Table 3.

Multivariate linear regression analyses to predict physical activity in children with juvenile idiopathic arthritis and controls

	B	95 % CI lower	95 % CI upper	P
PAL
Reference	1.75	1.67	1.83	<0.01
Controls	0.10	0.03	0.18	0.01
Age centered 10 years	0.04	0.02	0.07	<0.01
BMI centered 17 kg/m2	−0.01	−0.02	−0.00	0.04
Gender	−0.07	−0.13	−0.01	0.02
JIA season	−0.14	−0.23	−0.05	<0.01
MVPA
Reference	1.70	1.27	2.13	<0.01
Controls	0.41	−0.02	0.83	0.06
Age centered 10 years	0.20	0.07	0.33	<0.01
BMI centered 17 kg/m2	−0.02	−0.08	0.04	0.50
Gender	−0.12	−0.43	0.20	0.47
JIA season	−0.50	−1.00	0.01	0.06
Sedentary time
Reference	18.86	18.36	19.37	<0.01
Controls	−0.59	−1.09	−0.09	0.02
Age centered 10 years	−0.13	−0.28	0.03	0.10
BMI centered 17 kg/m2	0.15	0.08	0.21	<0.01
Gender	−0.02	−0.38	0.35	0.92
JIA season	0.78	0.18	1.37	0.01

The regression equation for PAL is as follows:

$\begin{array}{l}\mathrm{PAL}=\mathrm{reference}+0.10*\mathrm{control}+0.04*\mathrm{age}\left(\mathrm{centered}10\right)+-0.01*\mathrm{B}\mathrm{M}\mathrm{I}\left(\mathrm{centered}17\right)+-0.07\\ *\mathrm{gender}+-0.14*\mathrm{season}\end{array}$

The reference for this equation is a 10 year old boy with JIA, a BMI of 17 kg/m² of which the data was collected in the summer. So a healthy girl (no JIA) of 8 years old, a BMI of 20 has a predicted PAL of (1.75 + 0,10 * 1 + 0.04 * (8–10) + −0.01 * (20–17) + −0.07 * 1 = 1.73

JIA juvenile idiopathic arthritis, BMI body mass index, PAL physical activity level, MVPA moderate to vigorous activity expressed in hours/day. Sedentary time expressed in hours/day, CI confidence interval of B. Reference category: Boy of 10 years, with a BMI of 17, with JIA, who filled in the diary in the summer period