|
|
|
Electromyography (EMG) is the measurement of the electrical activity of a muscle. EMG can be measured by surface or needle electrodes. Surface electrodes measure the sum of the electrical activity from all the motor units, whereas needle electrodes measure the electrical activity of a single motor unit. A motor unit, the functional unit of neuromuscular control, consists of a single motor neuron and the muscle fibers it innervates. |
|||||||||||||||||||
 |
An EMG tracing is illustrated in the figure to the right. The top tracing is the left soleus and tibilas anterior and the bottom tracing is the right soleus and tibialis anterior. In both conditions, the first deflection is the stimulation (at time zero). In Electrodiagnostic Testing, the EMG is evaluated by
Latency is the time period between the stimulus and the muscle movement. It represents the time, in milliseconds, it takes to transfer the message of movement to the muscle, i.e. neural conduction. Whereas the amplitude measures the extent of the muscle contraction. A decrease in amplitude and/or an increase in latency are signs of disease. |
||||||||||||||||||
| Application to the clinical setting: Electromyography (EMG), in addition to its applications as a diagnostic tool, has an important role in rehabilitation from some spinal injuries and stroke. State of the art treatment uses EMG information from an unaffected limb’s response to a particular motor task and patterns, or programs a series of muscle stimulations to re-train the brain and spinal cord. In the case of post-polio syndrome, there may be one or more muscles of a muscle group that may be affected. Electrodiagnostic testing would be used to identify the effected muscles and establish the extent of the loss of function. EMG electrodes would be applied to several muscles. Specific motor tasks would be done. The electrical activity from the muscles would be evaluated for latency and amplitude. Once the limitations of the muscle or muscle group has been established, the physical therapist will then work to strengthen the functioning muscles around the affected muscle. If some motor units of the affected muscle appear functional, rehabilitation will include strengthening of those.
Nerve conduction velocity is the
speed in which a nerve propagates an impusle. Neurons are both
sensory and motor. Nerve conduction velocity can simply be measured
by stimulating one end of the nerve and measuring how long it takes to reach
the other end. Factors that can affect NCV include
Conditions that affect NCV include:
Normal values for nerve conduction velocity range from
55 to 70 meters/second for the sensory or motor nerves. Abnormal
NVC is < 45 msec. |
|||||||||||||||||||
 |
Nerve
conduction velocity was measured in our lab by stimulating the tibial nerve
and measuring the EMG activity of the soleus muscle. This has
been called the H-Reflex. The advantage of using the H-Reflex
instead of simple nerve conduction velocity is that the H-Reflex includes
spinal integration of the impulse. The tibial nerve is a mixed
nerve, having both motor and sensory neurons. With this model,
the following variables can be measured:
|
||||||||||||||||||
| The tracing on the right is the H-reflex of the soleus muscle following stimulation of the tibial nerve. The first deflection is the motor unit and the second deflection represents the impulse traveling through the sensory neurons to the spinal cord and back through the motor unit to the soleus muscle. |  |
||||||||||||||||||
|
The latency period for sensory and motor units are significantly different. Sensory neurons are larger and exhibit faster nerve conduction velocity whereas motor neurons are smaller and thus, slower. Normal and abnormal values are summarized in the table below for the tibial nerve we measured in lab. Keep in mind, each nerve will have different characteristics. The abnormal column represents the deviation, based on a percentage of the nerve characteristics.
Application to the clinical setting: Nerve conduction velocity tests are imperative in the diagnosis of many cumulative trauma disorders (CTDs). CTDs are a group of maladies caused by repeated motion, impact, and/ or vibration at a particular joint. Common CTDs involve the cervical spine, shoulder, elbow (cubital tunnel) and wrist (carpal tunnel). In these cases, patients complain of weakness or clumsiness and have difficulty performing fine motor tasks. While MRI, X-ray, and CT scans are valuable in determining if a fracture is present near the joint in question, only a nerve conduction test is definitive in diagnosing nerve impingement.
Balance is both static and dynamic. Static
balance, which represents postural stability, is independent of dynamic
balance. Static balance is measured by having the patient stand
on a force platform which measures sway or variation from the stable standing
position. Dynamic balance is measured by recovery time from perturbation. That
is, how much time it takes to return to a normal standing position after
being disrupted.
Balance depends on several control
mechanisms:
The contribution of each of these systems can easlily
be measured with simple tasks while standing erect on the force platform. Each
task involves one of the three systems. When one system is
manipulated, the other two are challenged to maintain posture or balance. The
tasks for each system are These simple tasks are illustrated
below. In order, they are: 1) eyes open, 2) eyes closed, 3)
head moving side to side, 4) head moving up and down, and 5) head moving
side to side with eyes shut.     
|
|||||||||||||||||||
|
The postural sway is measured during each task. The illustration on the right is a tracing of the center of pressure during the sway under the normal eyes open condition. This tracing has been characterized by a "peanut" shape. The figure below is the tracing when the eyes were shut and the head was moving up and down. This tracing loses the peanut shap and resembles more of an oval. Thus, the more the shape resembles the peanut, the more normal the static balance. |  |
||||||||||||||||||
 |
Sway is evaluated by
Normal values for sway area are <1000 mm2. Abnormal values for sway area are >2000 mm2. Normal ratio for S:L Sway >1.5 whereas abnormal values for S:L sway <1.0.> | ||||||||||||||||||
Dynamic
balance, on the other hand, is illustrated in the figure to the right.
This is a recording of the center of pressure as it moves to regain
its balance after being displaced by electrical shock. The figure
to the left is typical for a child; the middle figure is for an adult; and
the far right figure is for an elderly subject. Dynamic balance
is measured by:
|
 |
||||||||||||||||||
Application to the clinical setting:
Balance evaluation from data acquired by a force platform provides
objective measures to be used as screening tools for identifying individuals
at risk for falling. The ability to differentiate between vestibular,
proprioceptive, and vision impairments makes this a valuable tool for recommendation
of specific treatment and training programs. The force platform
can also measure the effectiveness of a treatment program.
Balance becomes an important factor for activities
of daily living in aging and neuromuscular diseased states. Balance
can be improved by training. Following the specificity of exercise
principle, static and dynamic balance can be improved by different forms
of exericse. In a recent study, TaiChi was found to improve
static balance and specific dynamic balance exercise was found to improve
dynamic balance. Swimming did not improve either form of balance. The
mimimum time to make a training change can be as little as 1 to 5 weeks. Neural
changes from training can take as few as 7 days to detect. The balance testing can be used for screening
and training. In an apparently healthy young individual, all
three systems should be functioning to maintain balance. When
all three systems are functioning properly, negative responses for each
task will be exhibited. However, if one system is not functioning
properly, the balance task that requries that system to maintain balance,
will exhibit a positive response. For example, a patient is being screened for
balance and exhibits the following response: The conclusion of this screening is that vision is the
primary system for balance. Neither the vestibular system or
the proprioceptive system are contributing significantly to balance. In
this case, balance can be improved by training the proprioceptive system. Medical
intervention is needed to change the vestibular system. To
train the proprioceptive system, balance practice on foam, with and without
the eyes closed, should be effective. |
|||||||||||||||||||
This page was last updated 22 July 2001
URL: http://www.indiana.edu/~k561
Webmaster: Janet P. Wallace, PhD, FACSM
Contact:wallacej@indiana.edu
Copyright 1998, The Trustees of Indiana University