Introduction to Motor Control

Question 1

what is the movement scientists perspective

Answer 1

• the brain is specialised in movement
• if we understand movement, we understand the brain

Question 2

why is movement so easy to us

Answer 2

• we do not recognise the difficulty of the movement control
• complicated high-level control strategies are used by our brain, even when we are thought-lessly doing simple motor tasks

Question 3

what are empirical observations of fitts law

Answer 3

• if you need accuracy, you become slow
• a highly precise relationship can be found between movement duration and log (2A/W)
• this relationship is NOT limited to specific body parts or specific movement

Question 4

what are empirical observations of two-third power law

Answer 4

• if you are making a curved movement you become slow - slower when more curved
• curvature (larger is flatter) and velocity may have a positive relationship
• this relationship is NOT limited to specific body parts or specific movement
  
  • can be found in running, drawing, dancing etc

Question 5

what is space

Answer 5

• set (of points) with some added structures
• e.g. plane: two dimensional space
• e.g. cube, sphere etc: three dimensional space
• e.g. world: three dimensional space + additional dimension of time
• e.g. space with a complex structure (non-Euclidean)
• end effector space / task space / operational space
  
  • viewed from eye level
• configuration space / joint angle space / body space
  
  • viewed from limb position

Question 6

what is a coordinate system

Answer 6

• set of numbers (called coordinates) that uniquely determine a point in space
• a single space can have multiple coordinate systems
• also called coordinate frame / reference frame

Question 7

what are the different coordinate systems

Answer 7

• cartesian
  
  • all axes (x, y, z) cross through the origin
• cylindrical
  
  • specifies point positions by the distance from a chosen reference axis, the direction from the axis relative to a chosen reference direction and the distance from a chosen reference plane perpendicular to the axis
• spherical
  
  • determine the position of a point in three dimensional space based on the distance from the origin and two angles

Question 8

what does egocentric mean

Answer 8

coordinate system attached to our body (first person view)

Question 9

what does allocentric mean

Answer 9

coordinate system attached to external world (third person view)

Question 10

what is linear vs angular kinematics

Answer 10

distances vs angles

Question 11

what is linear vs angular kinetics

Answer 11

forces vs torques

Question 12

what is the equation for force

Answer 12

F = Ma
force/dynamics = intertia x acceleration/kinematics
- newtons second law defines relationship between kinematics and dynamics
- this simple equation can be extended and applied to much more complex systems where F and a are vectors (of forces and torques) and M is a matrix (inertia matrix)

Question 13

what is control in this context

Answer 13

• feedforward (open) loop: plan in advance (i.e. pre determined actions)
• feedback (closed) loop: policies (i.e. case by case actions)

Question 14

what are the pros and cons of a feedforward controller

Answer 14

• pros
  
  • movement can be controlled without gathering / processing any external information: good for fast movement
• cons
  
  • error mode by controller accumulates over time
  • no flexibility to environmental changes
  • high cost of planning, no generalisation

Question 15

what are the pros and cons of the feedback controller

Answer 15

• pros
  
  • flexible: can deal with unexpected events
  • robust: control error does not accumulate over time
  • general: control is invariant to starting or goal position
• cons
  
  • processing sensory feedback is time consuming - not so good for fast movement
  • controller can become unstable when sensory feedbacks are noisy and the feedback gain (degree of how seriously I take the answer) is high
  • can also become unstable when feedbacks are delayed

Question 16

what is variability

Answer 16

• even when the same movement is repeated, controller behaviours under uncertainties result in different movement patterns
• this is called variability - by taking a closer look at the variability patterns, we can get a sense of the brain’s control strategy