Lumbar Multifidus Morphology in Youth Competitive Alpine Skiers and Associated Sex, Age, Biological Maturation, Trunk Stability, and Back Complaints

Background: The lumbar multifidus (LMF), as a dynamic stabilizer of the lumbar spine, may play an important role in the prevention of overuse-related back complaints. Hypothesis: LMF morphology is associated with trunk stability and differs between symptomatic and asymptomatic skiers. Study Design: Cohort study. Level of Evidence: Level 3. Methods: A total of 85 youth skiers (28 females, mean age, 14.7 ± 0.7 years; 57 males, mean age, 14.9 ± 0.5 years) underwent anthropometric assessments, an estimation of biological maturation, a magnetic resonance imaging- and ultrasound-based examination of LMF morphology, and a biomechanical quantification of deadbug bridging stabilization performance. Athletes were categorized as symptomatic if they had registered at least 1 significant overuse-related back complaint episode in the 12 months before the main examination. Results: Male skiers showed a greater LMF size (ie, anatomical cross-sectional area [ACSA]) than female skiers, except for vertebral body L5, where no difference was found (8.8 ± 1.8 cm 2 vs 8.3 ± 1.4 cm 2 , P = 0.18). Conversely, female skiers displayed longer fascicles than male skiers (5.8 ± 0.8 cm vs 5.4 ± 0.8 cm, P = 0.03). Skiers aged under 16 years (U16) skiers had greater values for LMF size and fascicle length than U15 skiers. Maturity offset was associated with L5 LMF size ( R 2 = 0.060, P = 0.01), fascicle length ( R 2 = 0.038, P = 0.04), and muscle thickness ( R 2 = 0.064, P = 0.02). L5 LMF size was associated with trunk stability ( R 2 = 0.068, P = 0.01). Asymptomatic skiers showed on average a 12.8% greater value for L5 LMF size compared with symptomatic skiers ( P = 0.04). Conclusion: There are sex- and age-related differences in LMF morphology in youth competitive alpine skiers. Moreover, the ACSA at the level of the lumbar vertebral body L5 undergoes changes during biological maturation, shows a small, but significant association with trunk stability, and differs between symptomatic and asymptomatic skiers with back complaints. Clinical Relevance: The observed association of muscle structure (ie, L5 LMF ACSA) with functional aspects (ie, trunk stabilization capacity) and clinical representation (ie, overuse-related back complaints) further highlights the important role of the multifidus muscle for training and injury prevention in youth competitive alpine skiers around the growth spurt.