Background: Hamstring tightness impairs flexibility, agility, and dynamic balance, increasing the risk of musculoskeletal injury. While Post-Isometric Relaxation (PIR) and the Jack Knife Technique (JKT) are common interventions, their comparative effectiveness remains unclear.

Objective: To compare the effects of PIR and JKT on hamstring flexibility, agility, and dynamic balance in healthy young adults with bilateral hamstring tightness.

Methods: Forty-six participants aged 18–25 with asymptomatic bilateral hamstring tightness were randomly allocated into two groups: PIR (n=23) and JKT (n=23). Interventions were administered thrice weekly over four weeks. Outcome measures included the Back Saver Sit and Reach (BSSR) for flexibility, T-Test for agility, and Y-Balance Test (YBT) for dynamic balance. Paired t-test, Wilcoxon test, independent t-test, and Mann-Whitney U test were used for analysis at α=0.05.

Results: Both groups showed significant within-group improvements (p<0.001). PIR showed greater improvement in flexibility (2.98±0.74 cm vs 2.34±0.59 cm; p=0.003), agility (1.18±0.18 sec vs 0.99±0.13 sec; p<0.001), and dynamic balance (1.83±0.29% vs 1.46±0.21%; p<0.001) compared to JKT.