Typical benefits of EMG are:
- · EMG allows to directly “look” into the muscle
- · It allows measurement of muscular performance
- · Helps in decision making both before/after surgery
- · Documents treatment and training regimes
- · Helps patients to “find” and train their muscles
- · Allows analysis to improve sports activities
- · Detects muscle response in ergonomic studies
1) Tissue characteristics
The human body is a good electrical conductor, but unfortunately the electrical conductivity varies with tissue type, thickness, physiological changes and temperature. These conditions can greatly vary from subject to subject (and even within subject) and prohibit a direct quantitative comparison of EMG amplitude parameters calculated on the unprocessed EMG signal.
2) Physiological cross talk
Neighboring muscles may produce a significant amount of EMG that is detected by
the local electrode site. Typically this “Cross Talk” does not exceed 10%-15% of the
overall signal contents or isn’t available at all. However, care must been taken for
narrow arrangements within muscle groups. ECG spikes can interfere with the EMG
recording, especially when performed on the upper trunk / shoulder muscles. They are
easy to see and new algorithms are developed to eliminate them.
3) Changes in the geometry between muscle belly and electrode site
Any change of distance between signal origin and detection site will alter the EMG
reading. It is an inherent problem of all dynamic movement studies and can also be
caused by external pressure.
4) External noise
Special care must be taken in very noisy electrical environments. The most
demanding is the direct interference of power hum, typically produced by incorrect
grounding of other external devices.
5) Electrode and amplifiers
The selection/quality of electrodes and internal amplifier noise may add signal
contents to the EMG baseline. Internal amplifier noise should not exceed 5 Vrms.
Most of these factors can be minimized or controlled by accurate preparation and
checking the given room/laboratory conditions.
PROCEDURE OVERVIEW:
During the test, one or more small needles (also called electrodes) are inserted
through the skin into the muscle.
· Needle electrodes to study electrical activity of motor units
· Surface electrodes to study the electrical activity of muscles
EMG measures the electrical activity of muscle during rest, slight contraction, and
forceful contraction. Muscle tissue does not normally produce electrical signals
during rest. When an electrode is inserted, a brief period of activity can be seen on the
oscilloscope, but after that, no signal should be present.
After all of the electrodes have been inserted, you may be asked to contract the
muscle, for example, by lifting or bending your leg. The action potential (size and
shape of the wave) that this creates on the oscilloscope provides information about the
ability of the muscle to respond when the nerves are stimulated. As the muscle is
contracted more forcefully, more and more muscle fibers are activated, producing
action potentials.
ANALYSIS:
A healthy muscle will show no electrical activity (no signs of action potential) during
rest, only when it contracts. However, if the muscle is damaged or has lost input from
nerves, it may have electrical activity during rest. When it contracts its electrical
activity may produce abnormal patterns.
An abnormal EMG result may be a sign of a variety of muscle or nerve disorders,
including polymyositis (an inflammatory muscle disease that causes decreased muscle
power), muscular dystrophy (a chronic genetic disease that progressively affects
muscle function), myasthenia gravis (a genetic or immune disorder that occurs at the
point where the nerve connects with the muscle), and myotonic (stiff) muscles.
DETERMINATION OF CONDUCTION VELOCITIES IN MOTOR NERVES
THEORY:
Nerve conduction velocity (NCV) test is a measurement of the speed of conduction of
an electrical impulse through a nerve. NCV can determine nerve damage and
destruction.
During the test, the nerve is stimulated, usually with surface electrode patches
attached to the skin. Two electrodes are placed on the skin over the nerve. One
electrode stimulates the nerve with a very mild electrical impulse with pulse duration
of 0.2 to 0.5 m/s and the other electrode records it. The resulting electrical activity is
recorded by another electrode. This is repeated for each nerve being tested.
The nerve conduction velocity (speed) is then calculated by measuring the distance
between electrodes and the time it takes for electrical impulses to travel between
electrodes. This elapsed time is called latency.
The measurement of conduction velocity in motor nerves is used to indicate the
location and type of nerve lesion.
PROCEDURE:
- The EMG electrode and the stimulating electrode are placed at two points on the skin, separated by a known distance (L1).
- A brief electrical pulse is applied through the stimulating electrode.
- The action potential picked up by the EMG electrode is displayed on the software screen along with the stimulating impulse.
- The latency, between the stimulating impulse and muscle’s action potential is measured. (T1)
- Now, the two electrodes are repositioned with the distance of separation as (L2) such that L2 <>
- The latency is now measured (T2)
- Record your findings on the data sheet
- Calculate the conduction velocity.
- Repeat the test for different nerves.
The speed of nerve conduction is related to the diameter of the nerve and the degree
of myelination (a myelin sheath is a type of "insulation" around the nerve). A
normally functioning nerve will transmit a stronger and faster signal than a damaged
nerve.
In general, the range of normal conduction velocity will be approximately 50 to 60
meters per second. However, the normal conduction velocity may vary from one
individual to another and from one nerve to another.
Abnormal results may be caused by some sort of neuropathy (damage to the nerve)
that can result from a contusion or traumatic injury to a nerve. Various diseases can
also cause the impulses to slow down.
Nerve conduction velocity is often used along with an EMG to differentiate a nerve
disorder from a muscle disorder. NCV detects a problem with the nerve whereas an
EMG detects whether the muscle is functioning properly in response to the nerve's
stimulus.
Diseases or conditions that may be evaluated with NCV include, but are not limited
to, the following:
· Guillain-Barré syndrome - a condition in which the body's immune system
attacks part of the peripheral nervous system. The first symptoms may include
weakness or tingling sensations in the legs.
· carpal tunnel syndrome - a condition in which the median nerve, which runs
from the forearm into the hand, becomes pressed or squeezed at the wrist by
enlarged tendons or ligaments. This results in pain and numbness in the
fingers.
· Charcot-Marie-Tooth disease - a hereditary neurological condition that affects
both the motor and sensory nerves. One characteristic is weakness of the foot
and lower leg muscles.
· herniated disc disease
· chronic inflammatory polyneuropathy and neuropathy - conditions resulting
from diabetes or alcoholism
· sciatic nerve problems
· pinched nerves
· peripheral nerve injury
Nerve conduction studies may also be performed to identify the cause of symptoms
such as numbness, tingling, and continuous pain.
