fraction of
the voltage depends on the joint angle θ . Thus, the voltage on the wiper
arm is ν = kE θ = k 1 θ volts. Note that a voltage proportional to θ requires a
potentiometer whose resistance varies linearly with θ . A goniometer designed
for clinical studies is shown fitted on a patient in Figure 3.3.

Advantages
1. A goniometer is generally inexpensive.
2. The output signal is available immediately for recording or conversion
into a computer.
3. Planar rotation is recorded independently of the plane of movement of
the joint.

∗ Representative paper: Finley and Karpovich, 1964.