Dimitar Stefanov

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Lecture 6

• Dimitar Stefanov

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Recapping

• Size principle of recruitment of motor units
• The size of the newly recruited motor unit
  increases with the tension level at which it is
  recruited
• (The smallest unit is recruited first and the
  largest unit is recruited last).

The muscle action potential (m.a.p.) increases
with the size of the motor unit with which it is
associated.
Two types of motor units (M.U.) Type I and Type
II
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Muscles consist of contractive elements, parallel
connective tissue, series elastic tissue
Force-length curve of a contractile element
Influence of parallel connective tissue that
surround the contractive elements. (parallel
elastic component).
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The muscle twitch
How the muscle responds to impulse stimulus?

• Lets consider electrically stimulated motor unit
• Lets apply a short-duration electrical stimulus
  of a motor unit (impulse stimulus)
• F(t) mechanical response of the motor unit to
  this impulse

where T is the contraction time for which the
tension reaches its maximum value F0 is a
constant (depends on the characteristics of the
motor unit)
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F(t) mechanical response of the motor unit to
this impulse
T is the contraction time for which the tension
reaches its maximum value F0 is a constant
(depends on the characteristics of the motor unit)

• T is larger for the slow twitch fibers
• F0 increases for the larger fast twitch units
• Muscles of the upper limbs have shorter T than
  the leg muscles.

Typical values of T for some muscles
T increases in all muscles as they were
cooled. Example Biceps brachialis, T54 ms at
370 and T124 ms at 230.
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Shape of the muscle force during the voluntary
contraction and relaxation
Turn-on time!
Turn-off time!
(200ms)
(300ms)
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Muscle modeling
Muscle simulations are used to predict tensions.
The muscle is presented as a configuration of
contractive component plus linear and nonlinear
elastic components, which characteristics are
well known. The mechanical behavior of the model
is similar to the muscle behavior.
Viscous elastic muscle model
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Electromyography
Definitions Electrical signals associated with
the contraction of a muscular is called an
electromyogram (EMG). The study of EMGs is
called electromyography.
History 1838 Matteucci first describes the
existence of electrical output from muscle. 1929
introduction to coaxial needle electrode It
has been noted that the relaxing muscle doesnt
produce voltage, the EMG signals are generated in
case of muscle contractions.
EMG increases in magnitude with the muscle
tension.

• Factors, which can influence the EMG signal
• Velocity of shortening or lengthening of the
  muscle
• Fatigue
• Reflex activity.

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The electrical signals generated in the muscle
fibers are called muscle action potential
(m.a.p.).
Electrodes placed on the surface of a muscle or
inside the muscle tissue (indwelling electrodes)
will record the algebraic sum of all m.a.p. s.
which are being transmitted along the muscle
fibers.
Muscle consists of motor units Each motor unit
is controlled by motor neuron Motor end plate
synaptic junction between the motor neuron and
the controlled motor unit. Depolarization of the
post synaptic membrane arises in case of
activation of the motor unit. End plate potential
(EPP) the potential that is recorded. Depolariza
tion wave result of the depolarization. The
depolarization wave moves along the direction of
the muscle fibers. The signal between the EMG
electrodes corresponds to the depolarization wave
front and to the subsequent repolarization wave.
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• Contraction of the muscle

• Contraction of the muscle
• Alpha motoneurons begin firing
• Process of recruitment (adding new
  motoneurons).
• Alpha motoneurons are recruited in a set order,
  from smallest to largest
• Contraction increases
• EMG signal increases.

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Potential of surface electrode (V)
d - depth of the wave below the skin surface a
area of the leading edge of the wavefront
where V potential at the point electrode m
magnitude of the depolarization wave K-
constant W solid angle subtended at the
electrode by the wavefront area
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• Depolarization process
• quite rapid process
• The leading edge of the wavefront is quite sharp
• The magnitude of the m.a.p quite big.

• Repolarization process
• quite comparatively slow process
• The leading edge of the wavefront is not sharp
• The magnitude of the m.a.p quite small.

• Most EMGs require two electrodes over the
  muscle site.
• The voltage waveform that is recorded, is the
  difference in potentials between the two
  electrodes.

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difference in potentials between the two
electrodes.

• The voltage waveform at each electrode is almost
  the same but is shifted in time.
• The similarity between both waveforms is higher
  when the electrodes are closer.
• The differential signal between electrodes is
  smaller in case of nearly located electrodes.

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EMG electrodes

• Application of the EMG signals
• Muscle diagnostics
• Control of prosthetics and orthoses
• FES

• Two groups of electrodes
• indwelling (intramuscular) electrodes.
• surface electrodes non-invasive recordings

Indwelling electrodes
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Concentric electrode

• The concentric needle consists of a cannula with
  an insulated wire (or wires) down the middle.
• The active electrode is the small tip of the
  center wire, and the reference electrode is the
  outside cannula.
• Concentric needles may have two central wires
  (bipolar), in which case the active and reference
  electrodes are at the tip and the outside cannula
  acts as the ground.

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Monopolar electrode
Two electrodes are used
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Single fiber electrode
The electrode consists of a 0.5-0.6 mm stainless
steel cannula with a 25 µm fine platinum wire
inside its hollow shaft. In a side port 3 mm
behind its tip, the cut end of the platinum wire
is exposed.
Very small pick-up range
Wire electrodes
Fine electrodes with about the diameter of human
hairs. Hypodermic needle is used to insert the
wire electrode.
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Microelectrodes
Capillary glass microelectrode
Insulated metal microelectrode
Solid-state multisite recording microelectrode
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Surface electrodes consist of disk of metal ,
attached to the skin, usually above the place
where the muscle is located. Silver/silver
chloride disks about 1cm diameter.

• Pick up gross motor unit activities

• Best used as reference electrodes when monopolar
  needles are used.

• Increased magnitude of the capture signals.

Duration of the muscle action potential (m.a.p.)
The electrode signal depends on the surface of
the electrode.
The duration of the m.a.p. depends on the
velocity of the propagation of the wavefront of
the m.a.p. Velocity of propagation of the m.a.p.
4 m/s
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There is a delay between the EMG and muscle
contraction (30-80 milliseconds).
Relationships between the EMG activity and the
muscle tension.
I. Isometric contractions Experiment weights
hung over pulley to act against the elbow flexors
in isometric contractions skin electrodes over
the elbow flexor. Results Linear dependency
between rectified EMG output and isometric
tension produced by the muscle generating the
EMG.
II. If the muscle is permitted to shorten or
lengthen then the relation between EMG voltage
and the muscle tension is non-linear EMG signal
depends on the muscle length.
III. Electrical activity of muscle increases with
fatigue (in both isometric and isotonic
contraction).
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Electromyographic kinesiology

• Study of the motions through the detection and
  analysis of electromyographic signals.
• Finding correlation between the EMG signals in
  moving muscles with the motion of the moved
  segments.
• Synchronous recording of movement parameters and
  the EMG signals.
• Examples.

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Recording of the EMG

• Clean EMG signal
• undestroyed and free of noise or artifacts.
• Undestroyed signal
• The large signals and the small should be
  amplified at one and the same level (linearly
  amplified, without overdriving of the amplifier
  and without distortions of the large signals).
• Dynamic range
• the largest EMG signal should not exceed that
  range
• Noise in the EMG signal
• biologic noise, noise from the power lines,
  noise from machinery, artifacts.
• Example of biological noise the EMG signal
  picked up by EMG electrodes on the thoracic
  muscles.

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Major considerations during the design of EMG
amplifiers

1. Gain and dynamic range
2. Input impedance
3. Frequency response
4. Common mode rejection.