Electrophysiologic evaluation of muscle fatigue development and recovery in late polio

Objectives: To study aspects of fatigue in late-polio patients and healthy controls. We hypothesized that late-polio subjects would develop more perip heral fatigue, assessed with surface electromyography (EMG), and that no ma jor differences would exist between the two groups in neuromuscular junctio n transmission. Design: Case-control study. Setting: University hospital laboratory. Subjects: Ten patients with a history of polio (mean age, 54 yrs, SD = 5, m ean time since polio onset, 49 yrs, SD = 7) and a matched control group (me an age, 52 yrs, SD = 8). Methods: A protocol with a stepwise force increase up to 80% of maximal vol untary contraction ending with an g-minute recovery period was performed tw ice, first with surface EMG and then with electrical stimulation and surfac e-recorded evoked M-response. Main Outcome Measures: Surface EMG analysis of voluntary activity and evoke d M-response, Results: No significant differences existed between groups in the relative decrease during the fatigue protocol. The recovery of force was slower in t he late-polio subjects. A reduction in the root mean square (RMS) value dur ing recovery was seen in the polio group, although a normalization of the m ean power frequency (MPF) was seen in both groups. Conclusion: The weakness during the fatigue procedure was not caused by neu romuscular blockade, because electrical nerve stimulation evoked a normal r esponse. The weakness after exercise was the result of a slow recovery that may reflect both central and peripheral fatigue.