Monoexponential relationships between diaphragm muscle maximal tetanic force and its electrical activity. The maximal isometric tension (Po) is normalized for muscle cross-sectional area. The diaphragm muscle electrical activity (EMGd) during assist-control mechanical ventilation (AMV) was estimated by measuring the area subtended by the moving average EMGd curve and its baseline, and is expressed as a percentage of spontaneous breathing. Po is maintained almost identically to that of the control after 3 days of AMV with diaphragm muscle activation between 30% and 80% of spontaneous breathing. Whether diaphragm muscle activation between 0% and 30% is effective to maintain Po remains unknown. Data obtained from : n = 6 for the control and controlled mechanical ventilation (CMV) groups; n = 5 for the AMV group.