Page 98 - Human Inspired Dexterity in Robotic Manipulation

96 Human Inspired Dexterity in Robotic Manipulation

Fig. 6.5 Schematic drawing showing the important function of the extensor hood
mechanism. Left: A simplified and geometric representation of the extensor hood.
Right: The two small tendons of the lateral bands glide off from the PIP joint during
finger flexion.

The second layer of the extensor hood is known as the central slip with
an insertion point at the base of the PIP joint. Its function is to help extend-
ing/flexing the PIP joint. One of its tendon branches is often connected to a
small intrinsic muscle, namely the lumbrical muscle. It has been reported that
the lumbricals work as flexor tendons at the MCP joints, but can help extend
the PIP and DIP joints via the extensor hood mechanism [12]. Due to its
variations in size and inserting locations, the function of the lumbricals
are not unanimously agreed on yet. So, we treat them as a part of the intrinsic
muscles and do not emphasize its uniqueness in Fig. 6.5.
In sum, the complex, web structure of the extensor hood smartly trans-
mits muscle forces to finger joints. This is an important biomechanical
advantage that we would like to copy from the human hand.

6.3.4 Tendon Sheaths

As shown in Fig. 6.6, the tendon sheaths are fibrous tissues that wrap around
the flexor tendons and have multiple insertions on the dorsal side of finger
bones. Although made of tough collagen-based tissues, these pulleys could
tear and rupture when they are continuously subject to intense flexion forces