344 Motor Control
Figure 12.17 Lifting an object. Period a is the preload phase, b the loading
phase, c the transitional phase, in which the object is actually lifted, followed
by the static or hold phase (after Johansson, 1996).studies is reported in the chapter by Proctor and Vu in this
volume.
In the experiment of Merz et al. (1981), participants con-
trolled the movement of a cursor on an oscilloscope screen by
means of lateral pressure on a knob. Their task was a tracking
task in which they had to keep the cursor aligned with a mov-
ing target. For two groups of participants the cursor moved to
the right when the knob was pressed to the right (compatible
relation), and for two groups of participants the relation was
reversed (incompatible). One group with each of the two lev-
els of compatibility had the knob placed at the bottom of a
steering wheel, while for the other two groups the steering
wheel was covered by a piece of cardboard. In the incompat-
ible conditions there was a strong practice effect. More
important, however, is that the performance deficit in the in-
compatible conditions disappeared when the steering wheel
was visible; performance in this condition was as good as in
the compatible conditions. Thus, the visibility of the steering
wheel enabled the subjects to change the incompatible rela-
tion to a compatible one in associating clockwise rotation of
the wheel, which is consequent upon leftward pressure, with
a rightward motion of the cursor on the screen.
Adjusting and Adapting to External Forces
When external forces vary, motor commands for intended
movements have to be modulated accordingly. Except for
movements with different directions relative to gravity, per-
haps the most frequent adjustments are required when we
deal with objects of different masses. Here, depending on the
mass, the kinematic characteristics vary. Specifically, with
increasing mass, peak acceleration and peak deceleration as
well as peak velocity tend to decline, while movement dura-
tion tends to increase. This is true both for lifting objects
(Gachoud, Mounoud, Hauert, & Viviani, 1983) and for mov-
ing them in a horizontal plane (Gottlieb, Corcos, & Agarwal,
1989). Thus, for objects with different masses, peak forces
are not perfectly scaled, which would result in an invariant
acceleration profile; instead, with increasing mass, accelera-
tion and deceleration become smaller, but an increasing dura-
tion serves to avoid a shortening of the amplitude of the
movements. In spite of these mass-dependent variations, the
basic shape of the velocity profile remains invariant; that is,
with the proper scaling of time and velocity, the profiles be-
come identical (Bock, 1990; Ruitenbeek, 1984).
In lifting an object, it is not only the so-called load force
which has to be adjusted to the mass (and weight) of the ob-
ject, but also the grip force (cf. Johansson, 1996, for review).
When the grip force is too weak, the object may slip; when it
is too strong, the object may break; and, in addition, too high
forces are uneconomical. Figure 12.17 shows the buildup of
both types of force when an object is lifted. First, grip force
starts to develop (preload phase), then load force (loading
phase). When the load force is sufficiently strong, the object
is lifted (transitional phase) and thereafter held in a certain
position.
During the loading phase a certain relation between grip
force and load force is established, which is generally some-
what higher than the minimal value required to prevent
the object from slipping; this safety margin is typically in the
range of 10–40%. Of course, the proper adjustment of the
grip force depends not only on the object, but also on its sur-
face characteristics. In fact, there is a delicate grip-force ad-
justment to the friction between fingers and object surface
that depends not only on the surface characteristics of the ob-
ject, but also on those of the skin, which change, for example,
after washing one’s hands.
Adjustment of grip force is required not only when an ob-
ject is lifted, but also when it is moved around, so that there
is an inertial load in addition to the gravitational load. In a
manner similar to the way load force and grip force increase
in parallel when an object is lifted, grip force is modulated in
parallel to inertial load while an object is moved (Flanagan,
Tresilian, & Wing, 1993; Flanagan & Wing, 1995). In mov-
ing an object, there is an important difference between
periodic horizontal and vertical movements. In horizontal
movements, inertial load is orthogonal to gravitational load;
inertial load reaches maxima both at the left and right move-
ment reversals, and grip force reaches maxima at these points
as well. Thus, there is a 1 : 2 ratio of the frequencies of peri-
odic movements and grip-force modulations. In contrast, for
vertical movements inertial and gravitational load add, so
that total load is particularly strong at the lower movement