Reliability of electromyographic fatigue curves

The analysis of electromyographic fatigue curves allows for research of muscle fatigue in vivo and has potential clinical usefulness. In general, during a fatiguing constant force sub-maximal isometric contraction, EMG amplitude increases and the frequency characteristics decrease. The purpose of this investigation was to assess the test-retest reliability of EMG fatigue curves. To this end, 21 subjects (3 male and 18 female, mean age ± SD = 33.1 ± 12.7 years) performed one minute isometric contractions of the knee extensors at 50% of maximal force on three separate days. All testing was performed at an angle 30 degrees from full extension. Each contraction was divided into 60 one-second segments. From each segment, the integrated EMG (iEMG) and median power frequency (MPF) were determined. Fatigue curves for each subject were generated by regressing the iEMG and MPF values against time. The EMG data from the maximal contraction, mean value during the fatiguing contraction, and the y-intercept of the individual regressions, were used to separately normalize the responses. The resulting slopes were analyzed with one-way repeated measures ANOVAs and by calculating intraclass correlation coefficients and standard errors of measurement (SEM). None of the ANOVAs were statistically significant (p>0.32), indicating that there were no significant mean differences across trials for any of the dependent variables. The ICC results for the iEMG data were 0.82, 0.81, and 0.77 for slopes normalized to the maximal value, the mean value, and the y-intercept, respectively. These values were not significantly different (p>0.20) from each other. The SEM values ranged from 21.9 to 33.3% of the mean values. For the MPF slopes, ICC = 0.52, 0.58, and 0.58 and the SEM values ranged from 62.0 to 69.2% of the mean values. The ICC for the MPF data normailized to the maximal response was significantly lower (p<.05) than those normalized to the mean and y-intercept, althought the differences were quite small (.06 ICC units). The iEMG ICC values were significantly (p<.05) larger then the MPF values, with the exception of comparisons between the iEMG y-intercept ICC and the MPF mean and y-intercept values (p=.069). These results indicate that higher reliability occurs with EMG fatigue curves derived from amplitude versus frequency data while normalization appears to have little effect. The relatively poor reliability for the MPF data may be due to fluctuations in electrode orientation relative to muscle fiber direction across test sessions.