Chapter 2. Prehension

104 THE PHASES OF PREHENSION
OBJECT ANGLES
Figure 4.17 Percent of trials with pronation of the forearm or
not, for each object orientation. Note when the object is oriented
parallel to the frontal plane of the body, there is no pronation, and
the palm is oriented away from the body. When the object is
rotated beyond 90 degrees, there is a transition to an entirely
different arm configuration, with glenohumeral rotation and
forearm pronation, orienting the palm towards the body (from
Stelmach, Castiello & Jeannerod, 1993; reprinted by permission).
from 70 degrees, the orientation of the grasping surfaces of the finger
pads rotated correspondingly, with the thumb closer to the midline
than the index finger. At some point, when the object was rotated be-
yond an orientation parallel to the frontal plane (or beyond perpendicu-
lar to the midline sagittal plane), there was a major realignment of the
arm, whereby the forearm pronated and there was rotation about the
glenohumeral joint (abduction and internal rotation). With this
reorientation of the palm so that it faced the midline, the relative
positions of the finger and thumb were switched, i.e., the index finger
was closer to the midline that the thumb. Summarized in Figure 4.17,
across all trials, for object angles of 70, 80, or 90 degress, almost all
grasps were executed without forearm realignment. At 100 degrees,
approximately half the trials demonstrated some forearm pronation and
an altered orientation of the palm of the hand for grasping with the
finger pads. For 110 and 120 degree object orientations, all subjects
systematically demonstrated this strong pronation of the forearm, with
the palm oriented towards the body. An examination of the kinematics