F WAV E
- The F wave is a late muscle potential that results from
a motor nerve volley created by supramaximally stim-
ulated anterior horn cells (Fisher, 1998). - Unlike the H reflex, the F wave can be elicited at
many spinal levels and from any muscle.
•F wave studies, like H reflexes, look at a long path-
way. Consequently, small focal abnormalities tend to
be obscured by the longer segments. - Abnormalities with F wave values may be due to nerve
injury anywhere along the long pathway evaluated.
NEEDLE EXAMINATION
- The NE evaluates the entire motor unit (lower motor
neuron pathway), but not the sensory pathway. - The NE includes the evaluation with needle EMG of
the muscles at rest (to detect axonal injury) and with
volitional activity (to evaluate voluntary motor unit
morphology and recruitment). - The NE needs to be timed such that abnormalities are
optimally detected. - If the NE is performed too early (i.e., less than 2 to
3 weeks after the initial injury), spontaneous muscle
fiber discharges (denervation potentials) may not have
had time to develop. - If the NE is performed too late (i.e., more than
6 months after the initial injury), reinnervation via
collateral sprouting may have halted spontaneous
muscle fiber discharges (Press and Young, 1997).
•At rest, the electrical activity of selected muscles are
studied for abnormal waveforms. - Of these abnormal waveforms, the most common
abnormal finding is the presence of fibrillations and
positive sharp waves. They are found when the muscle
tested has been denervated (Dimitru, 1995). - Fibrillations and positive sharp waves are graded on a
scale from 0 to 4+(Table 19-3). - Complex repetitive discharges (CRDs) are also
common. They represent a group of single muscle
fibers that are time-linked because of crosstalk
between neighboring muscle fibers.
•Fasciculation potentials can be found in a variety of
benign and pathologic conditions. They represent
spontaneous discharges of an entire motor unit.
Sometimes fasciculations can be grossly observed as
muscle twitches.- Benign fasciculations may be found in athletes fol-
lowing heavy exercise, dehydration, anxiety, fatigue,
caffeine consumption, or smoking.
•With activation of the muscle, motor units are ana-
lyzed and this offers an opportunity to distinguish
between neuropathic and myopathic processes
(Robinson and Stop-Smith, 1999). - NE may also help differentiate acute from chronic
neuropathic conditions. - The amplitude of fibrillations can grade nerve injury
as occurring for less than or more than 1 year (Kraft,
1990). This can be particularly helpful in distinguish-
ing an athlete’s acute or chronic nerve injury. - Chronic nerve injuries, without significant ongoing
denervation, will additionally show large-amplitude,
long-duration, polyphasic motor unit potentials (Press
and Young, 1997).
DYNAMIC EDX- Some authors advocate performing EDX after exer-
cise or with the limbs in provocative positions. These
techniques have not been validated with sound
research and are subject to measurement error. - Anecdotally, however, they appear to have a limited
use. - Peroneal nerve entrapments in runners were detected
only with EDX testing following exercise (Leach,
Purnell, and Saito, 1989). - Runners diagnosed with compartment syndrome and
potential nerve entrapment (e.g., superficial peroneal
nerve entrapment as it exits the fascia of the lateral
compartment) have also been postulated as needing
EDX testing following exercise (Bachner and
Friedman, 1995). - It has been suggested that electrophysiologic evidence
of piriformis syndrome is more apparent when an H
reflex is performed with sciatic nerve on stretch (hip
flexed to 90°, maximally adducted, and knee flexed to
90 °) (Fishman and Zybert, 1992). - These techniques need to be interpreted with caution
as many abnormal readings occur based on measure-
ment error alone.
QUANTITATIVE ELECTROMYOGRAPHY- Demonstrates sequence of muscle recruitment and
muscle force. - Only available in specialized gait laboratories.
114 SECTION 2 • EVALUATION OF THE INJURED ATHLETE
TABLE 19-3 Grading of Fibrillations and Positive Waves
GRADING CHARACTERISTICS
0 No activity
1 + Persistent (longer than 1 s) in 2 muscle regions
2 + Persistent in 3 or more muscle regions
3 + Persistent in all muscle regions
4 + Continuous in all muscle regions