Errors in measurements of mechanical work, net energy expenditure and mechanical efficiency (ME) were calculated, when subjects performed isolated eccentric or concentric muscle actions and combinations of these actions [stretch-shortening cycle (SSC) exercises] with a special sledge apparatus. The relative error of mechanical work was 6.1%. When estimating the error of energy metabolism from oxygen consumption the error would be about 4% (McArdle et al. 1981). The maximum error of ME was the sum of these two values (10.1%). Obviously the error of ME was less than 5%, because 30 muscle actions were averaged and, in addition, the errors of mechanical work and energy expenditure were not in the same direction every time. It was concluded that mechanical work can be determined accurately when the force is measured as a function of the moved distance of the sledge. Thus calculation of ME can be performed quite reliably in isolated eccentric and concentric exercises. The greatest problems were, however, in the SSC exercises, where the errors were higher, because of the problems of dividing the net energy expenditure into eccentric and concentric phases. Therefore, further developments must be made to minimize the errors in measurement and calculation during SSC-exercise.