Abdominal motor unit activity during respiratory and nonrespiratory tasks

Abdominal muscles serve multiple roles, but the functional organization of their motoneurons remains unclear. To gain insight, we recorded single motor unit potentials from the internal oblique (IO) and transversus abdominis (TA) muscles of three standing subjects during quiet breathing, a leg lift, and an expiratory threshold load. Inspiratory airflow, recorded from a pneumotachometer, provided tidal volumes and respiratory cycle timing. Fine wires, implanted under ultrasonic imaging, detected single motor unit potentials that were visually distinguished by their spike morphology. From the number of spikes, firing profiles, times of occurrence in the respiratory cycle, and their onset, instantaneous, mean, and peak firing frequencies we deduced that 1) breathing patterns varied across tasks, 2) different motor units were recruited for each task with essentially no overlap, 3) their firing displayed prominent expiratory activity during each task, and 4) the recruitment levels and discharge patterns of IO and TA were different. We conclude that the IO and TA motor pools receive a strong central respiratory drive, yet each pool receives its own distinct, task-dependent synaptic input.