We evaluated abdominal muscle contractility and fatigue by measuring twitch gastric pressure (Pgat) after percutaneous supramaximal electrical stimulation of the abdominal wall before and after sit-ups to task failure. Mouth pressures during maximal voluntary expulsive maneuvers (PEmax) at TLC and FRC with superimposed twitches, and maximum voluntary ventilation (MVV) were also assessed. Mean fresh Pgat was 36.1 +/- 3.0 cm H2O with a coefficient of variation that ranged between 3.0 to 4.8%. Pgat decreased by 25% (p < 0.001) and 37% (p < 0.001) at 1 and 30 min after sit-ups. During maximal voluntary contraction twitch occlusion never occurred. PEmax at TLC and FRC decreased by 15% (p < 0.001) and 11% (p < 0.017) at 1 min, and 8% (p < 0.036) and 9% (p < 0.030) at 30 min after sit-ups, respectively. Despite the abdominal muscle fatigue, MVV values at 1 and 30 min after sit-ups were not significantly different from the value obtained before the sit-ups. We conclude that (1) Pgat is a useful objective indicator of abdominal muscle contractility and fatigue; (2) during maximal voluntary expulsive maneuvers the abdominal muscles are never fully activated; (3) sit-ups lead to substantial low-frequency fatigue but little high-frequency fatigue of the abdominal muscles, which has little effect on maximal breathing capacity.