Sternocleidomastoid muscle deoxygenation in response to incremental inspiratory threshold loading measured by near infrared spectroscopy.

This study investigated the pattern of changes in muscle oxygenation, deoxygenation and blood volume in the sternocleidomastoid (SCM) in comparison with the parasternal (PS) and intercostal (IC) muscles during a bout of incremental inspiratory threshold loading (ITL) in healthy subjects using near-infrared spectroscopy. During progressive loading, the PS and IC showed a significant increase in oxygenated hemoglobin (5.9 ± 2.3 and 6.8 ± 2.4 μM, P<0.05) and the SCM showed an increase in deoxygenated hemoglobin (17.3 ± 3.8 μM, P<0.05). Total hemoglobin also steadily increased in the SCM whereas it decreased in the quiescent vastus lateralis muscle (20.7 ± 6.1μM vs. -6.6 ± 2.4 μM, P<0.05), which was used as the control muscle during the ITL. Our data suggests that the SCM is recruited progressively during progressive ITL and is accompanied by an increased blood volume and maintenance of O(2)Hb. Blood redistribution away from the nonactive limb muscles during the ITL may provide one source of maintaining inspiratory muscle blood volume and oxygenation during high respiratory motor output.