Aim of investigation: To evaluate whether a perceived decline in the level of physical activity after the onset of pain (PAD) is more appropriate in the explanation of disability as compared to the actual level of physical activity (PAL) in patients with sub-acute back pain. Methods: Patients with 4-7 weeks of non-specific low back pain (LBP) participated in this study. Their habitual physical activity level before the back pain started (H-PAL), their actual level of physical activity (PAL) and their perceived decline in the level of physical activity after the onset of pain (PAD) were assessed. The association between these physical activity related variables and perceived disability (QBPDS), fear of movement/(re)injury (TSK), pain catastrophizing (PCS) and pain intensity (VAS) was examined. The role of PAD as a mediator in the association between fear of movement/(re)injury and disability was examined by three linear regression analyses. Results: 123 patients (66 male and 57 female) with a mean age of 44.1 years (SD = 10.3) participated in this study. PAD was significantly correlated with disability, fear of movement/(re)injury, pain catastrophizing and pain intensity. PAD and PAL appeared more important in the explanation of disability in the subgroup of patients who were physically active before their back pain started. Generally, PAD indeed mediated the association between fear of movement/(re)injury and disability. Conclusions: The perceived decline in physical activity, rather than the current physical activity itself is important in the evaluation of the impact of activity related changes on disability in low back pain.