Abstract
The purpose of this study was to examine scalene (SA) and sternocleidomastoid (SM) activation during normoxic (norm‐ITL; FIO
2
= 21%) and hypoxic (hyp‐ITL; FIO
2
= 15%) incremental inspiratory threshold loading (ITL). Thirteen healthy participants (33 ± 4 years, 9 female) performed two ITL tests breathing randomly assigned gas mixtures through an inspiratory loading device where the load was increased every two minutes until task failure. SA and SM root mean square (RMS) electromyography (EMG) were calculated and expressed as a percentage of maximum (RMS
%max
) to reflect muscle activation intensity. Myoelectric manifestations of fatigue were characterized as decreased SA or SM EMG median frequency during maximum inspiratory pressure maneuvers before and after ITL. Dyspnea was recorded at baseline and task failure. Ventilatory parameters and mouth pressure (Pm) were recorded throughout the ITL. SA,RMS
%max
and SM,RMS
%max
increased in association with ITL load (
p
≤ .01 for both). SA,RMS
%max
was similar between norm‐ITL and hyp‐ITL (
p
= .17), whereas SM,RMS
%max
was greater during the latter (
p
= .001). Neither SA nor SM had a decrease in EMG median frequency after ITL (
p
= .75 and 0.69 respectively). Pm increased in association with ITL load (
p
< .001) and tended to be higher during hyp‐ITL compared to norm‐ITL (
p
= .05). Dyspnea was similar during both conditions (
p
> .05). There was a trend for higher tidal volumes during hyp‐ITL compared to norm‐ITL (
p
= .10). Minute ventilation was similar between both conditions (
p
= .23). RMS,
%max
of the SA and SM increased linearly with increasing ITL. The presence of hypoxia only increased SM activation. Neither SA nor SM presented myoelectric manifestations of fatigue during both conditions.
Sternocleidomastoid activation was greater during hypoxic than mildly normoxic incremental inspiratory loading. Scalene activation was similar during both conditions. Both sternocleidomastoid and scalene demonstrated significant increases in activation that were linearly related to respiratory loading during both hypoxic and normoxic ITL. Neither scalene nor sternocleidomastoid exhibited changes in EMG median frequency or absolute voluntary respiratory strength.