322 Motor Control
Figure 12.5 Example recording of a rapid finger flexion.as they can, several of them will gradually replace move-
ments of the finger with movements of the wrist. Less inci-
dentally, Lippold, Redfearn, and Vucˇo (1960) describe what
they call “migration of activity” from one muscle to other
ones during prolonged activity that induces muscular fatigue.
More generally, the many-to-few mapping of the motor trans-
formation leaves the option to select different subsets from
the many input dimensions when some of them are function-
ally impaired, be it a fatigued muscle or an immobilized joint.
MOTOR PREPARATION
The initiation of a movement is a gradual and continuous
process. In Figure 12.5 an example recording of a rapid
index-finger flexion of about 20° amplitude is shown, as are
in particular the position-time curve, the velocity-time curve,
the acceleration-time curve, and the EMG of a finger flexor
(agonist) and an extensor (antagonist). Faced with such
recordings, it becomes somewhat difficult to answer the
question of when the movement starts. Typically the start of a
movement is defined in terms of a threshold for one of the
kinematic signals. From Figure 12.5 it is apparent that defin-
itions based on the acceleration signal generally lead to
earlier initiation times than definitions based on the position
signal: There can be a sizeable acceleration while position
has hardly changed. Thus, any definition of the start of a
movement is to some degree arbitrary.
Muscle activity can be observed in advance of changes of
kinematic signals, and the definition of the start of a movement
can also be based on EMG traces. In many instances the agonist
burst is over before a change of position can be seen. Thus, it is
not too remarkable that the agonist burst is hardly or not at all
affected when the overt movement is unexpectedly blocked.
More remarkable is that the later bursts, which normally
serve to decelerate the limb and to stabilize the end-position,
still occur, although they serve no obvious purpose any more
(Wadman, Denier van der Gon, Geuze, & Mol, 1979).
The overt movement is preceded not only by muscle activ-
ity, but also by various kinds of preparatory processes which
can be evidenced at different levels of the motor system (see
Brunia, 1999, and Brunia, Haagh, & Scheirs, 1985, for
overviews of psychophysiological findings). For example, in
the electroencephalogram, movement-related activity can be
seen when the start of the movement is used as a trigger for av-
eraging. Even such simple voluntary movements as key-
presses are preceded by a slowly increasing negativity that
starts in the order of 1 s before the overt movement. This readi-
ness potential orBereitschaftspotentialwas first described by
Kornhuber and Deecke (1965). Initially it is symmetrical, but
in the last 100 or 200 ms it becomes asymmetrical, being
stronger over the hemisphere contralateral to the responding
hand. This kind of asymmetry can also be observed in reac-
tion-time tasks. Called the lateralized readiness potential, it
has become an important tool in information-processing re-
search (see chapter by Proctor & Vu in this volume).