We evaluated abdominal muscle contractility and fatigue by measuring twitch
gastric pressure (Pga,) after percutaneous supramaximal electrical stimula
tion of the abdominal wall before and after sit-ups to task failure. Mouth
pressures during maximal voluntary expulsive rp maneuvers (PEmax) at TLC an
d FRC with superimposed twitches, and maximum voluntary ventilation (MVV) w
ere also assessed. Mean fresh Pga, was 36.1 +/- 3.0 cm H2O with a coefficie
nt of variation that ranged between 3.0 to 4.8%. Pga(t) decreased by 25% (p
< 0.001) and 37% (p < 0.001) at: 1 and 30 min after sit-ups. During maxima
l voluntary contraction twitch occlusion never occurred. PEmax at TLC and F
RC 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 situps were not
: significantly different: from the value obtained before the sit-ups. We c
onclude that (1) Pga, is a useful objective indicator of abdominal muscle c
ontractility and fatigue; (2) during maximal voluntary expulsive maneuvers
the abdominal muscles are never fully activated; (3) sit-ups lead to substa
ntial law-frequency fatigue but little high-frequency fatigue of the abdomi
nal muscles, which has little effect on maximal breathing capacity.