Descriptive characteristics of the study sample divided into groups by quartiles of knee extensor strength
| Knee extensor strength quartiles | ||||||
| 1 | 2 | 3 | 4 | All | ||
| N | 10 095 | 9983 | 10 018 | 10 021 | 40 117 | |
| Knee extensor strength in Nm, mean (SD) | 177 (21.4) | 217 (8.1) | 246 (9.2) | 295 (27.6) | 234 (46.9) | |
| Weight in kg, mean (SD) | 61 (7.9) | 65 (7.7) | 68 (8.2) | 72 (9.3) | 66 (9.3) | |
| Length in cm, mean (SD) | 176 (6.4) | 178 (6.0) | 179 (6.0) | 180 (6.0) | 178 (6.3) | |
| BMI, mean (SD) | 19.8 (2.3) | 20.6 (2.3) | 21.2 (2.4) | 22.2 (2.7) | 21.0 (2.6) | |
| Alcohol consumed per week in g,* median (IQR) | 53 (23,101) | 54 (26,102) | 55 (27,102) | 55 (27,105) | 54 (25,102) | |
| At least one parent with higher education,* n (%) | 717 (7.1) | 750 (7.5) | 849 (8.5) | 891 (8.9) | 3207 (8.0) | |
| OA, RA or meniscal/cartilage pathology, n (%) | 72 (0.7) | 56 (0.6) | 69 (0.7) | 70 (0.7) | 267 (0.7) | |
| Smoking, per day* | ||||||
| Non-smoker | 3884 (39) | 3943 (40) | 3971 (40) | 4306 (44) | 16 104 (41) | |
| 1–5 cigarettes | 1110 (11) | 1079 (11) | 1201 (12) | 1147 (12) | 4537 (11) | |
| 6–10 cigarettes | 2192 (22) | 2151 (22) | 2126 (22) | 1935 (20) | 8404 (21) | |
| 11–20 cigarettes | 2398 (24) | 2343 (24) | 2204 (22) | 2147 (22) | 9092 (23) | |
| 21+ cigarettes | 371 (4) | 314 (3) | 358 (4) | 308 (3) | 1351 (3) | |
| Adult occupation* | ||||||
| Farmers/miners | 5789 (58) | 5690 (58) | 5621 (57) | 5619 (57) | 22 719 (57) | |
| Production/machine operators | 527 (5) | 521 (5) | 589 (6) | 607 (6) | 2244 (6) | |
| Service, physically demanding | 2876 (29) | 2871 (29) | 2893 (29) | 2876 (29) | 11 516 (29) | |
| Other | 762 (8) | 776 (8) | 810 (8) | 824 (8) | 3172 (8) | |
| Knee injury during follow-up, n (%) | 614 (6.1) | 645 (6.5) | 643 (6.4) | 769 (7.7) | 2671 (6.7) | |
*Missing data: alcohol consumption, 3.8%; parental education, 6.4%; smoking, 1.6%; adult occupation, 1.2%.
BMI, body mass index; OA, osteoarthritis; RA, rheumatoid arthritis; SD, standard deviation; IQR, interquartile range