Electromyography Flashcards
what is the defitinition of electromyography?
technique for evaluating and recording electrical activity of skeletal muscle
what are some of the uses of electromyography?
- diagnosing neuromuscular disorders
- studying kinesology
- controlling prothesis
what are some of the issues involved in measuring EMGs?
- micrometre scale: must utlise very tiny probes to get into the cell through the membrane
- vision: how do you identify the muscle cell? how do you make sure one probe is inside the cell?
- when you probe the cell, how do you make sure the cell doesnt leak fluid?
what is the method called that measures EMGs and negates all the potential negative issues involved?
patch clamping
what are the two methods involved with patch clamping?
either uses surface electrodes or invasive electrodes
what are the two surfaces used when measuring EMGs using a monopolar configuration and what are their charges?
-detection surface: total charge is the sum of the action potentials under the electrode and the sum of all resting membrane potentials
reference surface: charge is the sum of all cell resting membrane potentials in that region
what are the two main issues when recording EMG’s using patch clamping ?
- sensitive to impedance
- sensitive to noise
what is impedance?
the reduction in amplitude across a range of frequencies of an electrical signal
what contributes to high impedance and why when measuring emg’s?
- lots of hair: air gaps present between skin and electrode create high impedance
- dirt: micro dirt particles dissallow a perfect seal of the electrode to the skin
- numerous layers between the muscle and electrode
how can we negate the effects of noise when measuring emg’s?
- choose a place on the body with minimal hair
- clean the region of the body, using rubbing alcohol
- pick a region on the body will few layers between muscle and surface electrode
what is noise?
defined as underisable electrical signals, which distort or interfere with an original or desired signal. Noise could be transient or constant
what are some sources of noise, what are their features, and how can they be treated?
- movement artefact: slight shifts in electrodes due to muscle movement, low frequency noise. can be removed through signal processing
- Electromagnetic noise: Skin surface is constantly bombarded with sources of electromagnetic radiation, high frequency, amplitude is up to 3x as large as EMG its self, stay away from fluro lights and AC power cables, consider using shielding and filtering
- Cross talk: underisable EMG from another muscle not being monitored. Cannot be removed as it has the same frequency as the emg itself
- internal noise: other biological activity, ie changes in membrane potential of other cells, cannot be removed through filtering
what are some other sources of noise which are less prevalent?
-Instability of the signal: emg’s are unstable, numbers of active motor units, motor firing rate and mechanical interaction between muscle fibers all affect behaviour of EMG signal. nothing much you can do here
-Electrocardiogram (ECG) artefacts:
If performing EMG close to the trunk, the contraction of the cardiac muscles, can be a source of cross talk. Choose a muscle away from the torso!
why is bipolar configuration better to use than monpolar when measuring emg’s?
bipolar configuration has the advantage as it cancels out much of the noise compared with using a monopolar configuration
what are some common types of amplifiers used and what characteristics do they have ?
- Op amp: very inexpensive, high input impedance
- Differential Amplifier: good for amplifying the difference between signals. a little more expensive than the op amp. higher input impedance than op amp
- Instrumentation amplifer: very good at amplifying the difference between two very small signals, more expensive than op amp (10x), highest input impedance
