Electromyographic and neuromuscular variables in unstable postpolio subjects, stable postpolio subjects, and control subjects

Abstract

Objective: To compare strength and endurance variables obtained in the quadriceps muscles of postpolio and control subjects over a 7-year interval with macro and single fiber electromyography (EMG) variables. Design: A controlled inception cohort study. Setting: Neuromuscular research laboratory of a university hospital. Subjects: A cohort of 23 postpolio and 14 control subjects. All postpolio subjects had a history, physical examination, and EMG consistent with previous poliomyelitis, and had greater than antigravity strength in the quadriceps muscle tested. Unstable postpolio subjects acknowledged new quadriceps weakness over the 7-year period of the study ( n = 11), and stable postpolio subjects denied new weakness of the quadriceps over the same period ( n = 12). Main Outcome Measures: All subjects had tests of neuromuscular function of the quadriceps muscles at the onset of this study and yearly over a 7-year period. EMG variables were determined on a separate day after the seventh year of neuromuscular measurements. Neuromuscular variables measured were isometric knee extension peak torque, isometric endurance (time to inability to maintain knee extensor contraction at 40% of maximal torque), tension time index (TTI) (product of isometric endurance time and 40% of maximal torque), and recovery of torque at 10 minutes after the endurance test. EMG variables were macro EMG and single fiber EMG (jitter, fiber density, and percent blocking). Results and Conclusions: Unstable postpolio subjects did not lose strength more rapidly than stable postpolio subjects or control subjects. Unstable postpolio subjects were significantly weaker, had decreased TTI, larger macro EMG amplitude, greater jitter, blocking, and fiber density in comparison with stable postpolio subjects (all p < .05). Strength was negatively correlated with macro EMG amplitude in the stable postpolio group ( p < .05). The slope of the regression line of strength over 7 years did not correlate ( p > .05) with neuromuscular or EMG variables in control, stable, or unstable postpolio subjects.