Neuromuscular Biomechanics - EMG Flashcards
(36 cards)
Sport & Exercise uses for EMG (x4)
- is the muscle active
- when is the muscle active
- is a muscle more or less active
- does the muscle fatigue
How can EMG be used for the assessment of injury risk
- using muscle (concentric) pairs, are they both active/equally during contractions & the EMD of the muscles
- lower explosive force production of the hamstring relative to the quads could compromise knee joint stability & increase ACL injury risk
- also want EMD to be similar and forces exerted to be similar to reduce risk of injury
Ricci et al. (2013) example study for EMG injury risk
- quads were 79% stronger than hamstrings, explosive force 480% greater from 25-50 ms after first activation
- hamstring EMD was 95% greater than quads EMD resulting in 21 ms later onset of force in hamstring
how an EMG signal is generated
- the stimulation of the muscle fibre at the motor end-plate producing a reduction of the electrical potential of the cell (depolarisation)
- this then propagates over the entire fibre before being followed by a repolarization wave
4 Steps of EMG generation
- detection of the potential fluctuations (electrodes)
- signal transmission (hardwire/telemetry/data logger)
- signal modification (amplifier)
- storage of the resulting waveform (computer)
process of signal amplification
- signal is small, varying from 10 to 5mV, so signal needs to be amplified up to at least a level of 1V
- differential amplifier usual type of amplifier, which amplify the EMG signal linearly without amplifying noise or error in the signal
- sources of error in the EMG signal can be from sources other the muscle (e.g. machinery/amplifier itself)
- amplifier must have high input impedance (resistance) & good frequency response
Causative intrinsic non-controllable factors affecting EMG
Physiological
- number of active MU’s; MU firing rate and synchronisation; fibre type and diameter; blood flow; metabolic factors
Anatomical
- fibre diameter/type; depth and location of fibres; subcutaneous tissue; number of muscle fibres
Electrode types are an extrinsic (causative) factor affecting EMG. What types are there and what’re the +ve’s and -ve’s?
Indwelling electrodes
- used for deep muscle and isolated MU’s
- (-ve) invasive and difficult to use for dynamic actions
Active Surface Electrodes
- used for superficial muscles/large muscle groups
- (+ve) early pick up of signals; amplification; transmitted at low ohm level which is less sensitive to motion artifact; less skin prep; less sensitive to impedance of the electrode-skin interface
- (-ve) bulky; placement errors; cross-talk
Causative extrinsic factors affecting EMG
Impedance
-skin prep (light red colour); electrode impedance tester
Location of electrodes
-NOT on outside edges (cross-talk); NOt on motor point (greatest neural density); NOT on tendon (fewer & thinner fibres); between motor point (innervation zone) and tendon (point where muscle begins to twitch with lowest amount of current); on the muscle belly
Orientation of electrodes
- parallel with muscle fibres
How to determine innervation zone?
- use an electrical stimulator
- array sensor = the most accurate
- location of the highest value of frequency
When standardising EMG recordings, what factors do you have to consider?
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
Load/resistance
- use static resistance or external weights
Duration/repetitions
- strongly determines influence of fatigue (so use fixed durations/reps)
Preliminary status (e.g. fatigue)
- metabolic & CNS conditions & the time of day (e.g. same time of day, standardized warm up)
General EMG recommendations
- prefer isometric tests
- use dynamometers if a high standardization is needed
- single joint exercises have less variability
what is Nyquist Sampling Theory
A band limited continuous-time signal can be sampled & perfectly reconstructed from its samples if the waveform is sampled over twice as fast as its highest frequency component
Causative extrinsic factors: crosstalk characteristics
- when the detected signal contains noise from another muscle
- common in small, close muscles (e.g the forearm)
- range between 3-10%, can be up to 17%
- detected using cross-correlation
Methods of reducing crosstalk (Causative extrinsic factors of EMG)
- decreasing electrode size & spacing
- placing the electrode on the muscle belly
- double differential technique (3 electrodes)
Methods of reducing noise
Differential amplification/CMRR
- signal is detected at two sites, the signals are subtracted, and the difference is amplified
- as a result, any signal that is ‘common’ to both detection sites will be removed
- most noise is ‘common’ to both detection sites
Double differential technique
- the signal is detected at three sites & the subtraction procedure is performed twice
What is noise as an extrinsic factor affecting EMG signal
- noise = any signals that are not part of the physiological signal
- there is inherent noise within detection/recording equipment
- types of noise: ambient noise (50/60 Hz); motion artifact (0-20 Hz); physiological noise e.g. ECG (around 80 Hz); electromechanical noise
- can be caused by external pressure or change in position of detection site
- av. baseline noise should not exceed 3-5 microvolts, frequency between 10-250 Hz
- aim is to maximise signal-to-noise ratio
Types of filters used to reduce noise
Low-pass - lets lower frequency through High-pass - lets higher frequency through Band-pass - lets frequencies within certain range
Inherent filters used to reduce noise
Tissues - low-pass filter Electrode to electrolyte interface - high-pass filter Bipolar configuration - band-pass filter Amplifier - band-pass filter
what is a time-domain used for when processing an EMG signal?
- used to estimate amplitude
- Raw EMG data is rectified using…
AREMG - average rectified value over a time period (area under rectified EMG)
RMS - the square-root of the average power of the signal in a given time (measure of the number of recruited MUs)
what is a frequency-domain used for when processing an EMG signal?
- used as a fatigue indicator
- parameters mainly used…
Median Frequency - more sensitive to spectral depression
Mean Frequency - less variable
Zero crossings
Time to peak
how does frequency analysis of an EMG signal work as a fatigue indicator
- fatigue results in compression of the frequency spectrum towards lower frequencies.
This is due to… - fatigue of higher threshold MU’s
- decreased conduction velocity (metabolite accumulation)
- MU synchronisation
what is normalisation of EMG signals and what’re the +ve’s and -ve’s
- expression of muscle activity in relative terms
- main one used is PEAK EMG (isometric contraction)
(+ve’s) - allows comparisons with other muscles, other subjects & between studies
- eliminates any influence of the detection conditions
(-ve’s) - ability to consistently elicit an MVC varies between between individuals
- poor repeatability of EMG from MVC’s
- mechanics of movement but MVC is completed at fixed angle
what is the EMG-tension relationship
- as muscle tension & EMG signal are affected by the same factors (number and firing rates of the active muscle fibres), then a relationship should exist between them
- however the expected relationship only applies to the contractile elements
- both linear (Lippold, 1952) & non-linear (Vredenbregt & Rau, 1973) EMG-tension relationships have been found
