Click underlined text for links
George E. Carvell, PhD, PT   University of Pittsburgh
 
Motor Control Strategies Differ According to Task Demands
ALL MOVEMENTS 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). 
52.3 %
49.1 %
% AT
205 (33.4)
239 (30.9)
Deceleration time (msec)*
225 (22.6)
230 (25.5)
Acceleration time (msec)
1471 (344)
1444 (281)
Velocity (mm/sec)
430 (49.4)
469 (61.8)
Movement time (msec)**
Tennis Ball
Lightbulb
** p < .005
* P < .05
Table 4 Task Dependent Grasping: mean (SD)
Brooks2 Fig 1 p 16
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
Note longer movement time when grasping lightbulb. Most of the extra time is due to decel-eration of the hand.
WHEN IT PAYS TO GO BALLISTIC