Discussion
PsA, inflammation, peripheral joints, pain, buttock/pelvic pain, SIJ, enthesitis
INFLAMMATION, DISEASE, REMISSION, ENTHESITIS, JOINT INFLAMMATION
In this RCT, there was no clear evidence of an increased risk of inflammation after three months of HIIT, evaluated by objective measures such as US of the peripheral joints and entheses, as well as by MRI BME of the SIJ and spine, comparing the HIIT group with controls.It is crucial to implementing recommendations on physical activity and exercise to be aware of potentially harmful effects of this activity on inflammation in PsA. It has been hypothesized that mechanical stress in an inflammatory environment can promote the onset of enthesitis [In healthy individuals, both at low activity and after physical activity, US findings of active inflammation at the lower limb entheses have been observed [Reducing the inflammatory environment with treatments could reduce the risk of enthesitis. Thus, it seems rational for patients to be in remission before starting any vigorous physical activity. Properly adapted and supervised exercise could also minimize this risk [In our study, we did not evaluate for structural changes. However, the proportions with an increase of inflammation in the entheses were equal in both groups. This could be due to a low to minimal disease activity at baseline due to good medical treatment, thus protecting the patients from a flare after the HIIT. Also, the HIIT exercise was performed on a stationary bicycle that minimizes the mechanical stress to lower limbs and back. Finally, the timing of the US evaluation after the HIIT could be of importance as it was performed 1–2 weeks after the last HIIT.Only a few studies have evaluated the impact of physical activity on joint inflammation in PsA [MRI BME in spine and SIJ are not unique findings in PsA and SpA. MRI BME in the SIJ is frequently seen in other conditions as well, such as in patients with nonspecific back pain and in healthy subjects such as athletes and military recruits. And especially in postpartum women with buttock/pelvic pain a positive MRI BME in SIJ has been observed [Our patients with PsA had a low degree of BME in both the SIJ and spine at baseline and they had a moderate axial inflammation measured by ASDAS-CRP. Further, only 15% were HLA B27 positive. The axial phenotype of PsA is associated with HLA B27 [A strength of this study was the randomized and longitudinal design. Further, the use of objective evaluation of inflammation by US and MRI covering both the peripheral joints, entheses and axial skeleton, is reflecting the whole specter of potential regions with inflammation in PsA. Also, the MRI reader was blinded to group allocation, timepoint and clinical outcomes. The chosen mode of intervention, the HIIT, is a standardized method of physical exercise and may be a more reliable measure of physical exercise compared to self-report of activity.Adherence is a major barrier to interventions such as physical activity. However, the dropout rate was only 6–7% in both groups making the risk estimates more reliable. We have no information about the activity of the control group. However, the VO2 max (an objective measure of cardiorespiratory fitness) capacity increased significantly in the HIIT group compared to controls, indicating that the HIIT group did more physical exercise compared to controls, data not shown [Other limitations to this study imply the small sample size, especially regarding the subset of participants with MRI in which only two thirds of the study cohort are included. Accordingly, the CIs are wide, which may imply a lack of power to observe small changes and making the proportions and risk estimates imprecise.Further, the sonographer was not blinded to group allocation and clinical scores at follow up and US findings were not confirmed by another sonographer. However, previous studies in US found a good interrater reliability among skilled sonographers [The external validity may be questioned as patients volunteering to this kind of activity may not previously have experienced axial or entheses inflammation to a higher degree, as that might have prevented them from attending.