Physical – sport inactivity is often associated with loss of muscle mass, positive energy balance, fat gain, injuries, motor incompetence, and many others negative effects. The goal of this study was to evaluate loss of muscle function and performance as a response to total physical inactivity. Ten healthy male volunteers (age: 23.3 ± 2.2 years; body height: 179 ± 7.1 cm; body mass: 75.2 ± 9.3 kg) were measured before and after 35 days of horizontal bed rest. Energy intake was adapted from the beginning of the bed rest and associated with weekly changes in body mass and fat mass. Peak muscle power and maximal jump height was measured during vertical jump test, performed before and after bed rest. Maximum jumping power and jumping height decreased significantly after bed rest, for 10 W·kg-1 (19.2 %; P<0.001) and 11.2 (22 %; P<0.001), respectively. Inter subjects’ variability of the jumping height results was in the range from 8.6 % to 49.1 % and for jumping height from 6.8 % to 31.3 %. Results of this study confirm that vertical jump test demands vigorous human performance and should be conducted after bed rest studies in special care and considering huge inter subject variability in designing studies.