Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 313
tendon insertions (Kapandji, 1982). For example, although each
finger has four degrees of freedom, they are not individually
controllable. The flexors tend to work together (Tubiana, 1981):
when the index finger begins to close in toward the thumb in a
precision grip, the flexion of the proximal interphalangeal joint (see
Appendix A) releases tension on the retinacular ligament, allowing
flexion of the distal interphalangeal joint. Flexion of the proximal
interphalangeal joint puts the intrinsic muscles under tension, initiating
flexion of the metacarpophalangeal joint. In another example, the
intrinsic lumbrical muscle runs from the extrinsic flexor tendon to the
extensor tendon and plays multiple roles in flexion and extension
(Ranney & Wells, 1988). It is primarily responsible for inter-
phalangeal joint extension, which it does by decreasing the flexor
force. Because of its attachment to tendons moving in opposite
directions and the great number of muscle spindles (sensors), Ranney
and Wells suggested that the lumbrical muscle monitors the rate of
hand closing during grasp.
Other aspects of the muscular system show distinguishing
workspace characteristics. An obvious feature is the increased mobility
of thumb and its placement on the palm, both of which enable its
opposability to the fingers. While the tendency is to think only of the
thumb as being unique, there are distinguishing features among the
fingers besides their differential size and contribution to force
generation. While the extrinsic extensors going to all four fingers
have a common origin in the forearm, additional extrinsic muscles to
the index and 5th fingers give these fingers more independence of
movement (Kapandji, 1982). The 5th carpometacarpal joint, with a
mobility of about 25 degrees, has a unique intrinsic muscle that
allows the palm’s shape to be modified for complying to a variety of
object shapes. The differential action of the fingers is seen in the
composite distal palmar crease (Bunnell, 1944): the most distal of
these creases (i.e, the ‘heart’ line in palm reading) is related to the
action of the middle, ring, and little fingers in opposition to the thumb;
the more proximal crease (the ‘head’ line) is related to the action of the
index, middle, and ring fingers. Bunnell also notes that the palm,
having webbing between the fingers, is broader and longer than the
back of the hand.
A final feature of the hand’s anatomy is the skin and the
underlying fatty pads. Skin consists of multiple layers and tissues,
notably the outer epidermis and inner dermis (Thomine, 1981). Palmar
skin, which is thick, tough, resistant to pressure, and good for
complying with objects, can be contrasted with dorsal skin, which is