This study is the first to investigate the contribution of lower limb arthritis, particularly hip and knee arthritis, in poor outcomes in walking, cycling, function, and quality of life in children with JIA with peripheral arthritis. In JIA with lower extremity involvement, the most important determinants of walking endurance, function, and quality of life are hip and knee arthritis, whereas knee arthritis is crucial in cycling performance. The literature search showed us when investigating factors affecting poor outcomes in children with JIA, quantitative assessment methods, such as the number of swollen joints and tender joints, have often been used. It has been proposed to ultimately reduce the number of tender/swollen joints (or lower disease activity scale scores) [12, 13, 28]. On the other hand, the present study indicates that different joints affect the result differently with the qualitative assessment.
In this study, knee arthritis was observed at a rate of 28.1%, foot/ankle 15.6%, and hip 12.5% when clinical and subclinical arthritis were evaluated together. These rates correspond with the findings in the literature. In a study conducted in patients under one year of treatment, the prevalence of knee, ankle, foot, and hip synovitis was 8–20%, 6–12%, 5, and < 5%, respectively, and walking difficulty was observed at rates of 25–30% . The joint involvement that would best explain this difficulty in walking seen in the majority of the patients is not specified, and the diagnosis of arthritis is based solely on physical examination. However, it is important to distinguish subclinical synovitis in children, especially in feet and hip joints, and not to omit them while investigating the causes associated with the result. Each patient should be specifically evaluated for these joints by clinical and, if necessary, subclinical examination [15, 16].
It is well known that children with JIA have a decreased functional competence and quality of life than healthy children . In a previous large sample study, predictors of disability assessed by HAQ were female gender, symmetrical arthritis, hip joint involvement, long-term high ESR, and rheumatoid factor . Regarding the quality of life, the effect of the number of swollen joints was highlighted in another study . It is noteworthy that in the current study, a significant part of the variance in both functional loss and quality of life was explained by hip and knee arthritis.
The average of 6MWT in our study was at expected levels when compared with the previous study  and was lower than healthy Turkish children . The findings of our study highlight that out of all the factors, hip arthritis may have the greatest impact on this low score of 6MWT. In addition to hip arthritis, knee arthritis, gender, age, and weight explain 70.1% of the variance in 6MWT. This result suggests that walking endurance can be widely affected by active hip and knee arthritis. However, 6MWT is a widely used method to assess cardiovascular exercise capacity, and it has also been included in home exercise programs given to children with JIA in clinical trials [27, 33,34,35]. Hence, we also investigated a different exercise performance that was less likely to be affected by the effect of active arthritis in assessing actual cardiovascular performance. Although respiratory and metabolic requirements differ during cycling and walking exercise , we hypothesized that the effect of arthritis on cycling performance would not be as great as that on walking endurance.
In our study, a protocol that can provide a cycling exercise result with a wide variance was applied. CYC-E correlated moderately with 6MWT and remained at the submaximal exercise level determined by heart rate measurement. Ultimately, as intended, an intense home exercise cycling performance was provided . When we investigated the contribution of lower extremity involvement to cycling performance, it was seen that 29.9% of the variance was explained by the presence of knee arthritis and height. Although this data is similar to the effect of height determined in the study of Paap et al. , our study is unique in that it also includes the contributing joints. In conclusion, this study contributes to the literature by indicating that walking and cycling endurances are affected by different factors in these patients and hip arthritis has a greater influence on walking than cycling.
Although lower extremity deformity examined in this study is not found as an independent risk factor for poor outcome, it can be assumed that there is a questionable effect on walking endurance. On the other hand, it was not associated with CHAQ, unlike a previous study . The failure to identify lower extremity deformities as an independent factor may be since it is indeed not so or may be because our sample did not include a great sample size. Future studies are required to investigate the effect of the presence of deformity on adverse outcomes with larger samples and established disease. Apart from arthritis and deformity, one of the conditions most likely to be associated with reduced physical activity in children with inflammatory diseases is pain . However, the pain was not included in independent factors in this study, because it is a subjective complaint and our outcome variables could be influenced by any pain in children . Hence, a definite diagnosis of arthritis has provided more reliable observations in understanding the contribution of lower extremity arthritis to the results. In addition to all, there is a need to design and validate tools assessing optimal LED (especially other deformities of the foot) and home exercise performances . The pilot study that we conducted because of a lack of protocol that can be considered as a home program for cycling exercise in the literature, has ensured that this issue is relatively tolerated .
The results might have been different if the majority had been diagnosed with polyarticular or systemic JIA. The variances in the primer outcome measures might have probably not been explained that well. However, the main purpose of this study was to find out the value of lower extremity involvement type in itself. The results should not be generalized to all JIAs. Future research is needed to explain the variance of disability or QoL in the general JIA population.