Background: Several studies have shown physical activity (PA) to be inversely correlated to disease activity in rheumatoid arthritis (RA) patients. However, it is unclear whether improved PA leads to lower disease activity or if low disease activity predicts improved PA in rheumatoid arthritis patients. Furthermore, it is unknown how fibromyalgia (FM) affects this interaction. Objective(s): Our primary objective will be to compare the effect of an immediate improvement in physical activity after one week on the proportion of RA patients achieving low disease activity after 12 weeks of biological treatment. Furthermore, we will explore whether the presence of concomitant FM affects this clinical response (i.e., interaction between FM status and PA response).

Design: A prospective cohort study in the form of a target-trial attempting to address a causal question comparing the outcome among the participants with an immediate improvement in physical activity, relative to individuals without. Setting and patients: Biologically naive RA patients initiating biological treatment are consecutively enrolled. PA is quantified by accelerometry one week prior to, one week after, and after 3 months of biological treatment. Sample size: 100 RA patients starting biological therapy is planned enrolled in the study. Measurements: RA patients will be divided into two groups depending on their improvement in physical activity after onset of biological treatment. The percentage of time spent in moderate-to-vigorously physical activity (MVPA) i.e., the percentage of time a patient is in motion \[walking, running, bicycling, or swimming\] is measured prior to, one week, and approximately 3 months after biological treatment onset, respectively. The MVPA ratio (MVPA After biological treatment/MVPA Before biological treatment) will divide patients enabling a contrast between two groups: Those with high improvement considering MVPA (after 1 week; the upper tertile of MVPA ratio of the sample) and those without. Treatment response (achieving a disease activity score \< 3.2) after approximately 3 months from baseline will be analyzed as the contrast between groups.