Effect of Shoulder Flexion Range of Motion and Trunk Muscle Activity on Lumbar Lordosis in the Streamlined Posture in Healthy Young Men

Objective This study aimed to compare the lumbar spine, pelvic alignment, and trunk muscle activity in healthy participants with and without shoulder flexion restriction in a streamlined posture. Methods The lumbar lordosis angle, pelvic anteversion angle, and trunk muscle activity were measured in 31 young, healthy men in the resting standing position. The streamlined (SL) posture and correlation analysis were performed. Multiple regression analysis was performed to analyze variables that affect the lumbar lordosis angle in the SL posture. Results Correlation analysis revealed that the lumbar lordosis angle in the SL posture had a significant negative correlation with the muscle activity of the internal oblique/transversus abdominis muscle (IO/TrA) during the SL posture (r=-0.37, p<0.045). Multiple regression analysis revealed that lumbar lordosis angle in the SL posture was associated with lumbar lordosis angle in the upright position (β=0.46), IO/TrA activity (β=-0.37), shoulder joint flexion range of motion (β=-0.37), and longissimus dorsi muscle activity during the SL posture (β=-0.28), with a Durbin-Watson statistic adjusted coefficient of determination R² of 0.67 (p<0.001). Conclusion The lumbar lordosis angle in the SL posture was likely to be high when the range of motion of shoulder joint flexion was small, the activity of the longissimus dorsi muscle in the SL posture was high, and the activity of the IO/TrA was low. This suggests that limited shoulder flexion causes excessive lumbar lordosis, which may be a mechanism of low back pain in swimmers.