Chapter 2. Prehension

Chapter 5 - Movement Before Contact 153
Figure 5.17. Finger and thumb joint adjustments during rapid
pinch movements. On the left, the positions of the tip of the
finger and thumb at contact (circles) for 3 trials (a,b,c) illustrating
the covariable relation of the finger and thumb paths. On the
right, a schematic shows configurations of the thumb interphal-
angeal joint (TH) and the finger proximal interphalangeal (PIP)
and metacarpophalangeal (MP) joints. Top right figure shows a
typical configuration at contact. Middle figure shows hypothetical
configuration (solid lines) where pads miss making contact when
only TH and MP flex. Bottom figure shows observed results of
pads making contact because PIP and MP reciprocally adjust to TH
flexion (from Cole & Abbs, 1986; reprinted by permission).
1981), and task (Arbib et al., 1985), what the goal of the enclose mo-
tion is is still unclear. Insights into this question can be gained by
studying recent work in the area of rapid pinching tasks. Cole and
Abbs (1986) asked subjects to perform rapid pinch movements in or-
der to produce a specified finger-thumb contact force, as seen in
Figure 5.17. Only movement at the interphalangeal (IP) joint of the
thumb (TH) and at the proximal interphalangeal (PIP) and metacar-
pophalangeal (MP) joints were allowed due to constraints placed on
the hand. Subjects repeated the task and maintained the contact force
within the prescribed range, and while doing so, consistently brought