EMG

Question 1

Where does an EMG signal come from?

Answer 1

the signal is of the action potentials being delivered down the axons of the single motor unit and into the muscle fibres

Question 2

How quickly does an action potential move down both directions of a muscle fibre?

Answer 2

2-6 m/s

Question 3

What are the two types of EMG?

Answer 3

surface and fine-wire

Question 4

Why would you use fine-wire EMG over surface EMG?

Answer 4

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.

Question 5

What two muscle fibre factors influence EMG signal?

Answer 5

number of SMU’s being recruited and the discharge rate

Question 6

How does muscle fibre type influence EMG signal?

Answer 6

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

Question 7

What is cross-talk?

Answer 7

data recorded from muscles other than the target muscle, this occurs when the recording zone is larger than the target muscle

Question 8

How does subcutaneous tissue (i.e. fat) affect EMG signal?

Answer 8

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

Question 9

What are some technical determinants of EMG signal?

Answer 9

skin preparation (decrease oils) & other electrical noise

Question 10

What is the motor point?

Answer 10

the region of the muscle where the action potential is being delivered (neuromuscular junction)

Question 11

Why do we need to avoid the motor point when recording EMG signals?

Answer 11

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

Question 12

Where relative to muscle fibres should electrodes be placed?

Answer 12

parallel to the fibres

Question 13

Is there typically more motor unit activity during a concentric or eccentric contraction?

Answer 13

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

Question 14

Describe 6 factors that can influence the amplitude of an EMG signal.

Answer 14

1. skin preparation
2. outside electrical noise
3. positioning of the electrode (away from the motor point & parallel to fibers)
4. muscle fiber type (type 1 slow=summation, type 2 have more fibers in SMU)
5. subcutaneous tissue
6. cross-talk

Question 15

What is electromechanical delay?

Answer 15

the time lag between onset of muscle activation and muscle force production

Question 16

What biological factors influence an EMG signal?

Answer 16

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

Question 17

How does the type of muscle fibre affect an EMG signal?

Answer 17

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

Question 18

Why is electrode position important for an accurate EMG recording?

Answer 18

positioning the EMG over multiple muscles will not give a clear indication of a singular muscle (aka cross talk)

Question 19

Why might fine wire EMG be more accurate for singular muscles?

Answer 19

it has a smaller recording zone to surface EMG

Question 20

What are some technical factors that influence EMG signals?

Answer 20

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

Question 21

How could you normalise data in order to compare different muscles and different people?

Answer 21

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)