Muscle L4: Using electromyography to understand the control of movement Flashcards
Why is EMG (electromyography) used?
Advance our understanding of movement control
Some physiotherapists use EMG in the clinic for _______.
Biofeedback
What does a motor unit consist of?
motoneuron, motor axon and all of the muscle fibres it innervates
Motor units range in size allows _____, _____ and ____ force.
graded, sustained and controlled
Motor units get input from _______, ______ & ______.
descending pathways, spinal interneurons; afferent fibres
Measuring electrical activity generated by the __________ provides information about the neural drive to muscles.
muscle action potentials
What is depolarisation?
Many Na+ gates open, influx of ++
What is repolarisation?
K+ move out Na gates closed
What is hyperpolarisation?
Na+ out, K+ in via Na+/K+ pumps – influences maximal discharge rate
What is the neuromuscular junction?
The connection between the nervous system and the muscle is the neuromuscular junction (motor point, motor end plate, synaptic cleft)
Surface ECM: Where is the signal coming from? What affects an EMG signal?
Every discharge of an action potential in the motoneuron = action potential in the muscle fibres that belong to that motor unit
- Myoelectric action potentials (red arrows) are initiated by the Ach released by the motor neuron.
- The action potential sweeps across the muscle fibre and into the transverse (T) tubules.
- This initiates the release of calcium that regulate muscle activity.
Steps of muscle contraction
- Axon of motor neuron and synaptic terminal (release ACh)
- Changes in membrane potential of muscle
- Once muscle cell reaches threshold, generation of AP (run in both direction), along muscle fibre from neuromuscular junction (these AP are recorded in EMG)
- Down T-tubules to help release of Ca2+ from sarcoplasmic reticulum to allow process of actin and myosin to start to bind
A myoelectric action potential propagates at ~2-6 m/s along all muscle fibres from _____ same/different motor unit
same
It propagates in both directions from the ____ to the _______.
motor point; end of the muscle fibre
What are 2 ways to record action potentials?
- Surface electrode on skin (overlye activated muscle)
- Insert wires into muscle (record from much smaller area of muscle)
Electromyography records a complex summation of ________.
overlapping asynchronous action potentials
What are 4 biological determinants of EMG signal?
- Motor unit discharge
- Fibre angle, fibre type
- Change in muscle position
- Cross talking, other muscles
Biological determinants of EMG signal: How does motor unit discharge affect EMG?
- EMG is altered by number & discharge rate of motor units
- Motor unit discharge Also by the number of muscle fibres in the motor unit
- More drive, more action potentials, greater EMG amplitude -many biological and technical factors influence the EMG signal
EMG is altered by the ________ and _______ of motor units
number; discharge rate
Biological determinants of EMG signal: How does fibre angle, fibre type affect EMG?
- EMG is influenced by muscle fibre direction relative to electrode position and fibre type
- Aligned with muscle fibre
- Not perpendicular = signal cancellation remove good signals from recording
- Do this by looking @ this on ultrasound fibres & pennation angle
- More muscle fibres in a larger unit – more myoelectric action potentials.
- Slow twitch (Type 1, postural) have a slower myoelectric action potential conduction velocity
- Slow AP in slow twitch (fibre type dependent)
- If muscle fibre direction changes during a dynamic task- can change amount of area being recorded
EMG is influenced by muscle ________relative to electrode position and fibre type
fibre direction
- Must be align with muscle fibre.
- Make sure that its not perpendicular = signal cancellation remove good signals from recording
Slow twitch (Type 1, postural) have a ______ (slower/faster) myoelectric action potential conduction velocity. How does that affect EMG?
Slower
- Slow AP in slow twitch (fibre type dependent)
- If muscle fibre direction changes during a dynamic task- can change amount of area being recorded
Biological determinants of EMG signal: How does a change in muscle position affect EMG?
- An electrodes recording zone is the region or muscle from which the myoelectric signal can be recorded.
- Affects where skin lies over muscle
- If muscle/skin relationship changes can have inaccurate recording
- Consider if the electrodes will be above the muscle belly throughout motor task
- Cross talk - The recording zone of an electrode may be bigger than the muscle being measured
An __________ is the region or muscle from which the myoelectric signal can be recorded.
electrodes recording zone
Biological determinants of EMG signal: How does cross talk affect EMG?
The recording zone of an electrode may be bigger than the muscle being measured
How does cross talk relate to surface EMG and fine wire EMG? (based on SCM and scalenes)
Surface
Inspiration: EMG recorded conclusion- both SCM & scalenes produce a bit of force
- During surface electrodes, both SCM & scalenes are producing force
- During fine wire electrodes, only scalenes are producing force, not SCM
What are 3 technical determinants of EMG signal?
- Skin preparation
- Other electrical noise
- Electrode placement
Technical determinants of EMG signal: How does skin preparation affect EMG?
- Use sand paper, exfoliating cream- clean skin
- Shave skin
Technical determinants of EMG signal: How does other electrical noise affect EMG?
Normally around us (in environment) = 50Hz
Technical determinants of EMG signal: How does electrode placement affect EMG?
Does EMG differ between contraction types?
- During concentric contraction
- Single motor units are discharging at a higher rate than when eccentric
- Long muscle = same length = same torque = same but driving single motor units harder in concentric compared to eccentric
What are mechanism for higher muscle force for same neural drive?
- Active force is generated through the actin and myosin cross-bridge.
- In eccentric movements actin and myosin ‘hold on’, more stretch of the elastic components (eg titin) > more force for same energy (# action potentials)
- Decreased rate of cross-bridge detachments >> greater force production on the eccentric bout.
- EMG provides a good indication of neural activation of a muscle.
- No elastic info about how long myosin and actin stay together
- EMG cannot provide any information about passive forces.
- EMG does not = force Consider how muscle length and contraction type may change during the motor task.
____ force is generated through the actin and myosin cross-bridge.
Active
In ______ movements actin and myosin ‘hold on’, more stretch of the elastic components (eg titin) > more force for same energy (# action potentials)
eccentric
Decreased rate of cross-bridge detachments >> greater force production on the ______ bout.
eccentric
EMG provides a good indication of _____of a muscle. However, there is no ___info about how long ____and _____ stay together, ____ forces and ___________.
neural activation; elastic; myosin; actin; passive forces; how muscle length and contraction type may change during the motor task.
What is electromechanical delay? How it can be measured? How may it affect the way that we can assess movement?
EXAM QUESTION
- is the time lag between onsets of muscle activation and muscle force production and reflects both electro-chemical processes and mechanical processes
- Measuring onset of EMG does not tell you when the muscle force is produced.
