This study investigated 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). 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) took part in the study. Swiss Ball (SB) was more effective at stimulating LM than a Stable Surface (SS) in both groups: CLBP:SB:12.3(cm(2)) (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). There weren't any significant differences between groups to note. No differences between left and right side cross-sectional areas between or within groups were noted. Cross-sectional area of LM increased as the lability of the surface increased, showing 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.