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Performance and Motor Control Characteristics of Functional Skills

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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

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