Lecture 6Lecture 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

Force-length curve of a contractile element

Muscles consist of contractive elements, parallel connective tissue, series elastic tissue

Influence of parallel connective tissue that surround the contractive elements. (parallel elastic component).

The muscle The muscle twitchtwitchHow 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 valueF0 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 andT=124 ms at 230.

F(t) – mechanical response of the motor unit to this impulse

T is the contraction time for which the tension reaches its maximum valueF0 is a constant (depends on the characteristics of the motor unit)

Shape of the muscle force during the voluntary contraction and relaxation

Turn-on time!Turn-off time!

(200ms)

(300ms)

Muscle modelingMuscle modelingMuscle 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

ElectromyographyElectromyographyDefinitions:Electrical signals associated with the contraction of a muscular is called an electromyogramelectromyogram (EMG). (EMG).

The study of EMG’s is called electromyography.

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.

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.

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 unitsEach motor unit is controlled by motor neuronMotor 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:•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.

•Contraction of the muscle:Contraction of the muscle:

Potential of surface electrode (V)

where:V – potential at the point electrodem – magnitude of the depolarization waveK- constant

– solid angle subtended at the electrode by the wavefront area

d - depth of the wave below the skin surfacea – area of the leading edge of the wavefront

Depolarization processDepolarization 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 difference in potentialspotentials between the two electrodes.

•The voltage waveform at each electrode is almost the same but is shifted in timeshifted 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.

difference in potentialsdifference in potentials between the two electrodes.

EMG electrodesEMG electrodesTwo groups of electrodes:

1. indwelling (intramuscular) electrodes.2. surface electrodes – non-invasive recordings

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

Indwelling electrodesIndwelling electrodes

•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.

Concentric Concentric electrodeelectrode

Monopolar electrodeMonopolar electrode

Two electrodes are used

Single fiber electrodeSingle 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 electrodesWire electrodesFine electrodes with about the diameter of human hairs. Hypodermic needle is used to insert the wire electrode.

MicroelectrodesMicroelectrodesCapillary glass microelectrode

Insulated metal microelectrode

Solid-state multisite recording microelectrode

Surface electrodesSurface 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.)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 Relationships between the EMG activity and the muscle tension.muscle tension.

I. Isometric contractionsExperiment: 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).

• 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.

• SynchronousSynchronous recording of movement parameters and the EMG signals. •Examples.

Electromyographic kinesiologyElectromyographic kinesiology

Recording of the EMGRecording 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 Major considerations during the design of EMG amplifiers:EMG amplifiers:

1. Gain and dynamic range2. Input impedance3. Frequency response4. Common mode rejection.