Chapter 7
Performance and Motor Control
Characteristics of Functional
Concept: Specific characteristics
Skills of the performance of
various motor skills provide the basis for much of our
understanding of motor control
Speed-Accuracy Skills
When both speed and accuracy are
essential to perform the skill, this is called
speed-accuracy trade-off
When speed is emphasized, accuracy is
reduced and vice-versa
Speed-Accuracy Skills:
Fitts’ Law
Paul Fitts (1954) showed we could
mathematically predict movement time for speed
– accuracy skills
– If we know the spatial dimensions of two variables:
Movement distance
Target size
MT = a + b log2 (2D/W)
– Also demonstrated that an index of difficulty could be
calculated based on this equation: log2 (2D/W)
See Fig. 7.1 for examples of different ID’s for
manual aiming tasks, and predicted MTs
Application of Fitts’ Law to
Non-Laboratory Skills
Research has demonstrated that Fitts’ Law
predicts MT for various non-laboratory
motor skills, e.g.
– Dart throwing
– Peg-board manipulation task
Used in physical rehab assessment and training
– Reaching and grasping containers of different
sizes
– Moving a cursor on a computer screen
Speed-Accuracy Skills: Motor
Control Processes
General agreement that two motor control
processes underlie performance of speedaccuracy skills:
1. Open-loop control – At movement initiation
Initial movement instructions sufficient to move limb to the
vicinity of the target
2. Closed-loop control – At movement termination
Feedback from vision and proprioception needed at end of
movement to ensure hitting target accurately
Prehension
General term for actions involving reaching for
and grasping of objects
Three components
– Transport
Movement of the hand to the object
– Grasp
The hand taking hold of the object
– Object manipulation
The hand carrying out the intended use for the object (e.g.
drinking from it, moving it to another location)
Relationship of
Prehension Components
Important motor control question concerns the
spatial – temporal relationship between the
transport and grasp components
Initial views proposed the independence of the
components
Recent evidence shows
– strong temporal relationship
– the components interact synergistically
Relationship of
Prehension Components, cont’d
Research demonstrating temporal relationship of reach
and grasp
Goodale and colleagues (1991, 2005) showed:
Object’s size influenced
– Timing of maximum grip aperture
– Velocity profile of hand transport movement
Regardless of object’s size or distance
– Max. grip aperture (point of beginning of hand closure for
grasp) occurs at 2/3 movement time
Other research shows the relationship of movement
kinematics for prehension components exemplify
characteristics of a “coordinative structure”
Role of Vision in Prehension
Preparation and initiation of movement
– Assesses regulatory conditions
Transport of hand to object
– Central vision directs hand to object – provides timeto-contact info to initiate grasp
– Peripheral vision provides hand movement feedback
Grasp of object
– Supplements tactile and proprioceptive feedback to
ensure intended use achieved
Prehension and Fitts’ Law
Prehension demonstrates speed-accuracy
trade-off characteristics predicted by Fitts’ law
– Object width = Target width
Index of difficulty for grasping containers of
different sizes and quantities of liquid
– Developed by Latash & Jaric (2002)
– Critical component is % of fullness
– Ratio of mug size and liquid level
Handwriting
Different control mechanisms are involved with
what people write and how they write
People demonstrate much individual variation in
terms of limb segment involvement
Each individual’s motor control of handwriting
demonstrates “motor equivalence”
– Person can adapt to various context demands (e.g.,
write on different surfaces, write large or small)
Handwriting motor control demonstrates
characteristics of a coordinative structure
Handwriting, cont’d
Vision provides important info for the motor control
of handwriting
Write on a piece of paper:
– I like to sit and read books
Write the same sentence with your eyes closed
How do the similarities and differences with
eyes open and closed demonstrate the role
vision plays in the control of handwriting?
– See the experiment by Smyth & Silvers (1987) –
Results in Fig. 7.3
Bimanual Coordination Skills
Motor skills that require simultaneous use
of two arms
Skill may require two arms to move with
the same or different spatial and/or
temporal characteristics
– Symmetric bimanual coordination
– Asymmetric bimanual coordination
Bimanual Coordination Skills,
cont’d
Motor control characteristic: The two arms prefer to
perform symmetrically
– Demonstrates why it is difficult to rub your stomach and pat
your head at the same time, or draw a circle with one hand while
drawing a straight line with the other hand
Research demonstrations of temporal and spatial
coupling of the two arms
– Simple discrete skill: Classic experiment by Kelso, Southard, &
Goodman (1979) – See Fig. 7.4
– More complex discrete skill: Swinnen et al. (1990)
With practice, a person can learn to disassociate the
two limbs to perform an asymmetric bimanual skill
Locomotion
Central pattern generators (CPG) in the spinal
cord involved in the control of locomotion (i.e.
gait)
– Provide basis for stereotypic rhythmicity of walking
and running gait patterns
– But, proprioceptive feedback from muscle spindles
and GTOs also influence gait
Locomotion, cont’d
Rhythmic structure of locomotion
– Components of a step cycle (discussed in ch.5 in experiment by
Shapiro et al.)
– Rhythmic relationship between arms and legs
– Pelvis and thorax relationship during walking
Practical benefit of analyzing rhythmic structure of gait
patterns
– Allows for assessment of coordination problems of trunk and legs
(e.g. Parkinson’s Disease)
Another important motor control characteristic of
locomotion
– Head stability
– Consider why and implications of head stability problems
Locomotion, cont’d
Spontaneous gait transitions
– An important motor control characteristic of
locomotion (Initially discussed in ch.5)
– People spontaneously change from walking to
running gait (and vice-versa) at critical speed (specific
speed varies across people)
Why do spontaneous gait transitions occur?
– Various hypotheses
Most popular: Minimize metabolic energy use (i.e., VO2)
Some agreement that no one factor responsible
Locomotion and Vision
When we walk or run, vision is important to enable us to
contact objects and avoid contact with objects
Contacting objects
– Experiment by Lee et al. (1982) showed long-jumpers use tau as
basis for contacting take-off board accurately [See Fig. 7.5]
Avoiding contact with objects
– Vision provides advance info to determine how to avoid contact –
step over, around, etc.
– Vision provides body-scaled info to determine how to walk
through a door, or step on a step
Catching a Moving Object
Three phases
– Initial positioning of arm and hand
– Shaping of hand and fingers
– Grasping the object
Movement analysis evidence of the three phases –
Experiment by Williams & McCrirrie (1988)
– Figure 7.7 – Illustrates movement characteristics related to % ball
flight time
– Notable finding (not in figure) – Successful ball catchers initiated
final hand and finger shaping 80 msec earlier than non-catchers
Describe what you think are the roles of tactile,
proprioceptive, and visual information in the stages of
catching a moving object
Catching a Moving Object,
cont’d
Amount of visual contact time needed to catch a
moving object
Specific
Two critical time periods
– Initial flight portion
– Just prior to hand contact
amounts of time
not known
Between the two critical periods
– Brief, intermittent visual snapshots sufficient
Catching a Moving Object,
cont’d
Is vision of the hands necessary to catch a moving
object?
Key factor in answer is amount of experience
– Inexperienced – Yes
– Experienced – No
Describe how experience with using vision to
catch an object influences a person’s capability to
rely on proprioceptive feedback to position hands
to catch an object
Striking a Moving Object
Ball speed effect
– Skilled ”strikers” demonstrate similar ”bat”
movement time for all ball speeds, change
amount of time before initiating bat movement
Visual contact with moving ball
– Skilled ”strikers” do not maintain visual
contact with ball throughout ball flight but
visually ”jump” from early flight to predicted
location in area to strike ball