This study examined whether isokinetic strength of the hip and knee muscles would influence one-repetition maximum (1RM) performance across bilateral, unilateral, and split-load leg press variations. Thirty-one resistance-trained males underwent isokinetic testing of hip extensors and knee flexors/extensors, followed by the assessment of 1RM performance in multiple leg press modalities of a 45° plate-loaded linear leg press. Pearson correlations and multiple regression analyses were employed to explore predictive relationships between muscular force and 1RM outcomes. Significant correlations were found between dominant leg knee flexor strength and 1RM performance in the bilateral (r = 0.495, p < 0.01), the split-load (r = 0.473, p < 0.01), and the unilateral dominant leg press (r = 0.481, p < 0.01). Non-dominant knee extensor strength also correlated with 1RM across all modalities (r = 0.408–0.448, p < 0.05). Multiple regression analysis identified dominant knee flexor force as the strongest predictor of bilateral 1RM (β = 0.895, p = 0.017, R²adj= 0.291). For the unilateral non-dominant leg press, the combined strength of non-dominant knee flexors and extensors significantly predicted performance (β = 0.631, p = 0.046, R²adj = 0.227). Split-load performance was best explained by dominant knee flexor (β = 0.473, p = 0.007) and non-dominant extensor force (β = 0.728, p = 0.026). Non-dominant hip extensor strength showed moderate correlation with unilateral 1RM (r = 0.398, p < 0.05), but was not a significant predictor. These findings underscore the leg press as a knee-dominant exercise and highlight the importance of balanced strength development across limbs and muscle groups.