Distribution of sample with respect to gender, age, LBP, and physical workload
| Variable | MZ twins (n=1420) | DZ twins (n=2400) | MZ+DZ twins (n=3820) |
|---|---|---|---|
| Gender | |||
| Male (%) | 44 | 44 | 44 |
| Female (%) | 56 | 56 | 56 |
| Mean age, years (SD) | 33 (4.47) | 33 (4.42) | 33 (4.45) |
| Outcome variable: low back pain during past year | |||
| Cases | |||
| Any LBP in past year (%) | 100 | 100 | 100 |
| LBP ≤30 days (short) (%) | 79 | 78 | 78 |
| LBP >30 days (long) (%) | 21 | 22 | 22 |
| Controls | |||
| No LBP in past year (%) | 100 | 100 | 100 |
| Potential risk indicators (cases and controls): physical work load | |||
| Sitting only (%) | 26 | 24 | 24 |
| Sitting and walking (%) | 27 | 26 | 27 |
| Light physical (%) | 31 | 35 | 33 |
| Heavy physical (%) | 15 | 14 | 15 |
| Missing (%) | 1 | 1 | 1 |