This investigation examined inertial effects on work output during isokinetic concentric knee extension and eccentric knee flexion. Total work (W_total) included work due to kinetic energy (W_kin), with respect to gravity (W_grav), and against the dynamometer (W_dyn). Eighteen resistance-trained participants (9 males, 9 females) performed maximal voluntary concentric (90, 150, 210, 270 deg/s) and eccentric (-150, -90, -30 deg/s) actions with the dominant leg. Differences between work measurement type (WMT), i.e., gravity-corrected work and W_total, were assessed. ANOVA (2 WMT x 2 mode x 2 gender x 4 speed) revealed significant main effects (p < 0. 05) for both factors concentrically but only for WMT eccentrically. It was concluded that the effect of kinetic energy during isokinetic leg extension may elicit differences in measurement where the associated error (K_err) significantly increases with increasing velocity concentrically and decreases eccentrically.