Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 317
While these are merely anecdotal evidence for such effects on
prehension, kinesiologists and psychologists have begun to explore
the functional issues. Movement constraints act within and around the
intentions of the performer, which delineate goals for a movement.
An example would be the 'sherry-glass response' (Traub, Rothwell &
Marsden, 1980). Subjects were asked to maintain their thumb and
index finger a few millimeters from the rim of a full glass of sherry. A
perturbation was made to the wrist sufficient to cause the hand to hit
the glass and knock it over. A short-latency grab response (50 msec)
occurred in the thumb muscle in response to the perturbation, thereby
saving the glass from being knocked over. However, the response
was observed only when the subject's intent was to prevent the glass
from falling over. Traub et al. argued that this suggests the presence
of a grab reflex, where the digits respond within a time too short to be
voluntary, flexing in order to maintain contact but only if contact is the
person's in tent.
In a formal empirical situation, intent emerges from the
experimenter's request to the subject to perform a certain task. In
MacKenzie et al. (1987) for example, the subjects were asked to point
with a stylus 'as quickly and as accurately as possible' to a target of
varying size and at varying distances. The question being asked was
whether there was a reliable kinematic measure of the speed and
accuracy requirements of the task. In this case, Fitts' Law (Fitts
1954) was used, which states that movement time (MT) is directly
proportional to the index of difficulty (ID) of the task, or MT = a + b x
ID where the ID is the log2(2A/W), A is the amplitude of movement
(an extrinsic object property), and W is the width of target, or target
tolerance (an intrinsic object property). When plotting MT against ID,
a linear relationship is seen. MacKenzie et al. (1987) measured the
MT of the tip of the stylus, its time to peak resultant velocity, and the
percentage of movement time after peak resultant velocity. They
found a differential effect of target size and amplitude on these
parameters. For each target size, the acceleration time increased as the
ID (amplitude) increased. For each amplitude, there is no effect of
target size on the acceleration time. When the data was normalized,
the percentage of MT after peak resultant velocity (the deceleration
phase) increased for each amplitude as the ID increased (target size
decreased). These results indicate that the resultant velocity profile is
not symmetrical. Fitts' Law argues that the MT will increase as the
target size decreases; the reason that the MT increases is the result of a
relatively longer deceleration phase. As well, for longer movements,
there is a longer acceleration phase. The results show that the time