1. Single-Joint Movements
Lecture 11:
Single-Joint Movements
Why study single-joint movements?
The trade-off of studying a natural action versus controlling the experiment well
Progress in science from simple to complex
In clinical studies, some patients limited to nearly single-joint actions
Testing theoretical frameworks for motor control

2. and Performance Variables
Task Parameters
and Performance Variables
Task parameters: What the subject is expected to do (distance to the target, target size, required movement time, etc.)
Performance variables: What the subject actually does (movement amplitude, scatter in the final position, movement time, etc.)

3. When Does the Movement
Start and End?

4. The Triphasic EMG Pattern
In the figure, the triphasic electromyographic (EMG) pattern begins with a burst of activity in the agonist muscle (triceps), followed by an antagonist burst (biceps), which is sometimes followed by a second agonist burst. Note that the first agonist burst starts several tens of milliseconds prior to joint trajectory. − 5 1 2 3 . 4
Trajectory
EMG
Time
Biceps
Triceps
Angle
Elbow extension
Antagonist burst
Second agonist burst
First agonist burst

5. Typical Quantitative Characteristics of the Triphasic Pattern
Magnitude: EMG peak amplitude, integrals over different time intervals (Q30, QAG, QANT)
Timing: Delay of the antagonist burst, duration of EMG bursts

6. Movements Over the Same Distance at “Different Velocities”

7. V Increases (L and D Are Const.)
An increase in the rate of agonist EMG rise, peak value, and area
A decrease in the delay of the antagonist burst
An increase in the antagonist burst amplitude and area
An increase in the level of final cocontraction

8. Movements Over Different Distances
“as Fast as Possible”

9. D Increases (L and V Are Const.)
Uniform rates of agonist EMG rise; higher and longer first agonist EMG burst
Longer delays before the antagonist burst
Inconsistent changes in the antagonist burst amplitude and duration

10. What Is That?

11. L Increases (D and V Are Const.)
Higher and longer agonist EMG bursts
No changes in the rate of the EMG rise
Longer delay before the antagonist burst
No apparent changes in the antagonist burst characteristics
Increased final cocontraction

12. Isometric Step Contractions
at Different Rates

13. Isometric Step Contractions to Different Targets “Very Fast”

14. Isometric Step Contractions
Increased rate of rise of the first agonist EMG burst
Increased peak EMG of the first agonist burst
Very small delay of the antagonist burst
Increased rate of rise of the antagonist burst
Same target, faster increase in torque:
Same rate, increase in the target torque:
Longer first agonist burst, same rate of rise
Delayed antagonist burst

15. Isometric Pulse Contractions

16. Isometric Pulse Contractions
Increased rate of rise of the first agonist EMG burst
Increased rate of rise of the antagonist burst
Same target, faster increase in torque:
Same rate, increase in the target torque:
Longer first agonist burst, same rate of rise
Delayed antagonist burst

17. Dual Strategy Hypothesis
The CNS computes “excitation pulses” to motoneuronal pools.
The pulses are rectangular; their duration and height are manipulated.
Motoneuronal pools behave like low-pass filters.
There are two strategies:
Speed-sensitive (control over movement duration)
Speed-insensitive (no control over movement duration)

18. Changes in the Excitation Pulse
Speed-Insensitive Strategy
Speed-Sensitive Strategy

19. Dual Strategy Hypothesis
Problems With the
Dual Strategy Hypothesis
EMGs are consequences of both central commands and reflex loops.
If a movement is perturbed, EMGs are expected to change at a short reflex delay; changes in commands are expected to come later.
To a large extent, early EMG changes are defined by changes in the muscle length.

20. EMG Patterns Within the Equilibrium Point Hypothesis
The r command can change at the same rate (SI) or at different rates (SS).

21. EMG Patterns Within the Equilibrium Point Hypothesis
Moving “at the same speed”: same w
Moving “at different speeds”: different ws
EMG: “excitation pulse” + effects of reflex loops

22. Reaction to an Unexpected
Change in Load
Trajectory
Agonist EMG
Antagonist EMG
Light load
Heavy load
Time
The figure shows kinematic and EMG patterns for two movements, both performed over the same amplitude and under the same instruction to be “as fast as possible.” However, the inertial load was 4 times as high during the second movement. Note the difference in movement kinematics, which apparently will be reflected in different reflex effects on both agonist and antagonist a-motoneurons.