ecg and opamp

Lab #3: Electromyography (EMG)

The circuit we built for EKG can also be used to detect and amplify other biotentials, such as the EMG. Actually, you probably noticed that you can produce an EMG artifact on the EKG trace by contracting a muscle under the electrode. In today's lab we are going to try to eliminate the EKG and look at the EMG instead.

Recall that the function of a differential or instrumentation amplifier is to ignore or reject anything that is present on both input leads. Up till now, we have used this principle to reject the ambient 60Hz electrical noise. By repositioning the electrodes directly over a muscle we can also use the same CMR principle to reject the EKG too.

I used the tibialis anterior muscle on the shin because it usually gives a nice large and clear EMG signal, but you can try any muscle - e.g. biceps on your arm.

EMG of tibialis anterior muscle.

In order to reduce noise (both AC and EKG) to a minimum, it is best to attach the two electrodes as close together as possible. In fact you can even buy special EMG electrodes which come in pairs. The common (ground) electrode is best attached to an area where there is no muscle, e.g. over a bony prominence such as the knee cap or shin bone.

Bandstop & Bandpass filters

The frequency content of EMG is larger than the EKG: while there is very little EKG signal above about 40 Hz, the EMG goes up to 250 or even 500 Hz, so your filter cutoff will have to be adjusted upwards. Unfortunately, this means that the AC electrical noise will now not be stopped by the filter. For this reason, it is necessary to use a bandstop

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filter (or notch filter), which just eleimates a narrow range of frequencies:

The width of the notch obviously depends on the cutoff frequencies and orders of the filters. You can do this with purely passive components using a Twin-T circuit:

To block 60 Hz, R = 27kΩ and C = 0.1µF (fo = 1 / 2πRC)

In practice, it is sometimes difficult to get the component values exactly right, and it is difficult to tune this circuit, because all three resistors need to be adjusted simultaneously. The frequency of the notch of this alternative filter (from Horowitz) can be tuned with one preset, and its depth can then be tuned for maximum attenuation with the second preset:





There are three remaining limitations of this filter:

1. the input impedance is low at high frequencies (approximately R/4) 2. as with all passive filters, the source and load affect its properties 3. the sharpness of the notch is limited

Active filters can be used to get round this problem, but in practice are difficult to make because the components need to be matched very closely. Because removal of AC power noise is such a popular operation, chips are available that are specially designed for this purpose. This one has a notch width of only 0.3 Hz, with 60 dB per decade rolloff.

The opposite of a bandstop filter, by the way, is called a bandpass filter, and is used whenever we want to just let one frequency through (e.g. in the EEG project).

Making the EMG signal resemble the muscle force

EMG is used primarily to indicate the timing and force of the underlying muscle contraction. In order to do this, it is important to understand the difference between the two variables, force and EMG.

Raw EMG is biphasic The force generated by a muscle is always contractile





In order to convert the raw EMG into a signal resembling the muscle force, we need to do two things:

l Detection l Smoothing

Detection

Detection, or rectification, means allowing only the positive half of a wave through. It is the basis of AM radio receivers, where it is used to separate out the lower frequency audio from the much higher (radio) frequency carrier wave on which it is modulated.

Modulated (high frequency) carrier wave

Demodulated signal produced by detection

EMG has a high frequency content (up to 250-500 Hz) Muscle force has a low frequency content (up to 3-5 Hz)





Detection is performed very simply by using a diode.

Smoothing

The envelope of the resultant modulated wave is an analog of the modulating signal.It just needs smoothing out:

This is done by a very low pass filter. Note that this is a completely different efect from filtering before the detection, since the only frequency present at that time was the high frequency carrier wave.

For EMG, the frequency used to produce this linear envelope depends on the twitch time of the muscle. This varies a little depending on the type of muscle - in general smaller muscles (such as the muscles that move the eyeball) have faster twich times, whereas large muscles (e.g. gluteus maximus and quadriceps) have slow twitch times. For most of the muscles involved in walking, a low-pass filter of about 3 Hz has been found to produce an envelope similar in shape to the force pattern generated by the muscle.





Uses of the EMG signal

Once transformed in this way, the EMG signal can be used not only as a means for estimating the muscle force, but also to control actuators such as prostheses. This is called myoelectric control.

Laboratory We will use the AD620 Instrumentation Amplifier. This has several advantages:

l High CMRR (100dB) l Wide bandwidth (> 100 kHz) l Adjustable gain up to 1,000 or more

Basic Circuit





The gain of the differential amplifier is set by RG.

The gain is a trade-off between CMRR and frequency response:





Since EMG has a very low amplitiude (about 0.5 mV), the total gain of the amplifier will need to be about 2,000. Although we could do all this with the AD620, it is probably better to add a second stage op-amp with a gain of about 20. So, the AD620 gain should be about (2,000/20) = 100.

l Build the AD620 stage and test it l Build the op-amp stage and test it l Join the two stages together l Add AC coupling l Attach the electrodes from your EKG amplifier to a muscle l Try to detect the EMG from contracting the muscle - note that it is biphasic l And a notch filter l Add a diode detector l Add a low-pass filter (a passive RC network will do - cutoff 3 Hz) to smooth the rectified EMG and produce the linear envelope

Questions

l What was the final gain of your circuit? l What was your CMRR? l What value of RG was this? l What happened with an RG of 100 and 1000 ohms? l What was the frequency response of your notch filter? l Which muscles were easiest to obtain an EMG from?





Human Locomotion BE522: Electromyography

Excitation-contraction Coupling

l motor neurone stimulated ¡ upper-motor neurone from brain travels down spinal cord ¡ lower motor neuron in the ventral horn of the spinal cord is activated ¡ action potential passes down lower motor neuron axon to muscle (K moves in, Na moves out) ¡ axon branches to supply a set of (10:eyeball -1000:gastroc) muscle fibers called a motor unit ¡ motor unit action potential (MUAP) is conveyed to motor end plate on each muscle fiber ¡ summation of all active motor unit activity is the EMG

l depolarization ¡ causes release of packets of acetylcholine into the synapse on surface of the muscle fiber ¡ electrical resting potential under the motor end plate changes, initiates an action potential along

surface of muscle fiber ¡ action potential spreads inside the muscle fiber along transverse (or T-) tubules ¡ calcium released from sarcoplasmic reticulum

l sliding filament theory (Huxley 1960) ¡ calcium binds to troponin-tropomyosin (T-T system), changing its shape ¡ myosin (A bands – thick filaments) binding sites open on actin filaments (I bands – thin filaments) ¡ Myosin head with ADP+P binds to actin (cross-bridge) ¡ Power-stroke causes filaments to slide ¡ ADP+P moves off myosin head ("grab and swivel" ratchet mechanism) ¡ When mysoin is tilted forward a new binding site for ATP becomes exposed ¡ A new ATP molecule binds to myosin head, allowing it to release actin

l summation ¡ all or none effect - graded increase in contraction comes from

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n recruitment of more motor units (spatial summation) n increased firing rate of active motor units (temporal summation)

the sarcomere contractile unit

What is the smallest amount of muscle that contracts when a lower motor neurone fires

Which of these elements is NOT necessary for muscle contraction

Types of Muscle Fiber

?

?

Red

Slow Oxidative Intermediate White

Fast Glycolytic Diameter Small Intermediate Large

Z-line thickness Wide Intermediate Narrow Glycogen content Low Intermediate High Resistance to fatigue High Intermediate Low Capillaries Many Many Few Myoglobin content High High Low Respiration type Aerobic Aerobic Anaerobic Twitch time Slow Fast Fast Myosin ATPase content Low Low High Examples Gluteus Maximus

SoleusGastrocnemius Vasti

Hamstrings Rectus Femoris

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Which type of muscle do you think cardiac muscle closest to

Recording EMG

?

Problem Solution Low amplitude (5 mV) Common Mode Rejection (CMR) AmplificationSkin resistance (keratin, grease, oils) Cleaning/abrasion/shaving of skin, gel, High impedance amp.Movement of electrode (motion artefact) High-pass Filtering (> 10-20 Hz)Electrical noise Pre-amplifier, Low-pass Filtering (< 200-500 Hz), CageCumbersome cabling Thin cable (multiplexing), telemetrySome muscles deep In-dwelling (transcutaneous) fine-wire electrodesHigh frequency (risk of aliasing) Sampling rate >> Nyquist (e.g. 1000 Hz)Cross talk Small electrodes, spectral analysis

Raw EMG is biphasic

Rectification (detection)

Quantification of varying signal Root Mean Square (RMS)

EMG much higher frequency than muscle force Linear envelope (low-pass filtering at 3 Hz after detection) - also called "integrated emg"

... produces the same shape as the muscle force or joint moment curve

Variability from stride to stride Averaging - plot mean and standard deviationVariations in electrode placement/skin from trial to trial Nomalization to Maximal Voluntary Contraction (MVC)Electromechanical delay Difficult to interpret

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Which is best for removing motion artefact

Factors governing the relationship between EMG and muscle force (or moment)

l Length-tension Relationship (AV Hill, 1938) ¡ Isometric preparation (hold length constant, study force) ¡ Isotonic preparation (hold force constant, study length)

l Force-velocity relationship l Recruitment of more motor units

leads to non-linear increase in EMG

l As muscle contracts, its relationship to the elctrodes changes

l Eccentric (lengthening) contractions are associated with less EMG than concentric contractions

l Conclusion: EMG-Force relationship linear only under isometric conditions

Which conditions would result in the greatest EMG

Applications

l Compartmentalization - determining relative contribution of each muscle to moment

l Co-contraction - where nett moment may be zero l Timing of muscle contraction - inappropriate or premature activity (spasticity) l Fatigue assessment - from spectral analysis (FFT): Mean Power Frequency (MPF)

falls with fatigue l Diagnosis - muscle disorders such as Duchenne dystrophy, myaesthenia gravis, Lou Gherig's

?

?

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l During selective dorsal rhizotomy - surgery to cut nerves causing spasticity

Noraxon Telemyo

l 8 channels standard, upgradeable to 16, 24, 32 or 40 channels l No pre-amp - patented signal processing algorithm l Minimal or no skin preparation l Bandwidth (-3dB) 10Hz to 500Hz l Lightweight & portable l Raw and enveloped outputs l Wireless (RF) telemetry - 300 yard range

Score: correct on first try, from attempts. 0 0

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第 1 頁，共 1 頁Common Mode Rejection Ratio (CMRR)

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ecg and opamp

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