Table 1.

Participant characteristics.

Variable	Participant type
Variable	Healthy control (n = 39)	LBP (n = 37)
Age (years)^*	30.9 ± 8.26	35.2 ± 9.86
Gender (% female)^*	71.8	48.6
Race (%)
White	87.2	78.4
African-American	10.3	8.1
Ethnicity (% non-Hispanic)	94.9	89.2
Chronic pain duration (median, in months)		70.2
Past month chronic pain intensity (0–100)		44.1 ± 16.91
Resting plasma BE (ng/mL)	1.5 ± 2.41	1.8 ± 2.57
BDI^**	2.8 ± 3.04	7.5 ± 5.88
STAI^**	32.1 ± 6.97	38.1 ± 8.38
PANAS–Negative Affect	15.4 ± 3.99	16.8 ± 4.20

Values are presented as percentages or means ± SD. BDI, Beck Depression Inventory; BE, beta-endorphin; LBP, chronic low back pain; PANAS, Positive and Negative Affect Scales; SD, standard deviation; STAI, State Trait Anxiety Inventory.

p < 0.05.

^**

p < 0.01.