Table 3.

Crude and multivariable Cox regression models with HRs for development of back pain at any site

	Univariate		Multivariable*
	HR	95% CI	HR	95% CI
Age
Months	1.02	1.01 to 1.03	1.02	1.01 to 1.03
Sex
Male	Ref	–	–	–
Female	1.40	1.35 to 1.46	1.40	1.35 to 1.46
BMIz categories
Underweight	0.78	0.41 to 1.50	0.78	0.41 to 1.51
Normal range	Ref	–	–	–
Overweight	1.18	1.10 to 1.27	1.18	1.09 to 1.27
Obese	1.34	1.20 to 1.51	1.34	1.19 to 1.51
Socioeconomic status
Rural area	1.29	1.20 to 1.38	1.29	1.20 to 1.39
Urban quintile 1	Ref	–	–	–
Urban quintile 2	1.16	1.07 to 1.25	1.16	1.07 to 1.25
Urban quintile 3	1.28	1.18 to 1.38	1.27	1.18 to 1.38
Urban quintile 4	1.32	1.23 to 1.43	1.33	1.23 to 1.44
Urban quintile 5	1.41	1.31 to 1.52	1.43	1.33 to 1.55
Urban not classified	1.32	1.19 to 1.47	1.32	1.18 to 1.47
Nationality
Spanish	Ref	–	–	–
Other	0.75	0.69 to 0.82	0.73	0.67 to 0.79

*Adjusted for all the variables reported on in this same table.

BMIz, body mass index for age z-score.