KINE 3301
Biomechanics of Human Movement
EMG & Motor Neurons
Chapter 18
Motor Unit
A motor unit is one alpha motor neuron and all of the
muscle fibers it innervates.
EMG Differential Amplifier
The EMG
amplifier takes
the difference
between the
muscle fiber
action potential
from each
electrode.
Synchronization Causes Amplification & Cancellation
• Synchronization of
two muscle fiber
action potentials with
the same polarity
causes amplification.
• Synchronization of
opposite polarity
causes cancellation.
Muscle Twitch Response
Stimulation of a motor neuron with a single impulse
causes a twitch response.
Stimulation Rate and Tetanic Contractions
• Increasing the
stimulation rate (firing
rate) causes a stairstepping effect of force.
• Maximal stimulation
produces a tetanic
contraction.
EMG & MU Properties
EMG, nerve
conduction
velocity, firing
rate ranges,
twitch responses
and fatigue
curves by motor
unit type.
EMG – Force Data Collection
Instrumentation used to study muscle force-velocity and
force-length relationship.
Ramp Contraction
•
•
•
In a ramp contraction
motor units are
recruited according to
size (ST, INT, FT).
A newly recruited unit
fires slower than
currently active unit.
Motor Units are
recruited and derecruited at the same
force threshold.
Ballistic Contraction
The subject is instructed
to generate maximum
force as rapidly as
possible.
Theoretically all motor
units are recruited
simultaneously and
each fires at it’s
maximal rate at the
outset.
Effects of Rate of Force Development on Recruitment & Firing Rate
• On the left the
subject is
instructed to
generate a 50%
MVC as rapidly
as possible.
• On the right the
subject is
instructed to
generate a 50%
MVC in 2 sec.
Concentric & Eccentric
EMG - Torque
Concentric
Eccentric
iEMG (mV)
685.1
660.6
Average
Torque (N∙m)
48.6
61.1
Peak Torque
(N∙m)
84.7
98.2
Electromechanical Delay
(EMD)