EMG Flashcards
Where does an EMG signal come from?
the signal is of the action potentials being delivered down the axons of the single motor unit and into the muscle fibres
How quickly does an action potential move down both directions of a muscle fibre?
2-6 m/s
What are the two types of EMG?
surface and fine-wire
Why would you use fine-wire EMG over surface EMG?
When recording data from a target muscle that is smaller. The recording zone on fine-wire is much smaller which decreases the incidence of cross-talk.
What two muscle fibre factors influence EMG signal?
number of SMU’s being recruited and the discharge rate
How does muscle fibre type influence EMG signal?
slow-twitch (type 1) vs fast-twitch fibres (type 2)
slower-moving action potentials make it more likely that signals between SMUs will overlap and cause summation
type 2 SMUs contain more muscle fibers than type 1 which means more action potentials= greater signal
What is cross-talk?
data recorded from muscles other than the target muscle, this occurs when the recording zone is larger than the target muscle
How does subcutaneous tissue (i.e. fat) affect EMG signal?
adipose tissue under the skin and on top of the target muscle can filter the surface EMG signal due to the relative increase in distance between the muscle and the sEMG
What are some technical determinants of EMG signal?
skin preparation (decrease oils) & other electrical noise
What is the motor point?
the region of the muscle where the action potential is being delivered (neuromuscular junction)
Why do we need to avoid the motor point when recording EMG signals?
the bipolar electrodes measure the net signal (so both +ve & -ve signals) so measuring over the top of the motor point will result in a very low signal regardless of muscle activity
Where relative to muscle fibres should electrodes be placed?
parallel to the fibres
Is there typically more motor unit activity during a concentric or eccentric contraction?
concentric
there is more force for eccentric contraction as compared with the same drive in a concentric contraction therefore more motor unit activity is needed for concentric movements
Describe 6 factors that can influence the amplitude of an EMG signal.
- skin preparation
- outside electrical noise
- positioning of the electrode (away from the motor point & parallel to fibers)
- muscle fiber type (type 1 slow=summation, type 2 have more fibers in SMU)
- subcutaneous tissue
- cross-talk
What is electromechanical delay?
the time lag between onset of muscle activation and muscle force production
What biological factors influence an EMG signal?
number & discharge rate of motor units
muscle fibre type (slow twitch more likely to summate)
number of muscle fibres within a single motor unit
recording zone (single muscle, multiple muscles)
muscle depth & subcutaneous tissue
How does the type of muscle fibre affect an EMG signal?
slow-twitch fibres are more likely to summate than fast-twitch fibres as they are happening for longer
also type 2 motor units contain more muscle fibres, hence more action potentials
Why is electrode position important for an accurate EMG recording?
positioning the EMG over multiple muscles will not give a clear indication of a singular muscle (aka cross talk)
Why might fine wire EMG be more accurate for singular muscles?
it has a smaller recording zone to surface EMG
What are some technical factors that influence EMG signals?
cleaning of skin to avoid excess oils on the skin
outside electrical noise
electrode position relative to the motor point (where lots of neuromuscular junctions occur) & parallel to muscle fibres to avoid +/- summation
How could you normalise data in order to compare different muscles and different people?
using a maximum voluntary contraction
this allows you to compare the EMG readings against a 100% marker (i.e. the person uses 20% of VL compared to 10% VM maximum contraction)
