Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 309
grasps (Hazelton et al., 1975). Because the extrinsic muscles send
tendons through the wrist, the amount of force that the fingers are
able to apply is greater in wrist extension and/or ulnar deviation than
in other positions. Hazelton et al. (1975) noted that the relative
amount of force available at each finger during a hook grip remains
constant across wrist positions (25% at the index, 33.5% at the long
finger, 25% at the ring finger, and 16.5% at the little finger). Amis
(1987) found the mean contributions to be 30%, 30%, 22%, and 18%
to the overall grasp force. The lesson here (and further detailed in
Section 8.2) is that there is a differential in the individual finger's
potential contribution to force application, thus a factor in choice of
postures.
Skin has properties that affect the frictional component of force
generation. Typically, the law of static friction (Amonton's law)
states that the tangential force, FT, of friction is constrained to be no
greater than the product of the normal force N with the coefficient of
static friction, ps, or FT I ps N (thus creating the cone of friction
mentioned above). The coefficient of static friction, measured by
various researchers (Buchholz et al., 1988; Comaish & Bottoms,
1971; Westling & Johansson, 1984), involves an interaction between
the constraints of the skin surface and object surface (see below for
discussion of object properties). Comaish and Bottoms (1971) found
that the hands palmar surface has a higher coefficient of static friction
than the back of the hand. Buchholz, Frederick and Armstrong (1988)
show that the coefficient of static friction increases when moisture is
present, a situation that occurs particularly when the sweat glands of
the palm are activated. Comaish and Bottoms also found that with
some surfaces, the coefficient of static friction increased as the contact
area increased (which would imply that Amonton's law is not the best
model to use here, because it says the coefficient of static friction is
dependent on the magnitude of the forces, not the amount of
contacting area). Interestingly, the coefficient of static friction
decreases with increasing load, reducing the effectiveness of friction
for heavier objects (Buchholz et al., 1988; Comaish & Bottoms,
197 1).
The frictional component of force generation is further enhanced
when the pulps on the distal palmar surface of the fingers are used.
These are highly specialized for prehension in that they provide
friction, due to the epidermal ridges and to the sticky self-lubricating
excretions through the tops of the ridges, and that they to comply with
(instead of being displaced by) objects touched in the environment
(Glicenstein & Dardour, 198 1; Quilliam, 1978; Thomine, 198 1).