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Lecture 6
Dimitar Stefanov
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
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).
The muscle twitch
How the muscle responds to impulse stimulus?
Let’s consider electrically stimulated motor unit
• Let’s 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)
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,
T=54 ms at 370 and
T=124 ms at 230.
Shape of the muscle force during the voluntary
contraction and relaxation
Turn-on time!
(200ms)
Turn-off
time!
(300ms)
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
Electromyography
Definitions:
Electrical signals associated with the contraction of a muscular is called an
electromyogram (EMG).
The study of EMG’s 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 doesn’t 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.
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.
Depolarization 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.
•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.
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
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 EMG’s require two electrodes over the muscle site.
• The voltage waveform that is recorded, is the difference in
potentials between the two electrodes.
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.
Application of the EMG signals:
•Muscle diagnostics
•Control of prosthetics and orthoses
•FES
EMG electrodes
Two groups of electrodes:
1.
2.
indwelling (intramuscular) electrodes.
surface electrodes – non-invasive recordings
Indwelling electrodes
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.
Monopolar electrode
Two electrodes are used
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.
Microelectrodes
Capillary glass microelectrode
Insulated metal
microelectrode
Solid-state
multisite recording
microelectrode
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
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).
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.
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.
Major considerations during the design of
EMG amplifiers:
1.
2.
3.
4.
Gain and dynamic range
Input impedance
Frequency response
Common mode rejection.