A motor unit is composed of an α motor neuron, its axon, and the muscle fibers it innervates. There are three main types of motor units - S, FR, and FF - that are recruited in order of size according to the Henneman size principle, with S units recruited first and FF units recruited only during high force generation. Electromyography can be used to study individual motor units and their recruitment patterns during different levels of exercise.

1. Motor unit
SR2002
October 28, 2013
Dr. Arimantas Lionikas

2. Motor Unit (MU)
Plan
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Structural organization
Electromyography (EMG)
Recruitment threshold of MUs
MU types
MU recruitment during exercise
• Reading list:
1. Enoka R. Neuromechanics of human movement. 2002.
Publishers: Human Kinetics, p. 278-297
2. MacIntosh, B.R., Gardiner, P.F. McComas, A.J. Skeletal
muscle, 2nd edition. 2006. Publishers: Human Kinetics, p. 3239, 126-150.
3. McArdle W.D. et al. Exercise Physiology: energy, nutrition,
human performance. 2001. Publisher: Lippincott Williams &
Wilkins, p. p. 374-382, 394-400.

3. Structural organization
• Motor unit (MU) is
composed of α motor
neuron (α MN), axon
and muscle fibres
• α-MN innervates
<3000 muscle fibres
• α-MNs are in the spinal
cord
• Motor neuron pool is a
group of α MNs that
innervates a muscle
McArdle et al. 2001

4. Structural organization
• Axons of α motor
neurons reach
muscles through
peripheral nerves
• Action potentials
(APs) travel by
jumping between
Ranvier nodes
• Conduction velcoty
of APs is <120 m/s
• α motor neurons (α
MNs) receive action
potentials through
dendrites
McArdle et al. 2001

5. Action Potential
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Inside of the nerve and muscle cells is
electronegative compared to outside at rest; cell
membrane is “polarized” (Top figure).
Action potential is a rapid change in the membrane
potential, “depolarization”, followed by its
“repolarization” (Bottom figure).
Depolarization at one point of cell membrane excites
its adjacent portions resulting in propagation of action
potential along the membrane.
Na+ and K+ ions and voltage-gated sodium and
potassium channels play critical role in the
development of action potential.
Na+ concentration is much greater outside of nerve or
muscle cell membrane than inside (Na+in/Na+out=0.1).
K+ concentration is greater inside than outside
(K+in/K+out=35.0).
As voltage-gated sodium channels open, Na+ ions
rush in following the concentration gradient causing
“depolarization”. Infulx of Na+ ceases with inactivation
of the channels.
“Depolarization” triggers opening of potassium
channels leading to outward diffusion of K+ ions and
“Repolarization” of the membrane.

6. Electromyography (EMG)
EMG signal consist of:
1) muscle activity (m)
2) electric noise (n)
• Differential EMG
• Difference in electrical
potential is measured
between:
• 1) Muscle electrode 1
and reference electrode
(m1+n)
• 2) Muscle electrode 2
and reference electrode
(m2+n)
• Differential EMG is
effective in reducing
signal noise levels

7. Analysis of EMG signal
(“Raw” EMG signal)
(Negative values are turned into
positive values)
(Absolute values are averaged over
fixed intervals, for ex.: 100 ms)
• EMG signal increases with contraction force
• Increasing number of motor units (MUs) is
activated to increase contraction force
Enoka 2002

8. Needle electromyography
MU1
MU2
MacIntosh et al. 2005
MU1
MU2
• Needle
electromyography can
be used to record action
potentials of separate
motor units (MUs)
• MU potentials are small in
myopathy (small size of
muscle fibres)
• Shapes of MU action
potentials are abnormal in
partially denervated
muscles (nerve injury)

9. Recruitment threshold of MUs
• Motor units (MUs)
differ in recruitment
threshold
• Low threshold MUs
need low force (weak
neural drive) to be
activated
• High threshold MUs
need high force (strong
neural drive) to be
activated
• High force is produced
when large number of
MUs is activated
Kamen & DeLuca 1989

10. Classification of MUs
• 1) S type (slow, fatigue resistant, small)
Easily activated even by weak neural inputs
• 2) FR type (fatigue resistant, fast, medium size)
Require stronger neural inputs for activation than S type
• 3) FF type (fast, fatigable, large size)
Recruited only by very strong neural inputs
• Picture:
• Territories of muscle fibres
of different motor units in
the cross section of the cat
medial gastrocnemius
muscle
• Individual motor units
extend across large area
of muscle belly (number of
visible fibres / estimated
total number of fibres in
motor unit)
From MacIntosh et al. 2005

11. Important rule:
MUs receive common neural input and are recruited
according to their sizes !!! (Henneman's Size Principle)
Progressive increase in neural input
(frequency of action potentials)
Motorneuron
1st recruited
S
Muscle fibres
Slow
(S type)
2nd recruited
FR
Fast
Fatigue
Resistant
(FR type)
3rd recruited
FF
Fast
Fatigable
(FF type)
THREE major types of α motor neurons:
S type
are
small
“high” excitability
FR type
are
big
“average” excitability
FF type
are
very big
“low” excitability

12. MU recruitment
• Waking recruits primarily type S (slow) MUs
• Jumping requires recruitment of type II (fast)
motor units

13. MU recruitment (cont)
• Recruitment of muscle
fibres during exercise:
• Light intensity exercise:
Type I (slow)
• Medium intensity
exercise: Type I + type
IIA (FR)
• High intensity exercise:
Type I + Type IIA + Type
IIX (FF)
• Important observation:
Type I fibres are always
recruited during exercise

14. MU recruitment (cont)
• A number of activated
MUs increases with
effort and leads to the
increase in force
• Increments of MU force
become progressively
larger as big MUs are
activated at high forces
• Note: It is difficult to
grade muscle force
precisely when high
force is produced

15. Motor unit
Summary
• A Motor unit (MU) is composed of α motor
neuron, axon and muscle fibres
• Electromyography can be used to study
motor units (MUs)
• There are three main types of MUs (S, FR and
FF)
• MUs are recruited according to their sizes in
the following order: S => FR => FF