Chapter 2. Prehension

270 THE PHASES OF PREHENSION
In classifying human hand movements, Elliott and Connolly
(1984) identified 11 different dynamic movements. In order to
describe the movements, the hand reference frame shown in Chapter 2
is recalled, as seen in Figure 6.22. Three axes are aligned with the
palm of the hand, along anatomical directions. Oppositions occur
along these axes. The pinch is a term used by Elliott and Connolly to
describe movements towards and away from the palm, such as
removing a splinter or manipulating a thread. As seen in Table 6.6,
the movement translates the opposition vector of the object along the z
(ventro-dorsal) direction of the hand while the object is grasped in pad
opposition. The dynamic tripod, used for writing, combines pad and
side opposition and translates the opposition vector mostly along the x
(distal-proximal) axis. However, the side opposition between the
I:::’ ’ vy
Figure 6.22 Hand coordinate frame for describing dynamic
movements (from two perspectives). The x axis parallels the
length of the palm in a distal-proximal direction. The y axis
traverses the width of the palm in the radio-ulnar direction. The z
axis is perpendicular to the palm in a ventro-dorsal direction. Pad
opposition occurs along the x axis, side opposition occurs along
the y axis, and palm opposition occurs along the z axis.
thumb and radial side of the middle finger adds the ability to translate
the opposition vector along the y (radieulnar) axis. As can be seen in
the table, rock, twiddle, and radial roll are all rotations of the
opposition vector around the hand’s z axis. The difference between
them is in the oppositions being used, the relative motions of the
fingers, and the length of the opposition vector. The radial roll is for