Lecture 6 - Fundamental Considerations in the Measurement of Muscle Activity Flashcards
What are the components of a motor unit?
- Motor neuron
- Motor axon
- Motor end plate
- Muscle fibres
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).
– 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)
On the frequency spectrum what does a high pitch frequency indicate?
- A fast twitch activation within the muscle.
What is Frequency domain in EMG and what are the parameters used for this?
- Frequency-domain – fatigue indicator
-Median Frequency – more sensitive to spectral
depression- Mean frequency – less variable
- Zero crossings
- Time to peak
Why is the relationship between EMG and force not linear in dynamic contractions?
- Change in muscle length, force & moment arm
- Change in source of the EMG signal
`What is crosstalk in EMG, where it occurs, and how it can be reduced.
- 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
What is EMD and the % difference between the hamstrings and quads?
- 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)
What are the sporting applications of EMG?
- Is the muscle active
- When is it active
- Changes in muscle activity
- Does the muscle fatigue
- Is there coactivation.
What needs to be considered if you were to standardise ECG recordings?
- 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
What are the intrinsic (non-controllable) physiological and anatomical factors that effect EMG signal?
- 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
What are the extrinsic (controllable) factors that can effect EMG signal for location?
- 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
How can a difference in the EMD between the hamstrings and quads be a risk factor for injury?
- 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.
How is fatigue perceived on the frequency spectrum?
- Shown as lower frequency’s (Pitch) because:
- Higher threshold MU fatigue
- There is a subsequent contraction velocity and MU become synchronised.
What are the neural factors which influence force generation?
- 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)
What are the extrinsic (controllable) factors that can effect EMG signal for impedance and orientation of electrodes?
- 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
How can we manipulate neural factors during training?
- By adjusting training intensity
- Cadence / speed
- Rep numbers
What is a threshold analysis used for in EMG ?
- 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.
How are EMG signals generated?
- 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.
What is noise and how does it interfere with EMG data and give some examples of typical noise levels.
- 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.
What are Non-sporting application of EMG?
- 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
What is Normalisation, it’s advantages and disadvantages?
- 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
What are the types of electrodes for gathering EMG signals and some advantages or limitation of these?
- 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.
