EMG

Question 1

What is an EMG?

Answer 1

EMG: electricity(mV of an electrical signal), myo = filaments, graphy = recorded/written
- Allows you to see what muscles are controlling the movement
- Agonist is as fast as possible, then antagonist is switched on the stop/slow the movement
* recording changes in muscle electrical potential
* estimate muscle excitation, which is before the muscle is actually activated
* muscle timing and sequencing (control)

Question 2

What are some intrinsic Factors that are not always controllable in taking an EMG:

Answer 2

• Fibre type, diameter
  
  • Blood flow, subcutaneous tissue, active motor units

Question 3

Name some Extrinsic & controllable factors in EMG design:

Answer 3

• Sensor location, bandwidth, sampling rate
  
  • Muscle crosstalk, line noise, skin impedance

Question 4

What are the different types of electrode?

Answer 4

(1)Needle and (2)fine-wire are indwelling electrodes
- Consists of 2 fine-diameter insulated wires threaded through a hollow needle cannula(the greater the insulation the greater the recording volume)
- Directly into a muscle
- Can measure individual motor units - accurate to measure electrical activity
- Very invasive, not feasible for sporting movements
- expensive
(3)bipolar surface

Question 5

Describe sEMG:

Answer 5

Surface electromyography: (sEMG): used mainly in sports literature
* commonly active bipolar(2 electrodes) systems
* differential amplifiers - recording the differences between 2 muscle electrodes as big as possible
* surface muscles only - hard to get an accurate reading of a muscle under other muscles(e.g.: soleus which is under gastrocnemius
* global (whole muscle) pick-up
* inexpensive and easy to apply - easy to get a signal, difficult to get a good signal
* commonly used for sports movements

Question 6

What are the factors to consider when setting up an EMG?

Answer 6

1. electrode separation
   - Further apart they are the easier it is to pick up the action potential between them, but will be more background noise effecting the difference between them
   - The percentage of contamination(optimum distance) is best with 1cm between them(increase signal, not too much noise)
   - If the electrodes are not placed in parallel to the muscle fibres then the amplitude can be affected by 50%
   2.Cross talk between muscles
   - The surface based system may pick up signal from other muscles as there is no needle
   3.Belly of muscle
   - More muscle under EMG the more signal will be available to measure(in middle)
   4.Parallel to muscle fibres (skin preparation)
   - Action potential propagation needs to be in the same direction as electrodes(so it is visible)
   - Shaving and cleaning of skin to give the best possible connection between the electrodes and the muscle

Question 7

Where is the best place on a muscle to measure freq and amplitude?

Answer 7

Belly of muscle - best for EMG measure & in orientation of muscle fibre

Question 8

What is the role of a reference electrode?

Answer 8

Reference electrode - part of the leg without muscle, so that anything that electrode picks up can be emitted from the EMG

Question 9

Describe amplitude resolution:

Answer 9

The more increments used the more similar the signal will look to the original
- feature that allows the amplifier to attenuate these common signals is called common-mode rejection
- the extent to which signals common to both inputs are attenuated is described by the common-mode rejection ratio (CMRR)

Question 10

Why is time resolution important?

Answer 10

sampling frequency
* sEMG: 10 – 400 Hz (times per second)
* high → a lot of data/large files
* low → alias/measured signal doesn’t reflect the real signal
* depends on activity

Question 11

How can the signal of the EMG be affected?

Answer 11

The deeper the motor unit, the smaller the signal will be, also there will be a timing delay as takes time to reach the surface

Question 12

What is the electrical signal insulated by?

Answer 12

• insulated by:
  – connective tissue
  – fat
  – skin
  – other muscle?
  – (active materials, properties change with time)

Question 13

How is full wave rectification achieved?

Answer 13

1st step is full wave rectification, making the full wave
Positive(neg values*-1)

Question 14

What is low pass filtering?

Answer 14

Low pass filtering, removes high freq noise only keeping low freq
(linear envelope is kept) - to find p value

Question 15

What are some applications of EMG’s in life?

Answer 15

• clinical
• resistance training
• research
• simulation
• biofeedback

Question 16

What are the limitations to sEMG?

Answer 16

• other components included in signal
• excitation ≠ activation
• not muscle force

Question 17

Define:
neural-excitation
muscle excitation

Answer 17

• Neural Excitation–Electrochemical input from an α-motoneuron that depolarizes all of the muscle fibres that it innervates
• Muscle Excitation–Depolarization of the sarcolemma following neural excitation, delivered to the muscle via the neuromuscular junction