Module 4 - Electromyography Flashcards
What are some applications for Electromyography?
- Clinical (diagnostic and monitoring)
- Research
- Ergonomics
- Sport Sciences
- Control Signals for devices or games
What is a motor unit?
- Motoneuron and the muscle fibers it innervates
What is an innervation ratio?
- Number of muscle fibers innervated by a motoneuron
What is the Neuromuscular Junction?
- Special Synapse Between motor axon and muscle fiber
- Neurotransmitter: Golgi Tendon Organ
How does the number of motor units and innervation ratio vary between muscles?
- Small innervation ratio for eye and hand muscles
- Large for leg and trunk muscles
What are some examples of muscles and their motor units and innervation ratios?
Abductor Pollicis Brevis
- MU: 171
- IR: 90
Rectus Lateralis
- MU: 4150
- IR: 5
Medial Gastrocnemius
- MU: 580
- IR: 1934
How does the nerve communicate with a muscle?
- Synapse
What is the excitation-contraction coupling?
- Ach binding results in depolarization of motor end plate
- muscle fiber sarcolemma & action potential propagates from NMJ outwardly in all directions
What is the path of Excitation-Contraction Coupling?
- Generation of motoneurons action potential
- Release of Ach for presynaptic nerve terminal
- Binding of Ach to muscle fiber Ach receptor
- Depolarization of sarcolemma
- Conduction of action potential into the fiber by T-tubules
- Signaling of the SR by T-tubules to release Ca++
- Binding of Ca++ to troponin - allows actin-myosin interaction
- Force Generation (actin-myosin interaction)
- Pumping of Ca++ back in SR
- Ca++ concentration diminishes, muscle relaxes
What are the three muscle fiber types?
- FF type motoneurons innervate FG muscle fibers forming FF type motor units
- FR motoneurons innervate FOG type muscle fibers forming FR type motor units
- S motoneurons innervate SO type muscle fibers forming S type motor units
What is a twitch profile?
- excitation is rapid (5ms)
- Contraction and Relaxation Slow (100ms)
- If second impulse is superimposed, summation occurs
What is temporal summation?
- Second impulse superimposed before first one has fully relaxed
- Functional effect depends on the number of pulses and proximity
- Multiple pulses results in higher force than single one
What is an unfused contraction?
- temporal summation below 20Hz
Is it possible to distinguish an unfused contraction?
Yes
- Possible to distinguish pulses within the force record
- Distinguish individual twitches
Why is the net force smooth in human twitch profiles?
Motor units produce partially fused tetanus
- fire asynchronously with each other
How does Electromyography work?
Records Local Electrical Activity Associated with Muscle Activity
- Signal emitted each time a muscle contracts
- Muscle fiber action potential recorded
- Requires Electrodes and amplifier
What are the two types of electrodes used in EMG recording?
- Surface Electrodes
- Indwelling Electrodes
Describe the amplifier part of an EMG?
Gain and Dynamic Range:
- EMG signal up to 5mv (surface) and 10mV(indwelling)
- Gain of 1000 typical, noise of amplifier <20uV
- frequency response: 10-1000Hz (surface), 20-2000Hz(indwelling)
Is the Human Body a good conductor?
- YES
What can cause disruption or cross-talk with EMG?
- Power cords
- Fluorescent Lighting
- Electric Machine
Describe the process of EMG surface Recording.
- Surface electrodes are placed on the skin overlying the muscle of interest
- Frequently used
- Simple and non-invasive compared to indwelling
- Electrical activity is recorded from portion of muscle
What is the spatial summation of muscle electrical activity?
- depth of MU determines relative strength of signal and shape
- Deeper MU take longer to travel to surface
What is the temporal summation of muscle electrical activity?
- Conduction velocity influences shape of MU
What are the important technical considerations of EMG surface recordings?
- Position of Electrodes
- Inter-Electrode Distance
- Electrode Area
How does the Position of Electrodes impact the EMG surface Recording?
- Must be placed on one side of the innervation zone
- Best Practice: middle third of distance between innervation zone and muscle tendon interface
How does the Inter-Electrode Distance impact EMG surface recording?
- Smaller distances lead to higher frequency content
- Generally, 20mm is acceptable
How does the Electrode Area impact EMG surface recording?
- Smaller surface area leads to larger EMG signal
- Smaller duration of MUAP & less cross-talk
What factors influence EMG Signal?
- Quality of amplifier and recording equipment
- Electrodes configuration: area and shape
- Number of active motor units
- Depth and location of active fibers
- Amount tissue between surface of muscle and electrodes: spatial filtering
What man-made noises can be introduced from sources other than muscle?
- Mainly comes from power lines (60Hz in NA)
- Motor and other electric instuments
What biological noise can interfere EMG?
- Electrocardiogram when recording thoracic muscles
What are artifacts in relation to EMG?
- False signals generated by electrodes or cables
How can EMG artifacts be movement related?
- Movement of the cables or touching the electrodes
What are the limitations of EMG surface recording?
- Crosstalk
- Movement Artifacts
- Deep Muscles
- Individual MU
What is Crosstalk?
- Activity of nearby muscles contribute to EMG of the muscle under investigation
What are movement artifacts related to EMG surface recordings?
- Frequent during explosive movements or electrode contract
- Can usually be extracted because limited to low frequencies
How are deep muscles a limitation to EMG surface recordings?
- Cannot record EMG from deep axial muscles
How are individual MU’s a limitation to EMG surface recordings?
- Cannot identify single MU activity with a single differential surface EMG recording
Explain 2D array of Electrodes.
- Localization of EMG modulation
- Identification of Muscle Fibers
- MU firing patterns
In what orientation do you place high-density surface EMG?
- Position of the innervation zone
- Muscle fiber conduction velocity
Describe the EMG indwelling recording practice.
- Different shapes and often homemade: wires or needles
- Invasive
- Allows for recording specific muscle: crosstalk eliminated
- Remains the gold standard for single MU recordings
- Can record from deep muscles
- Possible determine recruitment order/rate coding/influence of MU in functional tasks
What is rate coding?
- Physiological strategy to alter force by altering activation frequency (temporal summation)
What are limitations of EMG indwelling recordings?
- Records only from a small portion of a muscle
- Technically difficult and invasive (compared to Surface)
- MU shape is time consuming to identify
Is muscle activity synchonized?
No
- Temporal and spatial summation of all MU’s active near the electrode during a task
What is an exception to non-synchronized muscle activity?
- Muscle Fatigue
- Electrical Stimulation (link to ECG)
Why do you need other sensors to understand muscle action?
- Can record identical EMG signals from different type of contrations
Does EMG measure muscle force?
No
- can try to establish muscle force relationship
What is an application for force relationship from EMG?
- Robotic Control of balance or assistive devices
Explain EMG’s oscillatory electrical activity.
- Positive and Negative components
- If we average raw single, value near zero
How do we rectify EMG signals?
- Envelope of rectified signals
- Integrated EMG, RMS, Power
What is the simplest comparison of EMG-Force Relationship?
- Isometric contraction
- One value of force can be compared to one value of EMG
- Avoid problems associated with muscle length and velocity & muscle movement under the electrodes
When is the EMG-Force relationship good?
- When electrode position is stable with respect to active muscle fibers
- When Motor Unit Activation pattern is stable
When is EMG-force relationship not predictive?
- May only be a coarse predictor of muscle tension
- Invalid if muscle length changes rapidly
What are some anatomical factors in the EMG-force relationship?
- Shape of neuromuscular systems
- Thickness of subcutaneous fat
- Size & distribution of motor unit territories
- Innervation Ratio
- Pennation Angle
What is the measurement system for EMG?
- Electrode Size
- Shape
- Placement
- Impedance
What are the geometrical properties of EMG recording?
- Muscle fiber shortening
- Movement of muscle relative to electrodes
What are the physical factors of an EMG recording?
- Conductivity of tissues
- Cross-talk
What are the physiological factors of an EMG recording?
- Conduction Velocity
- Number of MU activated
- Distribution of Discharge Rates
- Synchronization
What is EMG?
- Recording of Electrical Activity associated with muscle contraction
What are the key points of EMG?
- Does not record muscle force
- Can be done with surface or indwelling electrode
- Both techniques have advantages
- Depending on the question both can provide relevant information
