Eight Weeks of 448 kHz Radiofrequency Therapy Increases Multifidus Muscle Cross-Sectional Area and Modulates Thoracolumbar Range of Motion in Horses with Chronic Back Pain

We aimed to investigate the effects of an 8-week 448 kHz radiofrequency (RF) on multifidus cross-sectional area (CSA) and thoracolumbar differential range of motion (ROM). Eight Thoroughbred horseball horses received four RF treatments (T9–L6) at 2-week intervals. Multifidus CSA (T10–L5) and differential spinal ROM between the withers (W), T13, T18, L3 and sacrum (S) were measured before treatment, immediately after the first session, 24 h after the first session, and at 8 weeks (15 days after the last session). Linear mixed models with Tukey post hoc comparisons were used. Significant effects of timepoint for CSA were significant with CSA increased at 8 weeks at all sites (T10–L5; all p < 0.001). No significant Time × Side interactions were identified for CSA (all p ≥ 0.334), demonstrating bilaterally consistent responses. Absolute increases versus baseline were greatest at T18 (+2.52 cm2), L5 (+1.90 cm2), and T16 (+1.77 cm2) (all p < 0.001). Axial rotation ROM increased transiently between W–T13 at 24 h (+4.8°, p < 0.001) but returned toward baseline at 8 weeks. Caudal thoracolumbar motion (T18–L3) decreased at 8 weeks (−4.2°, p = 0.004), suggesting improved segmental stability. Flexion–extension increased immediately post-treatment at W–T13 (+2.29°, p = 0.009) before reducing below baseline at 8 weeks (−2.49°, p = 0.005). Lumbosacral ROM showed minimal change. RF therapy was associated with multifidus hypertrophy alongside region-specific modulation of spinal motion, characterised by transient mobility increases followed by reduced caudal thoracolumbar motion, consistent with improved neuro-muscular dynamic stabilisation via multifidus hypertrophy.