Abstract
Introduction Soft tennis involves repetitive, high-intensity trunk rotation during strokes and serves, with high popularity among Japanese high school boys. The sport's biomechanical demands place cumulative stress on the lumbar spine, and a high prevalence of low back pain (LBP) has been reported, particularly in elite-level athletes. Previous studies identified factors such as hip rotation range of motion (ROM), trunk strength, and training load as LBP-related in rotational sports. However, foundational insights into the structural characteristics of the lumbar region in this population remain limited, hindering the identification of anatomical risk factors for LBP. The lumbar multifidus (MF) is a key deep stabilizing muscle contributing to dynamic spinal control. It consists of superficial and deep layers, with the deep layer playing a primary role in segmental stability. Due to this role, the deep MF may be more susceptible to degeneration, such as fatty infiltration, under repeated sport-specific load. Objective This study aimed to non-invasively assess structural differences between superficial and deep layers of the lumbar MF in elite male high school soft tennis players, using ultrasound-based echo intensity (EI) as a quantitative index of tissue composition. Methods A cross-sectional study was conducted on 55 elite male high school soft tennis players. B-mode ultrasound (Noblus, Hitachi Aloka, Tokyo, Japan) images were acquired at the L4 level in a prone position, using standardized settings (depth 6.0-8.0 cm, frequency 10 MHz, gain 60 dB, and dynamic range 70 dB). Using ImageJ (National Institutes of Health, Bethesda, MD, US), the MF was bisected from the superficial border to the lamina to define superficial and deep regions of interest (ROIs), and each ROI was manually traced. EI was defined as the mean grayscale value (0-255) within each ROI. Two images per side (dominant and non-dominant) were averaged. Two-way repeated-measures analysis of variance (ANOVA) tested the effects of layer and side, with significance at p < 0.05. Results A significant main effect of layer was found (F(1, 54) = 120.64, p < 0.001, partial η² = 0.691), with the deep layer showing higher EI than the superficial layer. No significant main effect of side or interaction was observed (p > 0.05). The mean superficial vs. deep EI was 89.84 vs. 115.69 on the non-dominant side, and 89.59 vs. 113.68 on the dominant side. Conclusion This study is the first to demonstrate consistently higher EI in the deep MF compared to the superficial layer, regardless of side, in elite male high school soft tennis players. These findings suggest that the deep MF, which is vital for spinal stability, may undergo degenerative changes due to repeated loading. Such structural imbalance may compromise segmental spinal control and increase future LBP risk. This study underscores the value of layer-specific EI as a potential biomarker and highlights the need for longitudinal research to explore its predictive utility for LBP in rotational athletes.