Chapter 7Chapter 7

Performance and Motor Control Performance and Motor Control Characteristics of Functional Characteristics of Functional

SkillsSkillsConcept: Specific characteristics of the performance of various motor skills provide the basis for much of our understanding of motor control

Speed-Accuracy SkillsSpeed-Accuracy Skills

When both speed and accuracy are When both speed and accuracy are essential to perform the skill, this is called essential to perform the skill, this is called speed-accuracy trade-offspeed-accuracy trade-off

When speed is emphasized, accuracy is When speed is emphasized, accuracy is reduced and vice-versareduced and vice-versa

Speed-Accuracy Skills: Speed-Accuracy Skills: Fitts’ LawFitts’ Law

Paul Fitts (1954) showed we could Paul Fitts (1954) showed we could mathematically predict movement time for speed mathematically predict movement time for speed – – accuracy skillsaccuracy skills

–If we know the spatial dimensions of two variables:If we know the spatial dimensions of two variables:Movement distanceMovement distance

Target sizeTarget size

MT = a + b logMT = a + b log22 (2D/W) (2D/W)–Also demonstrated that an index of difficulty could be Also demonstrated that an index of difficulty could be

calculated based on this equation: calculated based on this equation: loglog22 (2D/W) (2D/W)

See Fig. 7.1 for examples of different ID’s for See Fig. 7.1 for examples of different ID’s for manual aiming tasks, and predicted MTsmanual aiming tasks, and predicted MTs

Application of Fitts’ Law to Application of Fitts’ Law to Non-Laboratory SkillsNon-Laboratory Skills

Research has demonstrated that Fitts’ Law Research has demonstrated that Fitts’ Law predicts MT for various non-laboratory predicts MT for various non-laboratory motor skills, e.g.motor skills, e.g.

–Dart throwingDart throwing–Peg-board manipulation task Peg-board manipulation task

Used in physical rehab assessment and trainingUsed in physical rehab assessment and training

–Reaching and grasping containers of Reaching and grasping containers of different sizesdifferent sizes

–Moving a cursor on a computer screenMoving a cursor on a computer screen

Speed-Accuracy Skills: Motor Speed-Accuracy Skills: Motor Control Processes Control Processes

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

1. Open-loop control1. Open-loop control – At movement initiation – At movement initiationInitial movement instructions sufficient to move limb to the Initial movement instructions sufficient to move limb to the

vicinity of the targetvicinity of the target

2. Closed-loop control2. Closed-loop control – At movement termination – At movement terminationFeedback from vision and proprioception needed at end of Feedback from vision and proprioception needed at end of

movement to ensure hitting target accuratelymovement to ensure hitting target accurately

PrehensionPrehension

General term for actions involving reaching for General term for actions involving reaching for and grasping of objectsand grasping of objectsThree componentsThree components

–TransportTransportMovement of the hand to the objectMovement of the hand to the object

–GraspGraspThe hand taking hold of the objectThe hand taking hold of the object

–Object manipulationObject manipulationThe hand carrying out the intended use for the object (e.g. The hand carrying out the intended use for the object (e.g.

drinking from it, moving it to another location) drinking from it, moving it to another location)

Relationship ofRelationship ofPrehension ComponentsPrehension Components

Important motor control question concerns the Important motor control question concerns the spatial – temporal relationship between the spatial – temporal relationship between the transport and grasp componentstransport and grasp components

Initial views proposed the independence of the Initial views proposed the independence of the componentscomponents

Recent evidence shows Recent evidence shows –strong temporal relationship strong temporal relationship –the components interact synergisticallythe components interact synergistically

Relationship ofRelationship ofPrehension Components, cont’dPrehension Components, cont’dResearch demonstrating temporal relationship of reach Research demonstrating temporal relationship of reach

and grasp and grasp

Goodale and colleagues (1991, 2005) showed:Goodale and colleagues (1991, 2005) showed:

Object’s size influencedObject’s size influenced–Timing of maximum grip apertureTiming of maximum grip aperture–Velocity profile of hand transport movementVelocity profile of hand transport movement

Regardless of object’s size or distanceRegardless of object’s size or distance–Max. grip aperture (point of beginning of hand closure for Max. grip aperture (point of beginning of hand closure for

grasp) occurs at 2/3 movement timegrasp) occurs at 2/3 movement time

Other research shows the relationship of movement Other research shows the relationship of movement kinematics for prehension components exemplify kinematics for prehension components exemplify characteristics of a “coordinative structure”characteristics of a “coordinative structure”

Role of Vision in PrehensionRole of Vision in Prehension

Preparation and initiation of movementPreparation and initiation of movement–Assesses regulatory conditions Assesses regulatory conditions

Transport of hand to objectTransport of hand to object–Central vision directs hand to object – provides Central vision directs hand to object – provides

time-to-contact info to initiate grasptime-to-contact info to initiate grasp–Peripheral vision provides hand movement Peripheral vision provides hand movement

feedbackfeedback

Grasp of objectGrasp of object–Supplements tactile and proprioceptive feedback to Supplements tactile and proprioceptive feedback to

ensure intended use achievedensure intended use achieved

Prehension and Fitts’ LawPrehension and Fitts’ Law

Prehension demonstrates speed-accuracy Prehension demonstrates speed-accuracy trade-off characteristics predicted by Fitts’ lawtrade-off characteristics predicted by Fitts’ law

–Object width = Target widthObject width = Target width

Index of difficulty for grasping containers of Index of difficulty for grasping containers of different sizes and quantities of liquiddifferent sizes and quantities of liquid

–Developed by Latash & Jaric (2002)Developed by Latash & Jaric (2002)–Critical component is % of fullnessCritical component is % of fullness–Ratio of mug size and liquid levelRatio of mug size and liquid level

HandwritingHandwriting

Different control mechanisms are involved with Different control mechanisms are involved with what people write and how they writewhat people write and how they write

People demonstrate much individual variation in People demonstrate much individual variation in terms of limb segment involvementterms of limb segment involvement

Each individual’s motor control of handwriting Each individual’s motor control of handwriting demonstrates “motor equivalence”demonstrates “motor equivalence”

–Person can adapt to various context demands (e.g., Person can adapt to various context demands (e.g., write on different surfaces, write large or small)write on different surfaces, write large or small)

Handwriting motor control demonstrates Handwriting motor control demonstrates characteristics of a coordinative structurecharacteristics of a coordinative structure

Handwriting, cont’dHandwriting, cont’d

Vision provides important info for the motor Vision provides important info for the motor control of handwritingcontrol of handwriting

Write on a piece of paper: Write on a piece of paper: –I like to sit and read booksI like to sit and read books

Write the same sentence with your eyes closedWrite the same sentence with your eyes closed

How do the similarities and differences with How do the similarities and differences with eyes open and closed demonstrate the role eyes open and closed demonstrate the role vision plays in the control of handwriting?vision plays in the control of handwriting?

–See the experiment by Smyth & Silvers (1987) – See the experiment by Smyth & Silvers (1987) – Results in Fig. 7.3Results in Fig. 7.3

Bimanual Coordination SkillsBimanual Coordination Skills

Motor skills that require simultaneous use Motor skills that require simultaneous use of two armsof two arms

Skill may require two arms to move with Skill may require two arms to move with the same or different spatial and/or the same or different spatial and/or temporal characteristicstemporal characteristics

–Symmetric bimanual coordinationSymmetric bimanual coordination–Asymmetric bimanual coordinationAsymmetric bimanual coordination

Bimanual Coordination Skills, Bimanual Coordination Skills, cont’dcont’d

Motor control characteristic:Motor control characteristic: The two arms prefer to The two arms prefer to perform symmetricallyperform symmetrically

–Demonstrates why it is difficult to rub your stomach and pat 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 your head at the same time, or draw a circle with one hand while drawing a straight line with the other hand drawing a straight line with the other hand

Research demonstrations of temporal and spatial Research demonstrations of temporal and spatial coupling of the two arms coupling of the two arms

–Simple discrete skill: Classic experiment by Kelso, Southard, Simple discrete skill: Classic experiment by Kelso, Southard, & Goodman (1979) – See Fig. 7.4& Goodman (1979) – See Fig. 7.4

–More complex discrete skill: Swinnen et al. (1990) More complex discrete skill: Swinnen et al. (1990)

With practice, a person can learn to disassociate the With practice, a person can learn to disassociate the two limbs to perform an asymmetric bimanual skilltwo limbs to perform an asymmetric bimanual skill

LocomotionLocomotion

Central pattern generators (CPG) in the spinal Central pattern generators (CPG) in the spinal cord involved in the control of locomotion (cord involved in the control of locomotion ( i.ei.e. . gait)gait)

–Provide basis for stereotypic rhythmicity of walking Provide basis for stereotypic rhythmicity of walking and running gait patternsand running gait patterns

–But, proprioceptive feedback from muscle spindles But, proprioceptive feedback from muscle spindles and GTOs also influence gaitand GTOs also influence gait

Locomotion, Locomotion, cont’dcont’d

Rhythmic structure of locomotionRhythmic structure of locomotion–Components of a step cycle (discussed in ch.5 in experiment by Components of a step cycle (discussed in ch.5 in experiment by

Shapiro et al.)Shapiro et al.)–Rhythmic relationship between arms and legsRhythmic relationship between arms and legs–Pelvis and thorax relationship during walkingPelvis and thorax relationship during walking

Practical benefit of analyzing rhythmic structure of gait Practical benefit of analyzing rhythmic structure of gait patterns patterns

–Allows for assessment of coordination problems of trunk and Allows for assessment of coordination problems of trunk and legs (e.g. Parkinson’s Disease)legs (e.g. Parkinson’s Disease)

Another important motor control characteristic of Another important motor control characteristic of locomotionlocomotion

–Head stabilityHead stability–Consider why and implications of head stability problemsConsider why and implications of head stability problems

Locomotion, Locomotion, cont’dcont’d

Spontaneous gait transitionsSpontaneous gait transitions–An important motor control characteristic of An important motor control characteristic of

locomotion (Initially discussed in ch.5)locomotion (Initially discussed in ch.5)–People spontaneously change from walking to People spontaneously change from walking to

running gait (and vice-versa) at critical speed (specific running gait (and vice-versa) at critical speed (specific speed varies across people)speed varies across people)

Why do spontaneous gait transitions occur?Why do spontaneous gait transitions occur?–Various hypothesesVarious hypotheses

Most popular: Minimize metabolic energy use (i.e., VOMost popular: Minimize metabolic energy use (i.e., VO22))

Some agreement that no one factor responsibleSome agreement that no one factor responsible

Locomotion and VisionLocomotion and Vision

When we walk or run, vision is important to enable us to When we walk or run, vision is important to enable us to contact objects and avoid contact with objectscontact objects and avoid contact with objects

Contacting objectsContacting objects–Experiment by Lee et al. (1982) showed long-jumpers use tau Experiment by Lee et al. (1982) showed long-jumpers use tau

as basis for contacting take-off board accurately as basis for contacting take-off board accurately [[See Fig. 7.5See Fig. 7.5]]

Avoiding contact with objectsAvoiding contact with objects–Vision provides advance info to determine how to avoid contact Vision provides advance info to determine how to avoid contact

– step over, around, etc.– step over, around, etc.–Vision provides body-scaled info to determine how to walk Vision provides body-scaled info to determine how to walk

through a door, or step on a stepthrough a door, or step on a step

Catching a Moving ObjectCatching a Moving ObjectThree phasesThree phases

–Initial positioning of arm and handInitial positioning of arm and hand–Shaping of hand and fingers Shaping of hand and fingers –Grasping the objectGrasping the object

Movement analysis evidence of the three phases – Movement analysis evidence of the three phases – Experiment by Williams & McCrirrie (1988)Experiment by Williams & McCrirrie (1988)

–Figure 7.7 – Illustrates movement characteristics related to % Figure 7.7 – Illustrates movement characteristics related to % ball flight timeball flight time

–Notable finding (not in figure) – Successful ball catchers initiated Notable finding (not in figure) – Successful ball catchers initiated final hand and finger shaping 80 msec earlier than non-catchersfinal hand and finger shaping 80 msec earlier than non-catchers

Describe what you think are the roles of tactile, Describe what you think are the roles of tactile, proprioceptive, and visual information in the stages of proprioceptive, and visual information in the stages of catching a moving objectcatching a moving object

Catching a Moving Object, Catching a Moving Object, cont’dcont’d

Amount of visual contact time needed to catch a Amount of visual contact time needed to catch a moving objectmoving object

Two critical time periodsTwo critical time periods–Initial flight portionInitial flight portion–Just prior to hand contactJust prior to hand contact

Between the two critical periodsBetween the two critical periods–Brief, intermittent visual snapshots sufficientBrief, intermittent visual snapshots sufficient

Specific amounts of time not known

Catching a Moving Object, Catching a Moving Object, cont’dcont’d

Is vision of the hands necessary to catch a moving Is vision of the hands necessary to catch a moving object?object?

Key factor in answer is amount of experienceKey factor in answer is amount of experience–Inexperienced – YesInexperienced – Yes–Experienced – NoExperienced – No

Describe how experience with using vision to Describe how experience with using vision to catch an object influences a person’s capability to catch an object influences a person’s capability to rely on proprioceptive feedback to position hands rely on proprioceptive feedback to position hands to catch an objectto catch an object

Striking a Moving ObjectStriking a Moving Object

Ball speed effectBall speed effect–Skilled ”strikers” demonstrate similar ”bat” Skilled ”strikers” demonstrate similar ”bat”

movement time for all ball speeds, change movement time for all ball speeds, change amount of time before initiating bat movementamount of time before initiating bat movement

Visual contact with moving ballVisual contact with moving ball–Skilled ”strikers” do not maintain visual Skilled ”strikers” do not maintain visual

contact with ball throughout ball flight but contact with ball throughout ball flight but visually ”jump” from early flight to predicted visually ”jump” from early flight to predicted location in area to strike ball location in area to strike ball