EMGThe Muscle Physiology of

Electromyography

Jessica Zarndt

Department of Kinesiology

UNLV

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

EMG and Muscle Physiology

I. Muscle Contraction• Brief Anatomical Review • Emphasis on the electrical potential

II. Physiological Explanation of an EMG signal• What corresponds to what we see on a signal

III. Physiological Factors that can Influence an EMG Signal

• How do things like fiber type, size and disease affect the EMG

Skeletal Muscle Organization

• Series Elastic Components– Tendons & Bones– Fascia, Endomysium,

Perimysium and the Epimysium

• Excitable Vs Non-Excitable– Muscle tissue IS– Connective is NOT

Skeletal Muscle Organization

• The Muscle Fiber (Cell) is excitable

• The Muscle Fiber is what Contracts

Skeletal Muscle Organization The Muscle Fiber

The Muscle Fiber at the electrophysiological level

• Resting Potential – the voltage across an unstimulated cell

• Muscle Cell = -90mV• Established by

1. Active Transport of Ions– The Na+/K+ pump– 3Na+ out / 2K+ in

2. Potassium Diffusion Potential– K+ diffuses in; sarcollemma is 100 x more

permeable to K+ than Na+

The Muscle Fiber at the electrophysiological level

• What does this mean?

– High [Na+] ~140mEq/L outside the c

EMG and Muscle Physiology

• How does the muscle fiber become excited & contract?1. Neuro-Stimulation

2. Electrochemical changes in the muscle

3. Proteins of the muscle move-the muscle moves

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

Action Potentialof a Neuron

• Resting Potential -70mv

• Excited to +35mv• The change in polarity

travels down a neuron to the next

• Neurotransmitter is released from terminal end

Action Potentialof a Neuron

Action Potentialof a Neuron

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

The Neuromuscular Junction

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

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

Electrochemical Changes in the Muscle

5) End Plate Potential

-Bidirectional

-Local

-Leads to AP if large enough

-Usually 50-70mV

Electrochemical Changes in the Muscle

6) When threshold is met in the End Plate, an Action Potential will initiate

- Threshold = -55mV

7) Action Potential

EMG’s

• 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

Action potential from one skeletal muscle cell An EMG

http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

EMG’S

At Resthttp://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

2

1

EMG’S

http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

EMG’S

• Evoked field potential from a single motor unit is actually (usually) triphasic

• Duration is between 3 and 15 msec

• Magnitude is between 20-2000 microvolts, depending on the size of the motor unit

• Frequency of discharge varies from ~6 – 30 per second

http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

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?

http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

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

Frequency (Hz)

Magnitude (dB)

1 2 4http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf

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!

http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf