Figure 2. a. Left panel - The relationship between maximal inspiratory pressure (MIP) obtained during the Mueller maneuver and the S-Index determined using the POWERbreathe® (n = 21; 7 subjects x 3 trials); r = -0.35 (p = 0.43). b. Right panel - A mean difference plot¹⁷ depicting the level of agreement between inspiratory pressures measured during a Mueller maneuver and estimated from S-Index values measured on a POWERbreathe® device.