Comparison of post-isometric relaxation and active static stretching in improving hamstring flexibility among allied health sciences students: a randomized controlled trial.

Hamstring tightness is a common musculoskeletal concern that can impair posture, restrict mobility, and predispose individuals to lower limb injuries. Various stretching techniques are employed to enhance flexibility, yet their comparative effectiveness remains debated. The aim of this study was to evaluate the effects of Post-Isometric Relaxation (PIR) and Active Static Stretching (ASS) on the hamstring flexibility of allied health sciences students. A parallel-group, assessor-blinded, single-center randomized controlled trial was carried out at the University of Health Sciences, Lahore's Department of Physiotherapy (ClinicalTrials.gov Identifier: NCT07153510). Fifty-two students aged 18-25 years with hamstring tightness and Active Knee Extension (AKE) > 20° from full extension were randomly allocated to PIR (n = 26) group and ASS (n = 26) groups. After baseline treatment, Group A, PIR underwent sub-maximal isometric hamstring contraction for 7 s, followed by 15 s of passive stretch, repeated 10 times. Group B, ASS group performed active hamstring stretch held for 30 s, repeated 10 times. Both groups received 12 supervised sessions over four weeks (three sessions per week). Primary outcome was hamstring flexibility (AKE). Secondary outcomes included resolution of hamstring tightness. The Statistical Package of Social Sciences (SPSS) version 25.0 was used to analyze the data. At baseline, both groups were comparable in AKE values (PIR: 150.73° ± 7.94; ASS: 148.31° ± 8.32; p = 0.288). Both groups showed notable within-group increases following the intervention, but the PIR group's flexibility gains were noticeably larger. (165.69° ± 6.61 vs. 158.69° ± 8.23; p = 0.001). A universal goniometer was used to objectively measure hamstring flexibility during the Active Knee Extension (AKE) test. When AKE < 20° from full extension, resolution was taken into consideration. Hamstring tightness resolved in 67.3% of participants overall, with higher improvement rates in the PIR group. No adverse events were reported. Both PIR and ASS effectively improved hamstring flexibility in young adults; however, Post-Isometric Relaxation produced significantly superior improvements. The results suggest that PIR may be a preferred clinical and preventive strategy for enhancing flexibility and reducing the risk of hamstring tightness-related dysfunctions in student and general populations.Trial Registration ClinicalTrials.gov Identifier: NCT07153510. Registered prospectively, Registration Date: 10th July, 2025.