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 peripheral fatigue, assessed with surface electromyography (EMG), and that no major differences would exist between the two groups in neuromuscular junction transmission.

DESIGN

Case-control study.

SETTING

University hospital laboratory.

SUBJECTS: Ten patients with a history of polio (mean age, 54 yrs, SD = 5; mean time since polio onset, 49 yrs, SD = 7) and a matched control group (mean age, 52 yrs, SD = 8).

METHODS

A protocol with a stepwise force increase up to 80% of maximal voluntary contraction ending with an 8-minute recovery period was performed twice, first with surface EMG and then with electrical stimulation and surface-recorded evoked M-response.

MAIN OUTCOME MEASURES: Surface EMG analysis of voluntary activity and evoked M-response.

RESULTS

No significant differences existed between groups in the relative decrease during the fatigue protocol. The recovery of force was slower in the late-polio subjects. A reduction in the root mean square (RMS) value during recovery was seen in the polio group, although a normalization of the mean power frequency (MPF) was seen in both groups.

CONCLUSION

The weakness during the fatigue procedure was not caused by neuromuscular blockade, because electrical nerve stimulation evoked a normal response. The weakness after exercise was the result of a slow recovery that may reflect both central and peripheral fatigue.