MC 2 (electromyography - lectures and labs) Flashcards
What is electromyography?
the registration and interpretation of the electrical activity associated with the contraction of skeletal muscle
How do an electromyograph and electromyogram differ?
electromyograph: instrument used to record EMG
electromyogram: the recording of the electrical signal generated by neuromuscular activity
Motor unit = ?
MU = motor neuron + muscle fibres it innervates
What is an innervation ratio?
the number of muscle fibres per motor neuron
larger muscles have greater innervation ratios
Describe a muscle fibre action potential (MAP)
>alpha motor neuron activated by CNS >action potential propagates down to motor plates >neuromuscular transmission elicits action potential in muscle fibre. >MAP propagates along fibre
How does action potential vary with fibre type?
- fast twitch = higher amplitude
- fast twitch = quicker
conduction
How does action potential vary with measurement site?
- further away = reduced
amplitude - further away = time delay
What is MUAP?
Motor Unit Action Potential:
the summation of electrical signals generated by all the muscle fibres of a motor unit
What is MUAPT
Motor Unit Action Potential Time:
the sequence of MUAPs from repeated neural stimulation
What is Henneman’s size principle?
Motor units are recruited in order of increasing size
What are the two configurations of electrodes when recording EMG signals?
Monopolar: one electrode
Bipolar: two electrodes, improves signal:noise by attenuating common noise across the two inputs
What is noise?
unwanted signal detected by the electrodes
e.g. other biological signals (ECG) or external sources (mains electricity)
Two electrode locations?
surface: for studies of surface muscle
indwelling: for studies of deep muscles and small or adjacent muscles where cross-talk may occur
What is cross-talk?
the signal from other muscles than what electrodes were intended for
What is the proper surface electrode location?
SENIAM guidelines - middle of the muscle between the origin and insertion - away from tendinous area and motor point - parallel to the underlying fibres
How would you prepare the skin for an EMG reading
Delsys adhesive interface and skin preparation techniques
> shave
> alchohol wipe
> abrade (sandpaper)
Why is the correct EMG prep procedure necessary?
better electrode contact with skin reduces noise
increases signal
greater signal:noise
reduced cross talk
How can you reduce cross talk?
- smaller electrode
- less adipose tissue
- careful prep
What are the 5 influences on an EMG signal?
- skin electrode interface: dead skin etc. provides
resistance to the signal - sub-cutaneous fat: less fat = greater signal:noise
- distance from electrode
- muscle length
- blood flow fatigue
What are we able to gather from raw EMG data?
- timing of activation
2. evidence of muscle activity
What does EMG NOT measure?
FORCE
What does a rectified EMG graph show?
absolute values (-2mV —– 2mV)
What does an integrated EMG (iEMG) graph show
the trend in muscular activity by the area under sections of a rectified EMG curve
What is the formula for average rectified EMG (arEMG)?
arEMG = iEMG / t
How do you achieve a smoother signal output line on a arEMG graph?
longer time windows
Can EMG be used as a form of biofeedback?
Yas.
Petrofsky (2001) showed EMG biofeedback to be effective in reducing the disturbance of walking patterns during stance phase (trendelenburg gait)
