This paper proposes a novel and simple method
to compute all possible solutions of the inverse kinematics
problem of the five-oblique-axis thumb model with intersecting
axes at the metacarpophalangeal joint. This thumb model is
one of the suggested results by a magnetic-resonance-imaging-
based study that, in contrast to those based on cadaver fingers
or on the tracking of the surface of the fingers, takes into
account muscle and ligament behaviors and avoids inaccuracies
resulting from the movement of the skin with respect to the
bones. The proposed distance-based inverse kinematics method
eliminates the use of arbitrary reference frames as is usually
required by standard approaches; this is relevant because the
numerical conditioning of the resulting system of equations
with such traditional approaches depends on the selected
reference frames. Moreover, contrary to other parametrizations
(e.g., Denavit-Hartenberg parameters), the suggested distance-
based parameters for the thumb have a natural, human-
understandable geometric meaning that makes them easier to
be determined from any posture. These characteristics make
the proposed approach of interest for those working in, for
instance, measuring and modeling the movement of the human
hand, developing rehabilitation devices such as orthoses and
prostheses, or designing anthropomorphic robotic hands.