2. • Electromyography (EMG)
– is a technique for evaluating & recording the electrical activity
produced by skeletal muscles
– is performed using an instrument called an electromyograph, to
produce a record called an electromyogram
• a resting muscle does not show recordable electrical
potential but with increase force of contraction,
amplitude of potential increases
• an electromyograph detects electrical potential
generated by muscle cells when these cells are
electrically or neurologically activated
3. Equipment
• electrodes
– Surface electrodes
– Needle electrodes
• a high-gain amplifier (10-5000Hz)
– connected to an oscilloscope
• oscilloscopic traces may be photographed or stored on magnetic tape
– EMG signals may be fed to an audio unit for an on the spot feel
of the signals
• an arrangement for recording the output
– EMG is best done in a specially constructed shielded room to
prevent interference
4.
5. Pattern of EMG Recorded Findings
Resting activity Muscle relaxed &
needle not moving
No activity
Insertion activity Needle is moved to
various sampling spots
within insertion tract
Brief action potentials
Motor unit potential Needle is not moved
while patient makes
slight contraction
A few motor unit
action potentials,
biphasic or triphasic,
short duration
Recruitment Subject makes
progressively stronger
muscle contraction
until reaching
maximum force
Increase number of
functioning
movements until the
baseline is obscured
6. Motor unit potential (MUP)
• the sum of the action potentials produced in the muscle
• characterized by its duration, number of phases,
amplitude, & rate of rise of first component
7. Feature of MUP
Duration • measured from the initial take-off to the point of
return to the baseline
• 5-15 ms
Phases • portion of the MUP between the departure & the
return to the baseline
• triphasic (positive, negative, positive)
• Polyphasic-MUP with more than four phase (5-15%)
Amplitude • measured from maximum peak of negative phase to
maximum peak of the positive phase
• 0.5mV to 2mV
Rise time • duration from the initial positive to subsequent
negative peak
• normal ↓ 500 µs
8. Factors that effect MUP
• Technical factors
– Type of needle electrode
– Characteristics of recording surface
– Electrical characteristics of cable
– Preamplifier & amplifier
– Method of recording
• Physiological factors
– Age of the patient
– Muscle examined
– Temperature
9. Analysis of EMG
• Qualitative analysis
– visual inspection of the record
• Quantitative analysis
– amplitude, duration, & frequency
– power spectrum analysis
10. Application of EMG
• kinesiology- degree & sequence of contraction of various
muscles participating in a movement
• mechanism employed by the body for grading the force
of muscular contraction
• clinical diagnosis & follow up myopathies & responses of
nerve & muscle to injury
11. Fibrillation Fasciculation
contraction of individual
muscle fiber
contraction of individual
motor units
cannot be seen through the
skin
visible through the skin
when muscle fibers lose
contact with their
innervating axon
occurs as a result of
additional nerve impulses
generated
10-100 µV in amplitude, 1-2
ms in duration, & 10 Hz in
frequency
50-500 µV in amplitude, 2-4
ms in duration, & 2-20 Hz in
frequency
Neuropathy, myopathy Neuropathy
12.
13. References
• Essentials of Clinical Neurophysiology, 3/E Karl & Thomas
• Principles of Neural Science, 4/E Kandel ER, Schwartz JH,
Jessell TM (editors)
• Understanding Medical Physiology, 4/E Bijlani &
Manjunatha
