Chapter 2. Prehension

116 THE PHASES OF PREHENSION
determination
I in extrinsic
desired trajectory in
intrinsic reference frame
directly from goal of
meit J
+ Step 3*
generation
of motor command -
(
r
motor command (e.g., muscle activity, joint torque) 1
Figure 5.3. Computational model of voluntary movement. Starting
from a desired goal, a trajectory is computed in an extrinsic
coordinate frame (step 1). The trajectory is transformed into an
intrinsic coordinate frame (step 2), which in turn is used to
generate motor commands (step 3). Alternatively, motor commands
can be generated directly from the extrinsically defined trajectory
(step 4), or even from the goal itself (step 5). Another alternative
would be to plan the trajectory directly in joint space (step 1’)
(from Uno, Kawato and Suzuki, 1989; adapted with permission).
involving a sequence of hand positions, and a speed or time
sequence of the movement along the path. This is step 1 in
Figure 5.3
2) Dordinates tra nsformation - the desired trajectory is translated
into an intrinsic kinematic coordinate frame of body coordinate
commands, such as joint angles . This is step 2 in Figure 5.3.
3) generation of motor command - body coordinate commands are
translated into an intrinsic dynamic coordinate frame of muscle
torque commands that can coordinate the activity of many
muscles. This is step 3 in Figure 5.3.