No abstract

This paper describes an in vitro method for comparing surface-detected electromyographic median frequency (MF) and conduction velocity (CV) parameters with histochemical measurements of muscle fiber type composition and cross-sectional area (CSA). Electromyographic signals were recorded during electrically elicited tetanic contractions from rat soleus, extensor digitorum longus, and diaphragm muscles placed in an oxygenated Krebs bath. Fibers were typed as slow oxidative, fast oxidative glycolytic, and fast glycolytic based on histochemical enzyme stains. Muscles with a greater percentage of fast glycolytic and fast oxidative glycolytic fibers exhibited greater initial values of MF and CV as well as a greater reduction in these variables over the course of the contraction. Regression indicated that fiber type composition could be predicted based on two MF parameters. A weighted measure of muscle fiber CSA was found to be linearly related to both initial MF and CV. The results of this study suggest that MF and CV parameters recorded during a muscular contraction are related to muscle fiber type composition and muscle fiber CSA.

No abstract

H+ accumulation at the sarcolemma is believed to play a key role in determining the electrophysiological correlates of fatigue. This paper describes an in vitro method to externally manipulate muscle pH while measuring the resultant effect on surface-detected median frequency (MDF) and conduction velocity (CV) parameters. Hamster muscle diaphragm strips (n = 8) were isolated with the phrenic nerve intact and placed in an oxygenated Krebs bath (26 degrees C). The muscle was clamped to a noncompliant load cell to measure isometric contractile tension. Tetanic contraction was developed via 40-Hz supermaximal stimulation of the phrenic nerve. Differential signals were recorded from three electromyogram (EMG) detection surfaces for computation of CV (via the phase shift in the EMG signals) and MDF. Repeated trials were conducted at bath pHs of 7.4, 7.0, and 6.6. Bath pH was altered by aerating predetermined concentrations of O2 and CO2 into the bath. Decreases in bath pH resulted in decreases in both initial MDF and initial CV. The differences in initial MDF and initial CV were significant (P less than 0.001) for each of the bath pH conditions. In general, the change in bath pH resulted in an equal percent change in initial MDF and initial CV. This suggests that the change in bath pH caused a decrease in CV without significantly altering the fundamental shape of the M wave. In contrast, the EMG was altered differently during stimulated contractions. During stimulation, the rate of decay of CV was 65% of the rate of decay of MDF.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract-The time-dependent shift in the spectral content of the surface myoelectricsignal to lowerfrequencieshas proven to be a useful tool for assessinglocalized muscle fatigue. Unfortunately, the technique has been restricted to constant-force, isometric con tractions because of limitations in the processingmethods used to obtain spectral estimates. A novel approach is proposed for cal culating spectral parameters from the surface myoelectric signal during cyclicdynamic contractions. The procedure was developed using Cohen class time-frequency transfonns to define the instan taneous median and mean frequency during cyclic dynamic con tractions. Changes in muscle length, force, and electrode position contribute to the nonstationarity of the surface myoelectricsignal.These factors, unrelated to localized fatigue, can be constrained and isolated for cyclic dynamic contractions, where they are as sumed to be constant for identical phases of each cycle. Estima tion errors for the instantaneous median and mean frequency are calculated from synthesized signals. It is shown that the instan taneous median frequency is affected by an error slightly lower than that related to the instantaneous mean frequency. In addi tion, we present a sample application to surface myoelectricsignals recorded from the first dorsal interosseous muscle during repeti tive abduction/adduction of the indexfinger against resistance. Re sults indicate that the variability of the instantaneous median fre quency is related to the repeatability of the biomechanics of the exercise.