Performance and Motor Control Characteristics of Functional Skills

1
Chapter 7

• Performance and Motor Control Characteristics of
  Functional Skills

Concept Specific characteristics of the
performance of various motor skills provide the
basis for much of our understanding of motor
control
2
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

3
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 IDs for
  manual aiming tasks, and predicted MTs

4
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

5
Speed-Accuracy Skills Motor Control Processes

• General agreement that two motor control
  processes underlie performance of speed-accuracy
  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

6
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)

7
Relationship ofPrehension 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

8
Relationship ofPrehension Components, contd

• Research demonstrating temporal relationship of
  reach and grasp
• Goodale and colleagues (1991, 2005) showed
• Objects size influenced
• Timing of maximum grip aperture
• Velocity profile of hand transport movement
• Regardless of objects 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

9
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
  time-to-contact info to initiate grasp
• Peripheral vision provides hand movement feedback
• Grasp of object
• Supplements tactile and proprioceptive feedback
  to ensure intended use achieved

10
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

11
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 individuals 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

12
Handwriting, contd

• 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

13
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

14
Bimanual Coordination Skills, contd

• 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

15
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

16
Locomotion, contd

• 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. Parkinsons Disease)
• Another important motor control characteristic of
  locomotion
• Head stability
• Consider why and implications of head stability
  problems

17
Locomotion, contd

• 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

18
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

19
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

20
Catching a Moving Object, contd

• Amount of visual contact time needed to catch a
  moving object
• Two critical time periods
• Initial flight portion
• Just prior to hand contact
• Between the two critical periods
• Brief, intermittent visual snapshots sufficient

Specific amounts of time not known
21
Catching a Moving Object, contd

• 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 persons capability
  to rely on proprioceptive feedback to position
  hands to catch an object

22
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