1 Department of Health, Kinesiology and Applied Physiology, Concordia University, 7141 Sherbrooke Street W, SP-165.29, Montreal, QC, H4B 1R6, Canada.
2 School of Health, Concordia University, Montreal, QC, Canada.
3 CRIR - Centre de réadaptation Constance-Lethbridge du CIUSSS COMTL, Montreal, QC, Canada.
4 Department of Health, Kinesiology and Applied Physiology, Concordia University, 7141 Sherbrooke Street W, SP-165.29, Montreal, QC, H4B 1R6, Canada. maryse.fortin@concordia.ca.
5 School of Health, Concordia University, Montreal, QC, Canada. maryse.fortin@concordia.ca.
6 CRIR - Centre de réadaptation Constance-Lethbridge du CIUSSS COMTL, Montreal, QC, Canada. maryse.fortin@concordia.ca.

Low back pain (LBP) is a disabling disease and a public health concern. Aquatic exercise is an alternative form of exercise with less spinal loading and difficulty performing movements, benefiting those with pain-related fear. This study aimed to investigate the effect of an aquatic exercise program (SwimEx) versus standard care (SC) on lumbar paraspinal muscle volume and composition, strength and patient outcomes in individuals with chronic LBP. This randomized controlled trial included 34 participants with chronic LBP. Participants were randomly allocated to each group (SwimEx, n = 18; SC, n = 16) and underwent a 10-week supervised intervention program twice per week. Magnetic resonance imaging was performed at baseline and 10-weeks to examine the impact of each intervention on multifidus (MF) and erector spinae (ES) muscle volume (cm³) and fatty infiltration (% FI) at L1-L2, L2-L3, L3-L4, L4-L5, and L5-S1. Mixed model repeated measures ANCOVA revealed no significant time*group interactions for MF and ES volume and %FI. SwimEX had significant increases in MF volume at L2-L3 and L3-L4, and ES volume at L1-L2. Furthermore, SwimEX also had a significant increase in MF %FI at L2-L3. Both groups displayed significant increases in lumbar strength. Correlations between muscle morphology and patient outcomes showed improvements in MF volume were moderately correlated with an increase in physical quality of life and decrease in anxiety/depression. Interestingly, improvements in MF volume, MF %FI, and ES %FI, were each moderately correlated with a decrease in sleep disturbance. In conclusion, aquatic therapy may help increase lumbar paraspinal muscle volume and strength in participants with chronic LBP. Our findings support the notion that improvements in paraspinal muscle health are related to improvements in patient-reported outcomes. More imaging studies are required to examine the impact of exercise on overall paraspinal muscle health in chronic LBP and investigate these associations.