Intercostal muscles are used during rotation of the thorax in humans

To test the idea that the lateral intercostal muscles may be more suited to aid in rotational than respiratory movements of the thorax, we inserted bipolar fine-wire electrodes in external and internal intercostal muscles in the right midaxillary line in nine sitting subjects and examined the pattern of contraction of these muscles during voluntary axial rotations of the thorax (30-35 degrees), resting breathing, and CO2-induced hyperpnea. The right external intercostal muscles were strongly recruited in rotations to the left but were not active in rotations to the right. In contrast, the right internal intercostal muscles were active in rotations to the right but not in rotations to the left. Rotations completed in 1 or 2 s were associated with an early burst of electromyographic activity, followed by a low plateau that persisted while the rotation was held. Rotations made very gradually over 5-10 s were associated with gradually rising electromyographic activity. The amplitude of activity recorded during 30-35 degrees rotations was equivalent to that measured when minute ventilation was increased by CO2 to 50 l/min. We conclude that the lateral intercostal muscles have a major role in producing axial rotations of the thorax.