Chapter 2. Prehension

220 THE PHASES OF PREHENSION
hot-water bottles and surrounded by electric heaters. After about 1/2
hour, the coefficient of friction on the forearm markedly increased,
almost doubling. With air drying and cooling, the coefficient of
friction fell to below resting levels; subsequent cleaning of the skin
with ether brought about a sharp increase in friction, then the
coefficient of friction gradually fell to resting levels. Thus friction
between skin and polythene is reduced if the skin is dry, greasy or
very wet, friction is increased if the skin is moist. Buchholz,
Frederick and Armstrong (1988) showed also that the coefficient of
friction decreased with increased levels of force in pad opposition, for
all moisture and material combinations tested.
From a tribology perspective, lubricants such as sweat act
differently depending on the shearing velocities between the two
contacting surfaces (Moore, 1975). At low shear velocities, it has
good adhesive qualities, useful in minimizing slip. At high shear
velocities, the lubricant reduces friction, thus reducing wear and tear
on the fingers.
Some authors (Kuno, 1934; Randall, 1947) suggested that direct
sweat gland stimulation by heat hardly ever occurs for humans under
natural circumstances. Others (e.g., Rothman, 1954) suggest that it is
not always possible to differentiate clearly between directhonnervous
glandular activity and that induced by nervous stimulation. For
grasping, it may be that both innervation of eccrine glands and direct
glandular activity due to frictional heating are critical. The motivation
to grasp may innervate selectively (through the autonomic nervous
system) those eccrine glands on the glabrous surfaces of the palm and
fingertips. Local heating and direct glandular activity are less likely to
be a sole function of thermal properties of the object-to-be-grasped,
since the skin temperature leading to sweating due to radiant heating is
so high that pain receptors would probably override. However, for
the phases of contour following and grasping objects, it is possible
that frictional forces could give rise to local heating of the skin
surfaces and systematic secretion by eccrine glands to facilitate
grasping goals. Alterations in lubrication may increase also the
adhesive properties of the grasping surface (sticky fingers),
minimizing slip. Research is needed to evaluate these hypotheses.
As summarized in Figure 6.5, there are synergistic events
occurring when fingerpads make contact with an object. The eccrine
glands on the palmar surface of the hand may be innervated centrally,
with the commands to grasp, or peripherally, with thermal or sensory
input. When the contact is made, friction is created due to the
resistance of the fingers moving along the object. This creates heat