This study was performed to determine whether deficits in muscle strength, work capacity, and ability to recover strength after exhaustion in symptomatic postpolio subjects were due to central inhibition (lassitude) or other physiologic differences. We studied 34 symptomatic polio subjects, 16 asymptomatic polio subjects, and 41 control subjects. Root mean squared electromyography (RMS-EMG) and the median frequency (Fm) of the power spectrum of the quadriceps muscle was determined with surface electrodes during maximal volitional contraction (MVC) of isometric knee extension, an endurance test to exhaustion at 40% of maximal torque, and strength recovery testing that was performed at regular intervals for 10 minutes postexhaustion. Initial Fm during MVC and during the 40% MVC endurance test were similar in all three groups. The Fm decreased during the endurance test and increased during recovery similarly in all three groups. The RMS-EMG also changed in parallel fashion in all three groups during the same procedures. During recovery, a similar degree of RMS-EMG facilitation was seen in all three groups. Thus, reduction in strength, work capacity, and ability to recover from fatiguing exercise do not appear to be related to central factors (lassitude); they are probably related to other factors, such as local muscle fatigue. Each group appears to fatigue and recover in a similar manner electrophysiologically, although symptomatic polio subjects have a deficit in strength recovery, apparently due to local muscular fatigue.