During inspiration, there is differential activity in the human parasternal intercostal muscles across interspaces. We studied if the earlier recruitment of motor units in the rostral interspaces, compared to more caudal spaces, during inspiration is preserved for the non-respiratory task of ipsilateral trunk rotation. Single motor unit activity (SMU) was recorded from the first, second and fourth parasternal interspaces on the right side in five participants in two tasks: resting breathing and 'isometric' axial rotation of the trunk during apnoea. Recruitment of the same SMUs was compared between tasks (n = 123). During resting breathing, differential activity was indicated by earlier recruitment of SMUs in the 1st and 2nd interspaces, compared to the 4th space in inspiration (P < 0.01). In contrast, during trunk rotation, the same motor units showed an altered pattern of recruitment as SMUs in the 1st interspace were recruited later and at a higher rotation torque than those in the 2nd and 4th interspaces (P < 0.05). Tested for a subset of SMUs, the reliability of the breathing and rotation tasks, and the SMU recruitment measures was good-excellent (ICC(2,1): 0.69–0.91). Thus, the output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task. Given that the differential inspiratory output of parasternal intercostal muscles is linked to their relative mechanical effectiveness for inspiration and that this output is altered in trunk rotation, we speculate that a mechanism that matches neural drive to muscle mechanics underlies the task-dependent differences in output of axial motoneurone pools.
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