Lecture 6 - Fundamental Considerations in the Measurement of Muscle Activity

Question 1

What are the components of a motor unit?

Answer 1

• Motor neuron
• Motor axon
• Motor end plate
• Muscle fibres

Question 2

What are the reasons for contradictory findings in the EMG - Tension relationship as they both are affected by the same factors (number and firing rates of the active
muscle fibres).

Answer 2

– Force output is the sum of forces from all contracting muscles + antagonist activity
– Control properties of EMG signal unstable between 80-100% MVC
– Cross-talk
– Muscle type (e.g. plantar flexors vs. biceps brachii)

Question 3

On the frequency spectrum what does a high pitch frequency indicate?

Answer 3

• A fast twitch activation within the muscle.

Question 4

What is Frequency domain in EMG and what are the parameters used for this?

Answer 4

• Frequency-domain – fatigue indicator

-Median Frequency – more sensitive to spectral
depression- Mean frequency – less variable
- Zero crossings
- Time to peak

Question 5

Why is the relationship between EMG and force not linear in dynamic contractions?

Answer 5

• Change in muscle length, force & moment arm

- Change in source of the EMG signal

Question 6

`What is crosstalk in EMG, where it occurs, and how it can be reduced.

Answer 6

• Crosstalk = when the detected signal contains noise from another muscle
• Common in small, close muscles e.g. the forearm
• Range between 3-10%, can be as high as 17%
• Detected using cross-correlation
• Can be reduced by:
  • Decreasing electrode size and spacing
  • Placing the electrode on the muscle belly
  • Double differential technique – 3 electrodes

Question 7

What is EMD and the % difference between the hamstrings and quads?

Answer 7

• EMD - Electro mechanical delay, the time between signal and contraction.
• The hamstrings have a 95% greater EMD than the quadriceps (22.6ms vs 44.0ms)

Question 8

What are the sporting applications of EMG?

Answer 8

• Is the muscle active
• When is it active
• Changes in muscle activity
• Does the muscle fatigue
• Is there coactivation.

Question 9

What needs to be considered if you were to standardise ECG recordings?

Answer 9

• Joint Angle – influences EMG amplitude because muscle mechanics change with length.
  The muscle may also migrate below electrodes.
• Range of Motion – same as above – use goniometers, training machines or mirrors
• Movement Velocity – higher velocity may mean more MU recruitment – use a metronome, treadmill or isokinetic devices
• Load/Resistance – use static resistance or external weights
• Duration/Repetitions – strongly determines influence of fatigue – use fixed durations and repetitions
• Preliminary status (e.g. fatigue) – metabolic and CNS conditions and the time of day – same time of day, warm-up subjects

Question 10

What are the intrinsic (non-controllable) physiological and anatomical factors that effect EMG signal?

Answer 10

- Physiological
• Number of active MU’s
• MU firing rate and synchronisation
• Fibre type and diameter
• Blood flow
• Metabolic factors

• Anatomical
  • Fibre diameter
  • Depth and location of fibres
  • Subcutaneous tissue

Question 11

What are the extrinsic (controllable) factors that can effect EMG signal for location?

Answer 11

• Location of electrodes
  • NOT on outside edges – crosstalk
  • NOT on motor point (greatest neural density)
  • NOT on tendon – fewer and thinner fibres
  • Between motor point (innervation zone) and tendon – point where muscle begins to twitch with the lowest amount of current
  • Muscle belly ideal

Question 12

How can a difference in the EMD between the hamstrings and quads be a risk factor for injury?

Answer 12

• The Hamstrings have a lower relative explosive force production which means that if there is a delay in timing the knee joint stability could be comprised resulting in risk of ACL.

Question 13

How is fatigue perceived on the frequency spectrum?

Answer 13

• Shown as lower frequency’s (Pitch) because:
• Higher threshold MU fatigue
• There is a subsequent contraction velocity and MU become synchronised.

Question 14

What are the neural factors which influence force generation?

Answer 14

• 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 action potentials (rate coding)

Question 15

What are the extrinsic (controllable) factors that can effect EMG signal for impedance and orientation of electrodes?

Answer 15

- Impedance (Signal activity)
• Skin preparation completed?
• Light red colour for on?
• Gel vs. no gel?
• Electrode impedance tester
• 5 minutes to achieve stable condition
- Orientation of electrodes
• Parallel with muscle fibres
- Reference electrode?
• On an electrically unrelated site

Question 16

How can we manipulate neural factors during training?

Answer 16

• By adjusting training intensity
• Cadence / speed
• Rep numbers

Question 17

What is a threshold analysis used for in EMG ?

Answer 17

• To accurately assess when the muscles is active /
  inactive.
• Do this by first finding the threshold before testing to se when testing if activity is above of below this point.

Question 18

How are EMG signals generated?

Answer 18

• A nerve impulse causes contraction of the muscle fibres
• The stimulation of the muscle fibre at the motor end-plate produces a reduction of the
  electrical potential of the cell (depolarisation)
• This which then propagates over the entire
  fibre before being followed by a repolarisation wave.

Question 19

What is noise and how does it interfere with EMG data and give some examples of typical noise levels.

Answer 19

• Noise = Any signals that are not part of the physiological signal.
• Ambient noise (50 or 60 Hz)
• Motion artifact (0-20 Hz)
• Physiological noise e.g. ECG (located at around 80 Hz.

Question 20

What are Non-sporting application of EMG?

Answer 20

• Ergonomics – Workplace assessment e.g. repetitive strain problems
• Clinical Diagnostics e.g. Gait irregularities, incontinence, rehabilitation.
• Helps in decision making both before/after surgery
• Product design e.g. orthotics, bracing, taping

Question 21

What is Normalisation, it’s advantages and disadvantages?

Answer 21

• Normalisation = Expression of muscle activity in relative terms

Advantages:
– Allows comparisons with other muscles, other subjects and between studies.
– Eliminates any influence of the detection conditions

Disadvantages:
– Ability to consistently elicit an MVC varies between individuals
– Poor repeatability of EMG from MVC’s
– Mechanics of movement – MVC at fixed angle

Question 22

What are the types of electrodes for gathering EMG signals and some advantages or limitation of these?

Answer 22

• Indwelling electrodes: deep muscles and isolated MU’s
• Active Surface electrodes (power supply):
  • Superficial muscles/large muscle groups.
  • Silver-silver chloride, Diameter = 1cm
  • Main advantages: Early pick up of signal, amplification, transmitted at low ohm level which is less sensitive to motion artefact.
  • Other advantages: less skin preparation, less sensitive to impedance of the electrode-skin interface
  • Limitations: bulky – pressure artefacts, placement errors, cross-talk.
  • Passive Surface electrodes (no power supply):
  • Limitations: skin preparation, placement errors, cross-talk.