Principles of Motor Control and Movement Accuracy
- Planning a movement while controlling speed and accuracy.
- Ballistic phase more important in tennis than in doing laundry correcting
trajectory is difficult. What do you do during practice to help this person
- To know Woodworth’s (1899) dual- component model
- To understand how movement amplitude and target width determine
movement time (Fitt’s Law) Fitt’s Law is the closest we are getting to an
exact science in motor learning!
- To the know the factors influencing Fitt’s Law
- Dual- component model
o Ballistic phase (Pre-planned)
o Current Control phase (on-line control)
- Displacement on the Y- axis; time on the X- axis
o The task was to move pen up and down to a target.
o We should see a perfect sinusoidal pattern. Why? Because we have a
phase of acceleration and a perfectly matched phase of deceleration.
But this not what we get!
o We observe corrections that were taking place during the movement.
Ballistic Phase: Based on information prior to movement
initiation. This is pre-planned.
The first component is similar to the expected pattern
(it is somewhat ballistic).
Current Control Phase: Based on feedback from movement.
- If the target is larger trajectory amendments decrease.
- If target is more precise trajectory amendments increase!
- Accuracy requirements have an influence on the current control phase.
- This is where Fitt’s comes in.
Which sensory information is least likely to contribute to the current control phase
of a movement?
- Golgi Tendon Organs
- Muscle Spindles
It depends on the task and the context.
The sensory information that is most likely to contribute to the CCP
depends on the task. (Vestibular is more important to the CCP than tactile Need to know where eyes
are relative to head; head relative to body, and limb relative to body)
Retno centric (visual information); head centric (where the eyes are relative to
the head); body centric (head relative to the body) needs vestibular information.
- Task is to go back and forth between the two targets as quickly as possible
with less than 5% error rate.
- MT = a + b [logbase2 (2A/W)]
o Hick’s Law + Fitt’s Law are the most robust, and ONLY laws in Motor
Learning. This field is still in its infancy.
o A = amplitude = distance between target
o W = width of target.
o When A decreases, MT increases.
o Within an individual, you can predict how the average person will
perform (how many saccades you can do between 2 targets based on
previous results). We need some information about the individual,
o Movement time is proportional to ID.
- Index of Difficulty: ID = Log (base2) (2A/W)
ID= Log(base2) (2x6/0.75) Log(base 2) = 16 ID = 4
ID = Log(base2) (2x2/0.25) Log(base 2) = 16 ID = 4
o Both impose the same demands on motor system!! They should both
yield the same movement times! Same ID.
o Practical example: When designing cockpit small button will yield
long movement time even if there is short amplitude. If there a bigger
buttons with shorter amplitude is better.
- Graph: Y axis: Time per movement; X axis: ID
o Results for various target sizes. What is the ID range where the task
is deemed difficult?
- ID needs to be in a range of 3-6 to find a solid reproduction of Fitt’s Law.
o If A is too small, you will be taping on the same place.
o Some IDs are limited by biomechanics.
- Having a control phase is a critical component to reproducing Fitt’s Law (Size
of target width)
- Cannot expect the linear ID slope to be reproduced perfectly in
o Slope and intercept will change within the self and across people,
based on fatigue, energy, limb used, etc.
o Fitt’s Law can only be reproduced only under specific conditions with
the same individual.
- Factors affecting slope (and intercept)
o Different limbs: Expect steeper slope using legs o Expect steeper slope for fine skill.