Electromyography Flashcards
What is electromyography (EMG) and what dies it measure?
It is measurement of electrical activity associated with muscle contraction (Electro=electrical activity, myo= muscle and graphy= measure).
What does EMG measure during movement?
How big the activation is by looking at the neural drive from the nervous system to the muscle.
Timing of the activation
Which muscles/coordination during movement (what’s working and when).
Muscle force which is a separate thing but is qualitative to EMG results.
Why do we recruit antagonistic muscles when performing exercise?
Joint stability.
2 types of electrodes
Surface Fine wire (intramuscular)
Surface and fine wire electrodes characteristics (3 each)
Surface • Suitable for larger, more superficial muscles • Easy to use • Large pick-up zone Fine wire • More precise • Inserted within the muscle using a needle • For deeper and smaller muscles.
What are electrodes?
Instrument of measuring EMG in the muscle through skin.
What is a bipolar electrode?
Difference in charge
Why is there a reference and a detection electrode?
The amplified signal is the potential difference between the two recording electrodes.
Subtracting the signal between electrodes eliminates majority of the ‘noise.’ Noise will effect each electrode to the same extent.
5 variables that will alter EMG results
An increase in motor units will increase EMG
An increase in motor units firing rate will increase EMG
An increase in motor units synchrony will increase EMG
The size of the motor unit determines amplitude
The speed of spreading influences motor unit amplitude shape
Skin preparation for an EMG data collection and the purpose.
Step 1: Shave the rejoin where you are collecting data from as it can inhibit the signal.
Step 2: Abrade the area to remove shaved hair and excess skin cells.
Step 3: Apply alcohol wipe to sterilize the testing area.
The purpose of this is to reduce the skin resistance (impedance) and
therefore allow greater amplitude and frequency
bandwidth of EMG to be detected
what are some electrode placements (4)?
Innervation zone (top of the muscle)
The midline of muscle belly (clearest signal)
Lateral edge of muscle
Myotendinous junction ( bottom of muscle) (ground electrode placed on bony landmark).
What happens if electrodes are too far apart?
May pick up signals from other muscles that are not being targeted
What are the 5 steps to electromyography processing?
1 Remove offset with the aim of getting the data to center around 0
2 Pass data through High pass filter, remove artificial movement (electrode slipping down leg during test etc) (motion artifact).
3 Rectify, converts data to absolute value (positive).
4 Smoothing, making the signal look cleaner so it is easier to interpretate typically with low-pass filter.
5 Normalization which is a process to facilitate comparison of signals between different parts of the body and different people
Limitations of surface and intramuscular EMG testing
Surface • Limited to large superficial muscles • Cross-talk Intramuscular • Invasive • Can be too selective • Dangerous under certain movement conditions
Why are surface EMG limited to larger, superficial muscles
- Electrodes have finite pick up radius
- Pick-up radius = interelectrode distance
- E.g. 2.5cm interelectrode distance will have 2.5cm pick-up radius
