GENERATION MECHANISM OF FAR-FIELD PNTENTIALS
Thoru Yamada, MD (University of Iowa, Iowa City, Iowa U.S.A.)
Far-field potentials (FFP) can be defined as volume conducted potentials recorded distant from the generator sources. The FFP are generated when the traveling impulse passes through the regions when 1) volume geometry surrounding the nerve abruptlv changes, 2) electrical conductivity or impedance surrounding the nerve changes or 3) orientation in the direction of nerve impedance changes. These mechanisms have been demonstrated by vitro and vivo models and also by computer simulations. FFP was earlier thought to be positive polarity but recent studies have indicated that FFP can be either positive or negative polarity depending on the recording electrode location in relationship with the dipole field orientation. Indeed, P9 FFP recorded over the scald and neck after median nerve stimulation has corresponding negativity (N9) over the stimulated arm. This negative potential is not traveling impulse but rather stationary field potential that is distributed widely over the entire stimulated arm with maximum amplitude over the shoulder. Based on latency, N9 and P9 dipole fields appear to be generated by volume geometry change when traveling impulse enters the body from the arm. It is plausible that each FFP had it's own characteristic distribution with Positive and negative dipole fields depending on how and when it is generated.
APPLICATIONS OF FREQUENCY SPECTRUM ANALYSIS OF EMG SIGNAL Younghee Lee (Yonsei University, Wonju, Korea)
Frequency spectrum analysis of EMG signal carry information useful to estimate muscle properties and to quantify muscle performance. It may help to identify various patients with neuromuscular disease, and it also have been used to evaluating patient with chronic low back pain. But most frequently, it has been used in quantifying the localized muscular fatigue.
The fatigability of several muscles that are known to have different muscle fiber composition were evaluated using frequency spectrum analysis. Also the effects of immobilization on behavior or frequency spectrum were evaluated. There were differences in fatigability, and in changes of fatigability by immobilization of different skeletal muscles according to their muscle fiber composition.
Some other applications of frequency spectrum analysis of EMG signal in rehabilitation research were discussed.