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Depolarization of Muscle Membrane
The EMG signal is derived from the depolarization of the muscle
membrane. The electrodes record the sum of all of the muscle fiber
action potentials from all the active motor units that pass through the
recording zone of the electrodes
EMG Signal
• The EMG
signal
represents the
sum of all the
muscle fiber
action
potentials
from all of
the active
motor units
that passes
through the
recording
zone of the
electrodes.
Motor Unit Synchronization
Un-Synchronized Action Potentials Reduce
EMG Amplitude.
Shaded areas indicate where the positive phase
of one action potential overlaps in time with
the negative phase of another action potential
leading to the cancellation of the surface EMG
(shown as the sum) [From Yao (2000) #1996]
Synchronized action potentials result in an
increase in the surface EMG. (shown as the
sum) [From Yao (2000) #1996]
Effects of Motor Unit Synchronization Upon EMG Amplitude and Force
Synchronization increases the EMG amplitude and the variability of the force. [From Yao (2000) #1996]
Effects of Synchronization Upon the Median Frequency of the EMG Signal
Motor unit
synchronization
lowers the median
frequency of the
surface EMG
signal. [From Yao
(2000) #1996]
In general,
there is a
linear relation
between EMG
and force.
Relationship Between EMG Signal and Motor Unit Force
•
•
•
•
The EMG signal of a ST motor
unit can be described as low
amplitude and long duration.
The EMG signal of a FT motor
unit can be described as high
amplitude and short duration.
When an ST motor unit fires, it
yields a relatively small amount
of force with a longer time to
peak force, when compared to a
FT motor unit.
Since FT units are highly
fatigable, the CNS is very
protective of them. FT units are
recruited when a rapid force is
needed or only at high force
levels.
Size Principle (Recruitment Threshold)
•
•
•
FT
motor
unit
•
•
•
ST
motor
unit
Small [Slow Twitch, Type I] are
recruited first
Intermediate [Type IIa] are then
recruited
Large [Fast Twitch IIb] are
recruited last
The vertical lines indicate when a
motor unit has fired. When the
lines are closer together, the
motor unit has increased its firing
rate.
Fast twitch units are the first to
be de-recruited, followed by
intermediate and finally slow
twitch units.
Notice that motor units are
recruited and de-recruited at
about the same force level.
Motor Unit Recruitment
Motor units are recruited according to size (I, IIa, IIb). However in ballistic
contractions the high threshold (MUP 2) units may show up in the EMG signal
first due the faster axonal conduction velocity [see Desmedt #1994].
Electromechanical Delay (EMD)
•
•
•
•
EMD is the delay between the start of EMG and the start of force.
The electrical event, depolarization of muscle membrane (EMG) occurs before the mechanical
event (force).
The delay comes from time needed to:
– Propagate the action potential along the T-Tubule system
– Release of calcium from the sarcoplasmic reticulum
– Formation of Actin-Myosin crossbridges
– Stretching of the series elastic element (SEC): titin, myosin head and arm, tendon
EMD times are effected by: level of force, muscle length, type of contraction
(isometric, concentric, eccentric) , premotor state of the muscle
[From Herzog #2318]
Electromechanical Delay in Gait
The EMG signal shown above is from the soleus of a cat during gait.
The force was measured directly from the achilles tendon. Notice that
the EMG starts about 70 ms prior to force and that the EMG ends
about 70 before the force ends.
Muscles have two
mechanisms to modulate
force: recruitment and firing
rate. Depending upon the
muscle and the force
requirements different
recruitment-rate patterns can
be observed. Muscle A is
the soleus, Muscle B is the
bicep. By 60% of MVC the
soleus has recruited all of its
motor units and must rely
upon increase firing rate to
increase force above 60%.
The bicep continues to
recruit additional FT motor
units until about 80% MVC.
Different firing rate control
strategies: In Figure A units
increase their firing sharply after
being recruited, show a plateau,
and then increase sharply and the
higher force levels. In Figure B,
unit A reaches max firing
immediately after being recruited,
units B and C display linear
increase in firing rates.
Recruit FT
Contraction
begins with
ST and INT
FT fatigues
FT 1
FT 2
INT 1
INT 2
ST 1
ST 2
EMG – Fatigue at 40% MVC
INT fatigues
EMG – Fatigue at 80% MVC
FT 1
FT 2
INT 1
INT 2
ST 1
ST 2
When motor units fatigue they
fire in shorter bursts and rest
longer between subsequent
bursts
A single Actin-Myosin bond yields less force concentrically (1-2pN) than eccentrically
(N*3-4pN), as a result, concentric EMG has a larger amplitude than eccentric EMG. The
increased EMG amplitude observed in concentric contractions is due to some combination
of increased recruitment and/or increased firing rate of currently active motor units.
[#1845]