Transcript (triceps), followed by an antagonist burst

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
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.)
When Does the Movement
Start and End?
Tang
ANGLE
VELOCITY
Tvel
ACCELERATION
T1
Tacc
T2
T3
0
TIME (s)
0.5
The Triphasic EMG Pattern
Elbow extension
35
Trajectory
EMG
First
agonist
burst
30
25
Second
agonist
burst
20
150
100
50
Triceps
15
10
0
Biceps
5
Antagonist
burst
Angle
0
−5
0
0.1
0.2
0.3
Time
0.4
−50
−100
0.5
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.
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
Movements Over the Same Distance
at “Different Velocities”
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
Movements Over Different Distances
“as Fast as Possible”
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
What Is That?
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
Isometric Step Contractions
at Different Rates
Isometric Step Contractions
to Different Targets “Very Fast”
Isometric Step Contractions
Same target, faster increase in torque:
 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 rate, increase in the target torque:
 Longer first agonist burst, same rate of rise
 Delayed antagonist burst
Isometric Pulse Contractions
Isometric Pulse Contractions
Same target, faster increase in torque:
 Increased rate of rise of the first agonist EMG burst
 Increased rate of rise of the antagonist burst
Same rate, increase in the target torque:
 Longer first agonist burst, same rate of rise
 Delayed antagonist burst
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)
Changes in the Excitation Pulse
Speed-Insensitive
Strategy
Speed-Sensitive
Strategy
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.
EMG Patterns Within the
Equilibrium Point Hypothesis

r1
r0
r2
SI-Strategy
r4
r3
1
r2
r1
r3
r4
r6
r5
r7
SS-Strategy
r1
r0
1
2
3
4
The r command can change at the same rate (SI) or at different
rates (SS).
EMG Patterns Within the
Equilibrium Point Hypothesis
 Moving “at the same speed”: same 
 Moving “at different speeds”: different s
 EMG: “excitation pulse” + effects of reflex loops
Reaction to an Unexpected
Change in Load
Trajectory
Light
load
Agonist
EMG
Antagonist
EMG
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.