Cardiopulmonary resuscitation (CPR) is a series of resuscitation actions that improve the survival chances after a cardiac arrest, maintaining tissue perfusion through sternal compressions. The aim of this study was to clarify potential differences in upper body muscle activity related to CPR in microgravity and hypogravity (Mars gravitational field). Thirty healthy male volunteers each performed 3 sessions of 30 external chest compressions (ECCs) on a mannequin, during which time the muscle activity of the pectoralis major, triceps brachii and rectus abdominis were recorded via superficial electromyography. Hypogravity and microgravity were simulated by means of a body suspension device and a counterweight system, to which the volunteer was connected via a harness. The standard terrestrial (1G) CPR position was adopted in simulated hypogravity, and the Evetts–Russomano method was used in simulated microgravity. Heart rate and perceived exertion were also measured via Borg scale. No significant difference was found between the ECCs per minute and per set of compressions when performed at 1G and in simulated hypogravity. However, the mean depth achieved during compressions showed a significant difference in hipogravity. After 3 sets of 30 ECCs, mean heart rate showed an increase from rest values to those obtained from the 3 gravitational fields, and also from 1G to microgravity, but not from 1G to hypogravity. Mean of perceived exertion presented a significant increase from 1G to hypograviy and to microgravity. Muscle activation during the performance of CPR at 1G and hypogravity was significantly higher for the rectus abdominis. All muscles were more active during CPR in microgravity when compared with 1G. These findings suggest that the rescuer should be physically well trained in order to deliver adequate CPR in extraterrestrial environments. The physical training should aim to improve muscular endurance and cardiorespiratory capacity to increase effectiveness of the rescuer emergency assistance.