Objectives: This study examined the effects of sitting surfaces on the cross-sectional area of lumbar multifidus (LM) in patients with Chronic Low Back Pain (CLBP) and healthy controls (HC). Design: Cross-Sectional Case Controlled Study. Setting: Isle of Man Institute of Sport. Participants: 40 age and sex matched, sporting participants aged 18-45 years, recruited from private physiotherapy practice patients (n = 20 CLBP, 16 male, 4 female, and n = 20 healthy controls, 16 males and 4 females). Main Outcome Measures: Cross-sectional area of LM was measured using rehabilitative ultrasound imaging. Results: Swiss Ball (SB) was more effective at stimulating LM than a Stable Surface (SS) in both groups: CLBP:SB:12.3 (cm2) (SD:3.6), SS:10.15 (SD:2.6), p <0.0001; HC:SB:12.5 (SD:2.7), SS:11.3 (SD:2.9), p <0.0001). No significant differences between groups were noted. No differences between left and right side cross-sectional areas between or within groups were noted. Conclusion: Cross-sectional area of LM increased as the lability of the surface increased, demonstrating that SB was more effective at stimulating LM activity than a non-labile surface. This confirms current clinical practice and supports the use of a labile surface in spinal rehabilitation. The lack of LM asymmetry within and between groups is discussed.