Table 4. Association (χ2) and prevalence ratio between dependent variable back pain and independent variables (behavioral).

Variable	n	%	Back pain (n)	Back pain (%)	χ2a	Prevalence ratio	95%CI
Behavioral
Physical exercise (n = 1,437)
Yes	1,287	89.6	727	56.5	0.103	1
No	150	10.4	74	49.3		0.95	0.9–1.01
Physical exercise weekly frequency (n = 1,295)d
Zero day per week	150	11.6	74	49.3	0.003b	1
One to two days per week	627	48.4	382	60.9		1.07	1.01–1.14
Three; four days per week	357	27.6	181	50.7		1.01	0.94–1.07
Five or more days per week	161	12.4	85	52.8		1.02	0.95–1.11
Practice competitive exercise (n = 1,273)c
Yes	560	44.0	302	53.9	0.128	1
No	713	56.0	415	58.2		1.02	0.99–1.06
Time spent watching television per day (n = 1,229)
Zero; three hours per day	698	56.8	359	51.4	0.001b	1
Four; seven hours per day	421	34.2	252	59.9		1.05	1.01–1.09
Eight or more hours per day	110	9.0	77	70.0		1.12	1.06–1.18
Time spent using computer per day (n = 1,102)
Zero; three hours per day	735	66.7	407	55.4	0.536	1
Four; five hours per day	231	21.0	132	57.1		1.01	0.96–1.06
Six or more hours per day	136	12.3	82	60.3		1.03	0.97–1.09
Time sleeping per night (n = 1,225)
Zero; seven hours per day	409	33.4	239	58.4	0.449	1
Eight; nine hours per day	624	50.9	341	54.6		0.97	0.93–1.01
Ten or more hours per day	192	15.7	105	54.7		0.97	0.92–1.03
Read or study in bed (n = 733)
No	285	38.9	144	50.5	0.021b	1
Yes	448	61.1	266	59.4		1.05	1.01–1.11
Postural
Sleeping posture (n = 1,308)
Supine	103	7.9	44	42.7	0.005b	1
Lateral decubitus	838	64.1	458	54.7		1.08	1.01–1.16
Prone	367	28.0	222	60.5		1.12	1.04–1.21
Sitting posture; write (n = 1,401)
Adequate	213	15.2	100	46.9	0.008b	1
Inadequate	1,188	84.8	678	57.1		1.06	1.01–1.12
Sitting posture on a bench (n = 1,394)
Adequate	183	13.1	93	50.8	0.126	1
Inadequate	1,211	86.9	690	57.0		1.04	0.98–1.09
Sitting posture; use computer (n = 1,394)
Adequate	301	21.6	150	49.8	0.018b	1
Inadequate	1,093	78.4	630	57.6		1.05	1.01–1.09
Posture; lift object from the floor (n = 1,339)
Adequate	105	7.8	67	63.8	0.069	1
Inadequate	1,234	92.2	680	55.1		0.94	0.89–1.04
Carrying school supplies (n = 1,439)
Backpack	1,332	92.6	749	56.2	0.190	1
Another (briefcase, purse, and others)	107	7.4	53	49.5		0.95	0.89–1.02
Way; carry backpack (n = 1,320)c
Adequate (symmetrical handles on both shoulders)	1,166	88.3	639	54.8	0.019b	1
Inadequate (asymmetric)	154	11.7	99	64.3		1.06	1.01–1.11

^a Bivariate analysis. Wald Chi-square test.

^b Significant association (p < 0.05).

^c Related only to those schoolchildren to which the variable applies.

^d Those containing the response “I don’t know, it depends on the week” were excluded.