Carvell, PhD, PTUniversity
Motor Control Strategies Differ According to Task Demands
ARE NOT CREATED EQUAL.
Goal-directed movements obey certain
rules related to speed, accuracy, amplitude, load, behavioral context, and environmental
constraints/affordances. Woodworth (see fig 1) described the relationship
between accuracy and speed. He demonstrated
that the best accuracy occurs when eyes are open and movements are slow.
Closing the eyes increases error that
is relatively constant at all speeds (proprioception equivalent at any speed?).
If the subjects did not attend to the task, performance decreases for all but
the fastest movements (all conditions have accuracy asymptote at fastest
speed). Fitts and
others have confirmed the speed-accuracy tradeoff and have described
differences for small vs. large movements: to increase accuracy move slower or
reduce range. Talent, motor learning, and practice improve performance at any
speed appropriate for
the task (see fig 7-4). Fast, accurate reaching is accomplished with an almost
linear trajectory characterized by
a single velocity peak. However, the task at hand alters the velocity profile.
Pointing to an object vs. grasping an object have similar velocity peaks but the
deceleration phase is much shorter when contact with object is not required;
this results in a
shorter movement time (MT). Likewise, reaching to grasp a tennis ball vs. a
lightbulb alters deceleration and MT(see table 4). Placing one's fingers
around the thin glass requires greater care (longer deceleration phase).
Deceleration time (msec)*
Acceleration time (msec)
Movement time (msec)**
** p < .005
* P < .05
Table 4 Task
Dependent Grasping: mean (SD)
Brooks2 Fig 1 p
Fig 7-4 In: L.M. Stallings, Motor
Learning from Theory
to Practice, St Louis, CV Mosby, 1982
Fig 7-4 p 164
Table 4 adapted
from Marteniuk p 372
longer movement time when grasping lightbulb. Most of the extra time is due to decel-eration of the hand.