BACKGROUND: The aim of the study was to investigate if different frequencies, loads and trunk postures of occupational lifting and carrying increases the risk of sub-chronic (1-30 days last 12 months) low back pain (LBP) to become persistent (>30 days last 12 months) among female health care workers.

METHODS: Female health care workers answered a questionnaire about occupational lifting or carrying frequency (rarely, occasionally and frequently), load (low: 1-7 kg, moderate: 8-30 kg and heavy: >30 kg) and trunk posture (upright or forward bent back), and days with LBP in 2005 and 2006.

RESULTS: The odds ratio (OR) for developing persistent LBP in 2006 from these characteristics of occupational lifting and carrying was investigated with multi-adjusted logistic regressions among female health care workers with sub-chronic LBP (n = 2381) in 2005. Among health care workers with sub-chronic LBP, increased risk of persistent LBP was found from frequently lifting or carrying with forward bent back of moderate loads (OR: 1.63; 95% CI: 1.15-2.33) and heavy loads (OR: 1.56; 95% CI: 1.04-2.34). No increased risk for LBP to develop into a persistent condition was found for frequent lifting with upright back, frequent lifting or carrying of light loads, or occasionally lifting or carrying of any loads.

CONCLUSIONS: Preventive initiatives for sub-chronic LBP to develop into a persistent condition ought to focus on reducing frequent lifting and carrying of moderate and heavy loads with forward bent back.