A CPR machine (100) is configured to perform, on a patient's (182) chest, compressions that alternate with releases. The CPR machine includes a compression mechanism (148), and a driver system (141) configured to drive the compression mechanism. A force sensing system (149) may sense a compression force, and the driving can be adjusted accordingly if there is a surprise. For instance, driving may have been automatic according to a motion-time profile, which is adjusted if the compression force is not as expected (850). An optional chest-lifting device (152) may lift the chest between the compressions, to assist actively the decompression of the chest. A lifting force may be sensed, and the motion-time profile can be adjusted if the compression force or the lifting force is not as expected.