The human hand consists of four fingers and one thumb.
All the fingers have the same structure, which consists of
three joints and three phalanxes. The design presented here
is based on the index finger, but it can be easily adapted to
the other three fingers as well. A simplified schematic of the
index finger is shown in Fig 1. The three joints are the
metacarpophalangeal (MCP) joint, the proximal
interphalangeal (PIP) joint and the distal interphalangeal
(DIP) joint. The MCP joint links the metacarpal bone and
the proximal phalanx; it has two degrees of freedom (DOF)
since it allows both the abduction/adduction and the
flexion/extension motion. The PIP joint links the proximal
and the medial phalanxes, while the DIP joint links the
medial and the distal phalanxes; each of them has one DOF,
which only allows flexion/extension. The flexion/extension
range of motion for the index finger is greater at the PIP
joint (from 100 to 110 deg) than it is at the DIP joint
(typically around 80 deg). Moreover the DIP joint may have
some passive hyperextension (around 15 deg), but the PIP
joint has essentially none in most individuals [12]