Muscle L4: Using electromyography to understand the control of movement

Question 1

Why is EMG (electromyography) used?

Answer 1

Advance our understanding of movement control

Question 2

Some physiotherapists use EMG in the clinic for _______.

Answer 2

Biofeedback

Question 3

What does a motor unit consist of?

Answer 3

motoneuron, motor axon and all of the muscle fibres it innervates

Question 4

Motor units range in size allows _____, _____ and ____ force.

Answer 4

graded, sustained and controlled

Question 5

Motor units get input from _______, ______ & ______.

Answer 5

descending pathways, spinal interneurons; afferent fibres

Question 6

Measuring electrical activity generated by the __________ provides information about the neural drive to muscles.

Answer 6

muscle action potentials

Question 7

What is depolarisation?

Answer 7

Many Na+ gates open, influx of ++

Question 8

What is repolarisation?

Answer 8

K+ move out Na gates closed

Question 9

What is hyperpolarisation?

Answer 9

Na+ out, K+ in via Na+/K+ pumps – influences maximal discharge rate

Question 10

What is the neuromuscular junction?

Answer 10

The connection between the nervous system and the muscle is the neuromuscular junction (motor point, motor end plate, synaptic cleft)

Question 11

Surface ECM: Where is the signal coming from? What affects an EMG signal?

Answer 11

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.

Question 12

Steps of muscle contraction

Answer 12

1. Axon of motor neuron and synaptic terminal (release ACh)
2. Changes in membrane potential of muscle
3. Once muscle cell reaches threshold, generation of AP (run in both direction), along muscle fibre from neuromuscular junction (these AP are recorded in EMG)
4. Down T-tubules to help release of Ca2+ from sarcoplasmic reticulum to allow process of actin and myosin to start to bind

Question 13

A myoelectric action potential propagates at ~2-6 m/s along all muscle fibres from _____ same/different motor unit

Answer 13

same

Question 14

It propagates in both directions from the ____ to the _______.

Answer 14

motor point; end of the muscle fibre

Question 15

What are 2 ways to record action potentials?

Answer 15

1. Surface electrode on skin (overlye activated muscle)
2. Insert wires into muscle (record from much smaller area of muscle)

Question 16

Electromyography records a complex summation of ________.

Answer 16

overlapping asynchronous action potentials

Question 17

What are 4 biological determinants of EMG signal?

Answer 17

1. Motor unit discharge
2. Fibre angle, fibre type
3. Change in muscle position
4. Cross talking, other muscles

Question 18

Biological determinants of EMG signal: How does motor unit discharge affect EMG?

Answer 18

• 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

Question 19

EMG is altered by the ________ and _______ of motor units

Answer 19

number; discharge rate

Question 20

Biological determinants of EMG signal: How does fibre angle, fibre type affect EMG?

Answer 20

• 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

Question 21

EMG is influenced by muscle ________relative to electrode position and fibre type

Answer 21

fibre direction

• Must be align with muscle fibre.
• Make sure that its not perpendicular = signal cancellation remove good signals from recording

Question 22

Slow twitch (Type 1, postural) have a ______ (slower/faster) myoelectric action potential conduction velocity. How does that affect EMG?

Answer 22

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

Question 23

Biological determinants of EMG signal: How does a change in muscle position affect EMG?

Answer 23

• 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

Question 24

An __________ is the region or muscle from which the myoelectric signal can be recorded.

Answer 24

electrodes recording zone

Question 25

Biological determinants of EMG signal: How does cross talk affect EMG?

Answer 25

The recording zone of an electrode may be bigger than the muscle being measured

Question 26

How does cross talk relate to surface EMG and fine wire EMG? (based on SCM and scalenes)

Answer 26

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

Question 27

What are 3 technical determinants of EMG signal?

Answer 27

1. Skin preparation 2. Other electrical noise 3. Electrode placement

Question 28

Technical determinants of EMG signal: How does skin preparation affect EMG?

Answer 28

* Use sand paper, exfoliating cream- clean skin * Shave skin

Question 29

Technical determinants of EMG signal: How does other electrical noise affect EMG?

Answer 29

Normally around us (in environment) = 50Hz

Question 30

Technical determinants of EMG signal: How does electrode placement affect EMG?

Answer 30

Question 31

Does EMG differ between contraction types?

Answer 31

* 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

Question 32

What are mechanism for higher muscle force for same neural drive?

Answer 32

* 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.

Question 33

\_\_\_\_ force is generated through the actin and myosin cross-bridge.

Answer 33

Active

Question 34

In ______ movements actin and myosin ‘hold on’, more stretch of the elastic components (eg titin) \> more force for same energy (# action potentials)

Answer 34

eccentric

Question 35

Decreased rate of cross-bridge detachments \>\> greater force production on the ______ bout.

Answer 35

eccentric

Question 36

EMG provides a good indication of \_\_\_\_\_of a muscle. However, there is no \_\_\_info about how long \_\_\_\_and _____ stay together, ____ forces and \_\_\_\_\_\_\_\_\_\_\_.

Answer 36

neural activation; elastic; myosin; actin; passive forces; how muscle length and contraction type may change during the motor task.

Question 37

What is electromechanical delay? How it can be measured? How may it affect the way that we can assess movement? ## Footnote **EXAM QUESTION**

Answer 37

* 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.