Electromyographic changes in the isolated rat diaphragm during the development of fatigue

To investigate whether the power spectrum of the electromyogram of a fatiguing muscle can be used to infer the degree to which the muscle is fatigued, we recorded isometric tension and two monopolar electromyograms from eight isolated rat diaphragm preparations suspended in an organ bath containing a balanced salt solution. Each preparation was excited with a fixed phrenic nerve impulse pattern made up of a 70-Hz train of impulses of supramaximal voltage delivered for 170 ms with a 500-ms recovery period. Tension fell rapidly over the first 60 s of the fatigue run and more slowly for the remaining 60 s analysed. The duration of extracellular action potentials increased and their amplitude decreased as the tension developed by the diaphragm decreased; conduction velocity along muscle fibres also decreased. The centroid frequency (fcen) of the power spectrum of the first action potential elicited by each train of stimuli decreased rapidly until tension fell to approximately 70% of the initial value; thereafter little change in fcen occurred, although tension continued to fall to 33% of its initial value. Our results demonstrated that under controlled conditions, fcen provided a sensitive index of fatigue in its early stages, but provided no information once fatigue was pronounced.