Introduction: Age-related differences in limited flexibility of the lower extremities, such as muscle tightness of the iliopsoas, hamstring, and quadriceps femoris muscles, and their correlations with low back pain (LBP) in juvenile athletes have not been well assessed. The purpose of this study was to investigate the relationships between LBP during the baseball season and tightness of lower extremity muscles with growth in young baseball players.

Methods: Participants comprised 1228 baseball players (220 pitchers, 1008 fielders) between 6 and 16 years old who underwent medical checkups (self-completed questionnaire and physical examination). We investigated the relationships between LBP during the season and measurements of flexibility of the lower extremity muscles, including the Thomas test (iliopsoas tightness), straight-leg-raising angle (hamstring tightness), and heel-to-buttock-test (quadriceps femoris tightness). We compared participants with and without seasonal LBP based on limited flexibility of the lower extremities on both sides using univariate analysis (chi-square tests) stratified by age groups (≤10 years, 11–12 years, 13–14 years, and 15–16 years). Multivariable logistic regression analysis was then performed, and odds ratios and 95% confidence intervals were calculated. We restricted the age group for the model based on the results of univariate analyses (P<0.05). In multivariable analysis, the following factors were analyzed as explanatory variables: total amount of practice per week greater than the median of this study population; Rohrer index greater than 145; and limited flexibility of the lower extremities defined as iliopsoas, hamstring or quadriceps femoris muscle tightness on either side. All tests used were two-sided, and values of P<0.05 were considered significant.

Results: Of the 1228 players, 255 (20.9%) reported experiencing seasonal LBP requiring rest during the previous year. Prevalence of LBP and tightness of the iliopsoas, hamstrings, and quadriceps femoris all increased with age. Univariate analyses revealed tightness of the hamstrings on the non-throwing-arm side and tightness of the quadriceps femoris on both sides were associated with seasonal LBP at 11–12 years old (P=0.027, P=0.005, and P=0.021, respectively). Multivariable analysis adjusting for factors associated with LBP showed significant associations between quadriceps femoris tightness (odds ratio 1.78, 95% confidence interval 1.13–2.81; P=0.013) and LBP in players at 11–14 years old.

Discussion: Tightness of the hamstrings and quadriceps femoris were identified as risk factors potentially associated with LBP among school-aged baseball players. These findings may guide the management of LBP among this age range of baseball players. In addition, particular attention should be paid to quadriceps femoris tightness in players at 11–14 years old.