1. Performance and Motor Control Characteristics of Functional Skills
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 ID’s 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 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

8. 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”

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 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

12. 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

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, 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

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, 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

17. 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

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, cont’d
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, 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

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