Introduction: Traditionally, assessment in axial Spondyloarthritis (aSpA) includes the evaluation of the capacity to execute tasks, conceptualized as physical function. The role of physical activity, defined as movement-related energy expenditure, is largely unknown and almost exclusively studied using patient-reported outcome measures. The aims of this observational cross-sectional study are to compare physical activity between patients with aSpA and healthy controls (HC) and to evaluate the contribution of disease activity to physical activity differences between groups.
Methods: Forty patients with aSpA were matched by age, gender, period of data acquisition in terms of days and season to 40 HC. Physical activity was measured during five consecutive days (three weekdays and two weekend days) using ambulatory monitoring (SenseWear Armband). Self-reported disease activity was measured by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). Differences in physical activity between patients with aSpA and HC were examined with Wilcoxon signed-rank tests and a mixed linear model. Difference scores between patients and HC were correlated with disease activity.
Results: Average weekly physical activity level (Med(IQR); HC:1.54(1.41-1.73); aSpA:1.45(1.31-1.67),MET) and energy expenditure (HC:36.40(33.43-41.01); aSpA:34.55(31.08-39.41),MET.hrs/day) were significantly lower in patients with aSpA. Analyses across intensity levels revealed no significant differences between groups for inactivity and time spent at light or moderate physical activities. In contrast, weekly averages of vigorous (HC:4.02(1.20-12.60); aSpA:0.00(0.00-1.20),min/d), very vigorous physical activities (HC0.00(0.00-1.08); aSpA:0.00(0.00-0.00),mind/d) and moderate/(very)vigorous combined (HC2.41(1.62-3.48); aSpA:1.63(1.20-2.82),hrs/d) were significantly lower in patients with aSpA. Disease activity did not interact with differences in physical activity between patients with aSpA and HC, evidenced by non-significant and very low correlations (range: -0.06-0.17) between BASDAI and HC-aSpA patients' difference scores.
Conclusions: Patients with aSpA exhibit lower physical activity compared to HC and these differences are independent of self-reported disease activity. Further research on PA in patients with aSpA should be prioritized.