Whyte, EF, Heneghan, B, Feely, K, Moran, KA, and O'Connor, S. The effect of hip extension and Nordic hamstring exercise protocols on hamstring strength: A randomized controlled trial. J Strength Cond Res 35(10): 2682–2689, 2021—Lower and between-limb asymmetrical eccentric hamstring isokinetic strength may be associated with hamstring injuries, which mainly affect the biceps femoris. The 45° hip extension exercise (HEE) strengthens the hamstrings in the lengthened position where the biceps femoris experiences greatest loading during high-speed running. However, the effect of an HEE program on eccentric hamstring isokinetic strength has not been investigated or compared with the Nordic Hamstring Exercise (NHE) program. Twenty-four male, varsity, Gaelic footballers (22.4 ± 2.7 years; 182.1 ± 6.0 cm; 84.4 ± 7.9 kg) were randomly allocated to an HEE or NHE 4-week program. Isokinetic (60°·s−1) eccentric peak torque, functional hamstring-to-quadriceps ratio, and between-limb eccentric strength (absolute and percentage) asymmetries were recorded before and after intervention. Muscle soreness scores were reported after exercise using a visual analogue scale. A mixed between-within analysis of variance investigated group (HEE vs. NHE) by time (pre vs. post) interaction effects, and any main effects. An independent-samples t-test compared muscle soreness scores between the NHE and HEE groups. No interaction or group main effects were found. Main effects for time were observed on dominant and nondominant limbs for eccentric peak torque (p < 0.001, η2 = 0.21, p = 0.01, η2 = 0.51, respectively) and functional hamstring quadriceps ratio (p = 0.03, η2 = 0.59, p < 0.0001, η2 = 0.26, respectively). There were no main effects for time between-limb eccentric peak torque or percentage asymmetries (p = 0.41, η2 = 0.03, p = 0.20, η2 = 0.07, respectively), or differences in muscle soreness scores (p > 0.05). A 4-week HEE program increases eccentric hamstring isokinetic strength similar to a NHE program, which may be useful as part of a hamstring injury prevention program.