Chapter 2. Prehension

404 Appendices
biceps, and shoulder and thoracic muscles. Using the shoulder and
thoracic muscles, force is transmitted by a shoulder harness and cable
to the terminal device. Body-powered devices are relatively
inexpensive, functional, reliable, and have some indirect sensory
proprioceptive feedback (Muilenburg & LeBlanc, 1989). They are
functionally better than other methods, because position and force
feedback is available from the harness. The indirect sensory feedback
is provided through extended physiological proprioception (Law,
198 l), which comes from proprioceptive information about limb
position and velocity. This seems to extend to include additional limb
segments provided a consistent relationship exists between the
position and velocity of the artifical segment and the position, velocity,
and applied force of the contiguous natural body part (Law, 1981).
Externally powered, myoelectric prostheses are controlled by
electromyographic (EMG) signals, where the force developed by the
muscle is used to drive motors or to set control switches.
Conventional myoelectric hands have a single channel from a pair of
opposing muscle groups, controlling a single degree of freedom in the
hand (Kyberd et al., 1987). The input is usually a simple on/off
signal, where flexor tension opens the hand and extensor tension
closes it. Depending on the level of the amputation, muscles use
include forearm muscles, or biceps and triceps. Sensory information
may come from visual and auditory cues, such as the sound of
motors, motor vibration, accurate memory of opening-closing time,
and movements of the center of gravity (Law, 1981).
Myoelectric digital on/off control uses a simple EMG threshold
detect to control a switch. When muscle tension reaches a critical level,
it causes the switch to change states, which in turn controls a motor to
direct the hand in one direction or the other. Pinch force between
finger can be indirectly related to the duration of the signal; the longer
the closing motor runs, the harder the fingers pinch. Proportional
myoelectric control is an alternative method of control. Integrated
EMG amplitude varies closely with actual tension generated by muscle
(see Sears & Shaperman, 1991). The EMG controls the motor
directly, designed so that the output of the motor is proportional to the
output of the muscle. With more muscle contraction, the faster the
motor operates. The user has more sensitive slow and fast control of
the hand, depending on the strength of the muscle contraction.
Myoelectric devices have certain advantages (Baumgartner, 198 l),
including no body harness, increased comfort, and more independent
control in bilateral amputations. The disadvantages are that they not
designed for heavy work, their weight is greater, costs are higher, and