1. EMG The Muscle Physiology of Electromyography Jessica Zarndt
Department of Kinesiology
UNLV

2. EMG
Electromyography (EMG) – the measurement of electrical activity that brings about muscle contractions
5. Plowman SA, Smith DL. Exercise Physiology for Health Fitness and Performance. Benjamin Cummings, 2003

3. EMG and Muscle Physiology
Muscle Contraction
Brief Anatomical Review
Emphasis on the electrical potential
Physiological Explanation of an EMG signal
What corresponds to what we see on a signal
Physiological Factors that can Influence an EMG Signal
How do things like fiber type, size and disease affect the EMG

4. Skeletal Muscle Organization
Series Elastic Components
Tendons & Bones
Fascia, Endomysium, Perimysium and the Epimysium
Excitable Vs Non-Excitable
Muscle tissue IS
Connective is NOT

5. Skeletal Muscle Organization
The Muscle Fiber (Cell) is excitable
The Muscle Fiber is what Contracts

6. Skeletal Muscle Organization The Muscle Fiber

7. The Muscle Fiber at the electrophysiological level
Resting Potential – the voltage across an unstimulated cell
Muscle Cell = -90mV
Established by
Active Transport of Ions
The Na+/K+ pump
3Na+ out / 2K+ in
Potassium Diffusion Potential
K+ diffuses in; sarcollemma is 100 x more permeable to K+ than Na+

8. The Muscle Fiber at the electrophysiological level
What does this mean?
High [Na+] ~140mEq/L outside the c

9. EMG and Muscle Physiology
How does the muscle fiber become excited & contract?
Neuro-Stimulation
Electrochemical changes in the muscle
Proteins of the muscle move-the muscle moves

10. 1. Nervous System Signal Originates in a Motor Neuron
Activated by conscious thought or afferent input (i.e. reflex)
Travels through the nervous system to the target muscle(s) via, depolarization (action potential) and neurotransmitters
Action Potential - a reversal in relative polarity or change in electrical potential of a cell
Neurotransmitters- chemical messengers

11. Action Potential of a Neuron
Resting Potential mv
Excited to +35mv
The change in polarity travels down a neuron to the next
Neurotransmitter is released from terminal end

12. Action Potential of a Neuron

13. Action Potential of a Neuron
Post Synaptic Stimulation
Pre Synaptic Stimulation

14. The Neuromuscular Junction
A specific synapse
Synapse = the junction at the terminal end of a neuron and another cell
The Neuromuscular Synapse
Motor Neuron and Muscle Cell

15. The Neuromuscular Junction

16. 2. Electrochemical Changes in the Muscle
1) Ca++ are released in the terminal end of Neuron
2) Neurotransmitter is released; Acetylcholine (Ach)
3) Ach travels to receptors on muscle end plate (~50million per fiber)
Muscle End Plate – area of muscle cell innervated by neuron
About 125 Vesicles Rupter w/ each AP
Acetylcholinesterase- splits Ach to acetate and choline-

17. Electrochemical Changes in the Muscle
4) Na+ channels open in the muscle cell
-Na+ flows into the cell
-Voltage begins to raise from -90mv
5) End Plate Potential- local positive potential inside a muscle fiber
Ca++, Potassium and other small + ions can pass through same channel, but predominatly Na
Why this happens is explained by resting potential origin; high + on the outside – on the inside

18. Electrochemical Changes in the Muscle
5) End Plate Potential
-Bidirectional
-Local
-Leads to AP if large enough
-Usually 50-70mV

19. Electrochemical Changes in the Muscle
6) When threshold is met in the End Plate, an Action Potential will initiate
- Threshold = -55mV
7) Action Potential

20. EMG’s
Action potential from one skeletal muscle cell
An EMG
EMG’s allow recording of the action potentials from an entire muscle (or at least significant portion of one)
The signal is a compound action potential

21. EMG’S 2 1
At Rest

22. EMG’S

23. EMG’S
Evoked field potential from a single motor unit is actually (usually) triphasic
Duration is between 3 and 15 msec
Magnitude is between microvolts, depending on the size of the motor unit
Frequency of discharge varies from ~6 – 30 per second

24. The result is a signal that looks a lot like noise….but is it?
EMG’S
Of course, when we measure an EMG, we are not recording the AP from a single motor unit, but rather we are recording from multiple cells/fibrils that:
Each generate an AP
The AP’s do not have to be in phase
Some may fire multiple times…others only once
The amplitudes of the AP’s can be different, too
The position of the electrode relative to other muscles may also cause interference
The result is a signal that looks a lot like noise….but is it?

25. FOURIER ANALYSIS
We can think of an EMG as the result of the superposition of many, many waves that may or may not be in phase
By changing from the “time” domain to the “frequency” domain, we can identify the individual waves that comprise the final signal
Magnitude (dB) 1 2 4
Frequency (Hz)

26. FAST FOURIER TRANSFORMS
Computational means of decomposing non-periodic signals into individual components
Fortunately, a lot programs are available to perform FFT’s (Matlab, for instance…also the Biopac software!)
This FFT of an EMG shows definite peaks at 33, 45, 55, 65, 75, 90, and 94 Hz.
This implies that there are large numbers of motor units firing at these frequencies!