Characteristics of the study group at baseline.
| Characteristic | Regular Exercise (n = 3031) | No Regular Exercise (n = 8925) | P Value |
| Female sex, % (n) | 52 (1587) | 56 (4950) | 0.002 |
| Current smoking, % (n) | 9 (255) | 13 (1171) | <0.001 |
| Previous back surgery, % (n) | 16 (492) | 20 (1754) | <0.001 |
| Workload, % (n) | <0.001 | ||
| Not employed | 55 (1554) | 72 (5836) | |
| Easy work | 18 (501) | 11 (887) | |
| Medium work | 19 (530) | 11 (845) | |
| Strenous work | 8 (219) | 6 (514) | |
| Age, y, mean (SD) | 65 (10) | 69 (10) | <0.001 |
| BMI, mean (SD) | 27 (4) | 28 (4) | <0.001 |
| ODI, mean (SD) | 39 (15) | 45 (15) | <0.001 |
| Back pain, VAS, mean (SD) | 51 (27) | 57 (26) | <0.001 |
| Leg pain, VAS, mean (SD) | 60 (25) | 63 (25) | <0.001 |
Abbreviations: BMI, body mass index; ODI, Oswestry Disability Index; VAS, visual analog scale.
Note: Categorical variables are presented as % (n) and continuous variables as mean (SD). Differences in characteristics were compared using the no exercise group as reference. Base line values of BMI, function ODI and pain on the VAS.