Electromyography 1.2 - Measuring Neuromuscular Activity Flashcards
the varying rates of MAP transmission may determine characteristics of the EMG signal. state and explain 3?
1) AP’s moving slow contribute to low-frequency components of surface EMG
2) as AP propagation is an ionic process, the AP conduction velocity along the muscle fibre is dependent on the rate at which these ions can be exchanged
3) the passive membrane part of the exchange rate, along with the active metabolic mechanism for pumping Na+ back out the fibre
state how (A) atrophied muscle fibres and (B) longer muscle fibres affect conduction velocity
both make it slower
how does superficial/deep location affect EMG
some temporal dispersion issues define the shape of the MUAP as measured by EMG –> deeper muscle fibres contribute less to the EMG signal
define - ‘the MU Discharge of Firing Rate’
the motor unit discharge of firing rate refers to the nervous system being able to control the frequency of Motor Unit recruitment
state, and explain, what it is meant by the key term - ‘Doublets’
- motor units may also fire in two short latency bursts to overcome force before beginning a regular firing rate
- Doublets (twin bursts) have the potential to produce more force than expected from the addition of two motor unit force twitches
state, and explain, the relationship between EMG amplitude and force
non-linear
moreover, compensatory activity of antagonist in agonist movement can occur so cannot assume that increases in EMG activity are indicative of parallel increases in force
Talk about ‘Monopolar Recordings’ of EMG (4 points)
- one electrode on muscle belly & one on bony, inactive site
- use din static contractions & clinical investigations (indwelling electrodes)
- less stable & poor choice for non-isometric movements
- are appropriate for assessments of H & T reflexes & M-waves
what is ‘Common Mode Rejection’ ? (2 points)
- the feature that allows amplifiers to attenuate common signals
- the common mode rejection ratio (CMRR) is expressed in a linear or logarithmic scale
care must be taken when setting an ‘Amplifier Gain’, but why? (4 points)
- ‘clipping’ occurs when EMG signal is larger than amplifier gain set (amplifier is saturated)
- clipping occurs when the amplitude of the output exceeds the bounds of the power supply
- however, if gain is too low, resolution of the signal after Analogue-to-Digital (A-D_ conversion will be small
- ideally, gain should be set so that the amplitude of the signal is matched to the range of the A/D converted
what technical issues arise with surface electrodes? (3 points)
- usually, there is a 30 mV potential difference between inside and outside of skin layers
- when the skin is stretched, the potential dec^ by approx 25 mV, resulting in a 5 mV artefact
- these artefacts can be reduced with the use of silver electrodes & abraising the skin to reduce the usual 50 kΩ impedance across the skin surface
what method can be used which has been shown to successfully minimise artefacts ? (3 points)
- locating the first-stage amplifier as close to the electrode as possible
- friction & movement in cable movement can be reduced by using a gain operational amplifier at each electrode (termed active electrodes)
- these electrodes position the amplifier very close to the sensor and therefore produce cleaner signals
what is the estimated limit of deepness of surface electrode readings?
10 - 20 mm (Barkhaus et al., 1994)
what else do surface electrodes have difficulty in measuring?
smaller muscles as difficult to say whether signal is coming from the smaller muscle or from adjacent muscles
what did Lexell et al., (1983) say about deeper motor units?
Lexell et al., (1983) provided some evidence that deeper motor units may be smaller than their superficial counterparts. Therefore, surface EMG recordings must be interpreted in the context of a possible bias toward recording from larger and more glycolytic motor units
what do fine wire electrodes facilitate?
fine-wire electrodes facilitate the recordings of deeper and smaller muscles
what is the configuration of fine-wire electrodes? (2 points)
their configuration generally consists of two fine diameter insulated wires that are threaded through a hollow needle cannula ]
the tips of the wires constitute the recording surfaces, and either these are cut flush of the last millimeter or so of insulation is removed from the wire
how are fine-wire electrodes usually used? (3 points)
the distal cm of these wires is bent backwards so that the wires and needle can be inserted into the muscle
after the needle is inserted, it is carefully removed, leaving just the wires as the recording electrodes
the cannula can either be removed or left off to the side during the experiment, and the ends of the wires are then connected to an amplifier
what are ‘Needle Electrodes’? (4 points)
1) frequently used to monitor activity of more individuals motor units rather than a whole muscle
2) concentric electrodes consist of a small wire placed in the middle of a hollow cannula
3) the wire is set in place with an epoxy
4) the cannula is then cut at an acute angle (approx. 15 degrees) leaving a shiny wire surface that is then referenced to the cannula for a bipolar recording
