Common rostrocaudal gradient of output from human intercostal motoneurones during voluntary and automatic breathing

In voluntary breaths, driven through the motor cortex, the pattern of activation in human inspiratory intercostal muscles is unknown. We measured single motor unit behaviour in the first, third, and fifth parasternal intercostal muscles in 5 subjects for 'quiet' and matched 'voluntary' inspirations. In voluntary breaths, the average onset, peak and end discharge rate of 264 inspiratory single motor units was greater in the first interspace compared to caudal spaces (p < 0.05). Relative to the onset of inspiratory flow, the time of recruitment of single motor units and the onset of multiunit activity were also earlier in the first compared to the fifth interspace (p < 0.05). For 215 'common' motor units, peak discharge frequencies were ∼20% higher in voluntary compared to quiet breaths (p < 0.05), due in part, to small differences in the pattern of breathing. A rostrocaudal gradient of motor unit activation across parasternal intercostal muscles was preserved in voluntary and involuntary tasks. A common mechanism may mediate this pattern of recruitment.