In sport training and rehabilitation practice, it is usual to use methods of mechanical muscle power output measurement, which are based mainly on indirect force measurement. The aim of this study was to verify the validity of indirect measurement for mechanical muscle power output with bench press exercise. As a criterion of validity, we selected a combination of kinematic and dynamic analyses. Ten men participated in this study. Average age of tested subjects was 28.0 ± 3.4 years. At mechanical power output measurement, these subjects lifted at maximum possible speed loads of 18, 26.5, 39.2 and 47.7 kg. Validity of mechanical power output measurement by means of a method using indirect force measurement was estimated using Spearmen's Correlation Coefficient. Factual significance of differences in average values of power output, force and velocity, measured by a method using indirect force measurement, in comparison to the selected criterion, was evaluated by means of effect of size. Power output measurement method using indirect force measurement showed lower values of force in relation to the criterion in the whole scope of selected loads. Velocity values in the whole scope of selected loads did not show any significant difference between the criterion and the verified method. The mechanical muscle power output measured by the method using indirect force measurement is lower in relation to the criterion, especially in the low scope of loads, where also validity rate was low (R = 0.5).