Abdominal motor unit activity during respiratory and non-respiratory tasks

To reveal whether the transversus abdominus (TA) and internal oblique (IO) motoneurons receive common neural inputs, we analyzed recruitment levels and firing profiles of single motor unit potentials detected with fine wires implanted in the TA and IO muscles under ultrasound imaging. TA and IO EMGs were digitized. Spikes generated by the same unit were identified on the basis of wave form, were isolated and analyzed. Phases of respiration were measured from the inspiratory flow signal, recorded via a pneumotachometer. Data were acquired over 5 breaths while the standing subject breathed quietly, expired against 7.5 cm rfcO, or held his leg extended 10 cm forward. The number of motor units activated varied across tasks. The firing rates of all units were enhanced during expiration and suppressed during inspiration. Only one unit in each muscle (TA? and IO₄) was active in each of the 3 tasks. The modes for the duration of interspike intervals for TA? differed across tasks, whereas the modes for the IO₄ intervals were locked at 150 ms (or an instantaneous firing frequency of 7.5 imp/s) regardless of task. We concluded that recruitment in both TA and IO is task-dependent; and even though TA and IO units receive a strong, common drive from central expiratory neurons, their firing profiles are controlled by different inputs.