This study examined respiratory muscle strength using the POWERbreathe® inspiratory muscle trainer (i.e., ‘S-Index’) before and after repeated-sprint cycling for comparison with maximal inspiratory pressure (MIP) values obtained during a Mueller maneuver. The S-Index was measured during six trials across two sessions using the POWERbreathe® and MIP was measured during three trials in a single session using a custom-made manometer in seven recreationally active adults. Global respiratory muscle strength was measured using both devices before and after the performance of sixteen, 6-s sprints on a cycle ergometer. Intraclass correlation coefficients for the POWERbreathe® S-index indicated excellent (p < 0.05) trial-to-trial (r = 0.87) and day-to-day (r = 0.90) reliability yet there was no significant correlation (r = -0.35, p = 0.43) between the S-Index measured using the POWERbreathe® and MIP measured during a Mueller maneuver. Repeated-sprint cycling had no effect on respiratory muscle strength as measured by the POWERbreathe® (p > 0.99) and during the Mueller maneuver (p > 0.99). The POWERbreathe® S-Index is a moderately reliable, but not equivalent, measure of MIP determined during a Mueller maneuver. Furthermore, repeated-sprint cycling does not induce globalized respiratory muscle fatigue in recreationally-active adults.

• The S-Index as measured by the POWERbreathe® is a reliable measure of respiratory muscle strength

• The S-Index does not accurately reflect maximal inspiratory pressure obtained from a Mueller maneuver

• Repeated-sprint cycling does not induce respiratory muscle fatigue as measured by the POWERbreathe® and the Manometer