KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 117

5.2 FORCE TRANSDUCERS AND FORCE PLATES

In order to measure the force exerted by the body on an external body or
load, we need a suitable force-measuring device. Such a device, called a force
transducer , gives an electrical signal proportional to the applied force. There
are many kinds available: strain gauge, piezoelectric, piezoresistive, capaci-
tive, and others. All these work on the principle that the applied force causes
a certain amount of strain within the transducer. For the strain gauge type, a
calibrated metal plate or beam within the transducer undergoes a very small
change (strain) in one of its dimensions. This mechanical deflection, usually
a fraction of 1%, causes a change in resistances connected as a bridge circuit
(see Section 3.2.3), resulting in an unbalance of voltages proportional to the
force. Piezoelectric and piezoresistive types require minute deformations of
the atomic structure within a block of special crystalline material. Quartz,
for example, is a naturally found piezoelectrical material, and deformation
of its crystalline structure changes the electrical characteristics such that the
electrical charge across appropriate surfaces of the block is altered and can
be translated via suitable electronics to a signal proportional to the applied
force. Piezoresistive types exhibit a change in resistance which, like the strain
gauge, upset the balance of a bridge circuit.

5.2.1 Multidirectional Force Transducers
In order to measure forces in two or more directions, it is necessary to use a
bi- or tridirectional force transducer. Such a device is nothing more than two
or more force transducers mounted at right angles to each other. The major
problem is to ensure that the applied force acts through the central axis of
each of the individual transducers.

5.2.2 Force Plates*
The most common force acting on the body is the ground reaction force,
which acts on the foot during standing, walking, or running. This force vector
is three-dimensional and consists of a vertical component plus two shear com-
ponents acting along the force plate surface. These shear forces are usually
resolved into anterior — posterior and medial — lateral directions.
The fourth variable needed is the location of the center of pressure of
this ground reaction vector. The foot is supported over a varying surface area
with different pressures at each part. Even if we knew the individual pressures
under every part of the foot, we would be faced with the expensive problem
of calculating the net effect of all these pressures as they change with time.
Some attempts have been made to develop suitable pressure-measuring shoes,

∗ Representative paper: Elftman, 1939.