Abstract: Benjanuvatra, N, Lay, BS, Alderson, JA, and Blanksby, BA. Comparison of ground reaction force asymmetry in one- and two-legged countermovement jumps. J Strength Cond Res 27(10): 2700–2707, 2013—This study examined whether ground reaction force (GRF) asymmetry of 2-legged countermovement jumps (CMJ) is related to 1-legged CMJ asymmetry. The GRF asymmetry of a 2-legged CMJ has been suggested as a preferred test to the 1-legged CMJ for functional strength and power deficit assessment. Twenty-eight men and 30 women performed 5 trials each of a 1-legged CMJ with the right limband the left limb, and a 2-legged CMJ. Vertical GRFs were collected from each lower limb using 2 force platforms. Although several GRF variables were calculated, vertical impulse correlated most strongly with jump height in all conditions (p < 0.05), and they were used in subsequent analyses. A moderate correlation was found for impulse asymmetry between the 1- and 2-legged CMJs for women (r = 0.45, p < 0.05), but not for men (r = 0.06, p = 0.76). In contrast, cross-tabulation analyses of subjects presented with the same dominant characteristics in the 1- and 2-legged CMJs revealed poor associations for both men (Freeman-Halton exact p = 0.61) and women (Freeman-Halton exact p = 0.19). Only 11 women recorded the same dominant limb for both 1- and 2-legged CMJs. This suggests that impulse asymmetries found in the 1- and 2-legged CMJ were unrelated. As the 1-legged CMJ relies on the extension forces generated entirely from 1 limb, variations in jump heights and GRF impulses by left and right limbs separately were more indicative of functional strength differences between sides. Hence, it is recommended that the 1-legged CMJ is used when examining functional strength asymmetry in the lower limbs. In contrast, factors causing asymmetry in GRF impulses during 2-legged CMJs are more complicated and require further investigation.