Examination of motor unit control properties of the vastus lateralis in an individual that had acute paralytic poliomyelitis.

The purpose of the study was to examine motor unit (MU) recruitment and derecruitment thresholds and firing rates of the vastus lateralis between 2 healthy (HE) individuals (women, ages = 19 and 23 years) and 1 individual (man, age = 22 years) who acquired acute poliomyelitis (PO). Each participant performed submaximal isometric trapezoid muscle actions of the leg extensors from 20% to 90% maximal voluntary contraction in 10% increments with a sensor placed on the vastus lateralis to record electromyography. Electromyographic signals were decomposed into the firing events of single MUs. Linear regressions were performed on the firing rates at recruitment and peak firing rates versus the recruitment thresholds and the derecruitment versus recruitment thresholds. In addition, data were pooled together from all contractions to examine differences between PO and HE with independent samples t-tests calculated for firing rates at recruitment, peak firing rates, recruitment thresholds, derecruitment thresholds, and duration of MU activity. The results demonstrated systematic differences in MU control strategies between the PO and HE. There were differences in the recruitment thresholds (P < 0.001; HE = 30.5% ± 22.2% maximal voluntary contraction; PO = 14.5% ± 5.0% maximal voluntary contraction), firing rates at recruitment (P < 0.001; HE = 7.4 ± 2.5 pulses per second; PO = 6.2 ± 1.7 pulses per second) and peak firing rates across the force spectrum (P = 0.001; HE = 22.2 ± 5.8 pulses per second; PO = 20.3 ± 2.3 pulses per second), altered derecruitment versus recruitment relationships (HE slope = 0.82 derec/rec, PO slope = 1.78 derec/rec), and duration of MU activity (P < 0.001) between the PO (18.6 ± 2.4 seconds) and HE (15.3 ± 3.0 seconds). Future research should examine the possible differences in MU behavior between PO and HE as a result of fatigue to further elucidate disease-related changes in MU properties.