Objective: We aimed to investigate the effective frequency of jack-knife stretching in preadolescent male football players. Methods: We enrolled 47 male preadolescent football players (average age: 12.4 ± 0.6 years old). All participants took surveys at baseline and after intervention for 6 months. The survey items were height, weight, body mass index, anteflexion in sitting, and quadriceps and hamstring tightness. The participants were divided into two groups according to the stretching exercise frequency, where group A performed stretching at least once every 3 days and group B performed stretching less than once every 3 days. After exclusion, 17 participants were enrolled in each group.Results: There was a significant increase in anteflexion in the sitting position in group A but not group B; moreover, straight leg raising was significantly larger in Group AConclusions: Performing jack-knife stretching at least once every 3 days’ intervals could improve hamstring flexibility in preadolescent male football players.Level of Evidence Level Ⅱ

PurposeTo investigate the incidence of lumbar degeneration findings and low back pain (LBP) over a 5-year period in children and adolescent weightlifters using magnetic resonance imaging (MRI) and medical questionnaires. Furthermore, to reveal the temporal changes in the lumbar vertebrae caused by long-term hard weightlifting training during the growth period.MethodsTwelve children and adolescent weightlifters who participated in weightlifting for >2 years (six boys, six girls, 11.4±2.0 years) were enrolled. Participants underwent annual medical questionnaire surveys including practice frequency, competition history, presence of LBP, and lumbar examinations using MRI during the 5-year follow-up.ResultsLumbar disc degeneration was detected in all participants after 4 years, and lumbar disc herniation findings were detected in 33% of participants after 5 years; one underwent herniotomy during the follow-up period. Lumbar spondylolysis was detected in 58% of patients at 5 years. Although there were three participants who had LBP in the final year, none had LBP that prevented them from returning to weightlifting.ConclusionA 5-year cohort study of 12 children and adolescent weightlifters detected lumbar degeneration in all participants. Long-term hard weightlifting training during the growth period may increase the risk of developing current and future LBP.