Introduction: The crossover effect, where training or stretching one limb improves the performance of the opposite limb, has been observed in both strength and flexibility studies. While much research has focused on short-term effects, evidence on long-term outcomes remains limited. To date, no studies have explored the lasting crossover effects of an eight-week unilateral static stretching program on the hamstring muscle.

Aim: The aim of this study was to investigate the long-term crossover-effect of static stretching on hamstring muscle length by comparing the non-stretched contralateral hamstring to the stretched dominant hamstring. An additional aim was to compare the non-stretched hamstring muscle length to the hamstring muscle length of a non-stretching control group.

Method: Thirty healthy men and women with less than 70 degrees range of motion (ROM) in hip flexion, measured with passive straight leg raise (PSLR), were included. Fourteen participants were randomized to the static stretch group (SSG), with a mean age of 42.1 (SD 13.4) and sixteen were assigned to the control group (CG), with a mean age of 36.9 (SD 13.7). The SSG performed 7,5 minutes of unilateral passive stretching on their dominant hamstring muscle, seven days a week, for eight weeks. The CG group did not perform any stretching. Participants were tested at baseline, w. 4, w. 8 and w. 20. Outcome measures were PSLR and active knee extension (AKE). Statistical analyzes was performed using paired samples T-test, independent samples t-test and Cohens´d for effect size. 

Result: Significant results were detected in both the stretched and the non-stretched leg in the SSG and in the CG, as measured by PSLR, at all time points. For AKE, the stretched leg in the SSG showed significant results at w.4, while the non-stretched leg showed significant results at w.4 and w.8. However, no significant results were observed at w.20. The CG group showed no significant results in AKE at any point. No significant differences were detected between the SSG non-stretched leg and the CG or the SSG stretched and non-stretched legs leg at any time during the intervention period or at follow up.

Conclusion: Unilateral static stretching of the dominant hamstring significantly increased contralateral hamstring flexibility, as measured by PSLR and AKE. No significant differences were observed between the stretched and contralateral leg, indicating a crossover effect. The control group also showed increased flexibility, possibly due to a short-term crossover effect or test familiarization. No significant differences were detected between CG and SSG non-stretched leg.