Chapter 2. Prehension

Appendix D - Prosthetic and Robotic Hands 405
greater servicing is required. Most important however is the lack of
sensory feedback, other than auditory and visual clues, requiring a
high level of concentration. Proportional myoelectric control have
additional advantages over digial myoelectric control, and these
include better control of force and speed, and less muscular effort
expended. A disadvantage of proportional myoelectric control is that
currently the circuits are larger. However, it operates on a very low
threshold EMG signal, and therefore operates with less muscle effort
and continues to run until the muscles are completely relaxed.
Stein and Walley (1983) found that tasks with a myoelectric
prosthesis took twice as long as a hook and five times as long as with
a normal hand. Yet 60% of below-elbow amputees preferred to use the
myoelectric prosthesis compared to a conventional prosthesis. Yet,
although the subjects were able to accomplish the tasks faster with the
hook vs the hand, extreme body movements had to be used due to the
harnessing, such as rotating the trunk in order to rotate a heavy object.
With the myoelectric prosthesis users, this was not observed.
However, in a survey of 33 patients on proportional versus digital
control, Sears and S haperman ( 199 1) found the proportional
myoelectric hand to be quicker than a hook and digital myoelectric
hand, with the amount of effort reduced. Patients rated it low for its
weight and bulkiness, but felt that it appeared more natural during use.
Former digital hand wearers gave the proportional myoelectric hand
the highest performance ratings, but the lowest rating on convenience.
Since the user can control speed and force, it was felt that there was
better control over fastenings in dressing, better dexterity in using
pencil, better control in using tools, and noticed difference in ability to
hold fragile objects.
Two other external control mechanisms have been used. One is
servo-control, where the hand opening is proportional to a
transducer’s motion. The transducer is controlled mechanically, with
very little froce and excursion. The other is switch control. It is similar
to servo-control, in that some minimal force and excursion capability
is required from a remaining body part. For example, a shoulder
flexion of 1.3 cm excursion can be used to activate the switch (Peizer,
198 1).
D.2.2 Split hook
As a prosthetic device, the split hook is strong, lightweight,
simple, functional and reliable (Law, 1981). It has functional shapes
(narrow tips, flat gripping surfaces, specialized tool-holding grips),