Measurement of voluntary muscle activity

Question 1

What are the non-sporting applications of EMG?

Answer 1

-Ergonomics
Clinical Diagnostics
-Helps in decision making before and after surgery
-Product design (orthotics, bracing and taping)

Question 2

What are the uses of EMG in sport?

Answer 2

• Identify if a muscle is active
• When is the muscle active, pattern of activation in skill acquisition. (proximal to distal sequencing)
• Is a muscle more or less active (as a result of training/detraining)
• Does the muscle fatigue at all?
• Does co-activation occur?

Question 3

What are the neural factors which influence force generation?

Answer 3

The force that a muscle exerts depends on

• the number of motor neurons that are activated (recruitment)
• The size of the activated motor neurons
• The rates at which they discharge their action potentials.

Question 4

How is an EMG signal generated?

Answer 4

> Nerve impulse causes contraction of the muscle fibres
Stimulation of the muscle fibre at the motor-end plate produces a reduction of the electrical potential of the cell (depolarisation) which is followed by a repolarisation wave.

Question 5

How does the EMG signal produce data?

Answer 5

1. detection of the potential fluctuations - electrodes
2. Signal transmission - hardwire/telemetry/data logger
3. Signal modification - amplifier
4. Storage of the resulting waveform - computer

Question 6

What are the intrinsic factors affecting the EMG signal? (non-controllable)

Answer 6

PHYSIOLOGICAL
-number of Active MUs
-MU firing rate and synchronisation 
-Fibre type and diameter
-Blood flow
-Metabolic factors 
ANATOMICAL  
-Fibre diameter 
-Depth and location of muscle fibres
-Subcutaneous tissue

Question 7

What are the Extrinsic Factors affecting EMG signal?

Answer 7

> impedance (skin preparation, light red colour, Gel vs no gel)
Orientation of electrodes
Reference Electrode
Location and placement of electrodes - consider migration of muscle belly

Question 8

What are the different electrode types?

Answer 8

• Indwelling electrodes
• Active Surface electrodes
• Passive Surface electrodes

Question 9

How can neural factors be manipulating during training?

Answer 9

Training Intensity - weight of lift
Cadence - speed of lfit
Number of reps

Question 10

How are EMG recordings standardised?

Answer 10

-Movement Velocity - A+B could both squat the same load. However A squat in 0.5s compared to B squat 2 seconds. Higher velocity = more MU recruitment.
Perform a MVC to allow comparisons of movement as a percentage of individuals maximum.

Question 11

What is crosstalk and how can it be avoided?

Answer 11

When detected signal contains noise from another muscle.

Can be reduced by decreasing electrode size, placing the electrode on the muscle belly.

Question 12

What is noise and how can it be avoided?

Answer 12

Any signals that are not part of the physiological signal. To reduce noise, careful taping should be performed. High quality shielded cables, pre amplifiers.

Question 13

How is the EMG signal processed?

Answer 13

Raw data is rectified. This is where all negative values below baseline are added to that above baseline. Rectification achieved via RMS or AREMG.

Question 14

What is Normalisation?

Answer 14

Normalisation is the expresion of muscle activity in relative terms.

Question 15

What are the advantages and disadvantages of MVC, in terms of normilisation?

Answer 15

> Advantages - allows comparisons with other muscles, other subkects and between studies.
Eliminates any influence of the detection conditions.
Disadvantages - Ability to consistently elicit an MVC
Poor repeatability of EMG from MVC’s
MVC is performed in a fixed movement - not functional.

Question 16

What is Threshold Analysis and why do we need it?

Answer 16

Threshold analysis is a defined range implemented to accurately assess when the muscle is active or inactive. It is needed to assess when the EMG signal is above this threshold. Activity is >baseline by 2 SD.

Question 17

What is the EMG-tension relationship?

Answer 17

Muscle tension and EMG signal are affected by the same factors - number and firing rates of active muscle fibres. Relationship only applies to contractile elements.

Question 18

What are the possible reasons for contradictory findings of EMG-tension relationship?

Answer 18

> Force output = sum of forces from all contracting muscles including antagonist activity.
Cross-talk
Control properties of EMG signal are unstable between 80-100% of MVC
Muscle type, fibre distribution

Question 19

EMG-tension relationship during dynamic contractions?

Answer 19

The relationship between sEMG and force is not lineasr in dynamic contractions. The reasons for this are due to a change in muscle length, force and moment arm. Also there is a change in source of the EMG signal.