Task dependency in back muscle fatigue--correlations between two test methods.
BACKGROUND
Various test methods which engage the back muscles in different tasks have been used in studies of back muscle fatigue with electromyography. The present objective was to study task dependency in lumbar muscle fatigue by comparing two test methods.


METHODS
In this cross-sectional study, 22 healthy subjects performed a seated (45s) and a prone test (to the limit of endurance) of back muscle fatigue in randomised order. Fatigue of the lumbar muscles was assessed using electromyography spectral variables and ratings of back muscle fatigue (Borg scale). Linear regression of the median frequency during contraction, and conventional statistical tests of group differences and correlations were used.


RESULTS
Significant differences (P<0.001) between the seated and the prone test were found for the initial median frequency, the slope, the median frequency decrease during the whole contraction, and for the ratings. However, correlation coefficients between the seated and the prone test were low for the median frequency decrease (r=0.42), absent for the slopes of median frequency (r=-0.08), higher for the Borg ratings (r(s)=0.51; P<0.05) and highest for the initial median frequency (r=0.69; P<0.05). Within each test, correlations between the Borg ratings and the electromyography variables were essentially absent (r<0.19).


CONCLUSIONS
Electromyography variables assessed in one type of task in a fatiguing test may not be valid for other types of fatiguing tasks, for example in daily life work situations. Thus task dependency has to be considered when using surface electromyography in determining lumbar muscle fatigue. Ratings of fatigue, however, seem to be less task dependent than the electromyography variables.
Elfving
Britt
B
Department of Neurobiology, Care Sciences and Society, Division of Physiotherapy 23 100, Karolinska Institutet, 141 83 Huddinge, Sweden. britt.elfving@ki.se
Dedering
Asa
A
D000328:Adult; D001415:Back / Q000473:pathology*; D001696:Biomechanical Phenomena; D004576:Electromyography / Q000379:methods*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D017116:Low Back Pain / Q000175:diagnosis / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000473:pathology; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction; D018763:Muscle Fatigue; D009132:Muscles / Q000473:pathology; D012044:Regression Analysis; D015203:Reproducibility of Results