OBJECTIVES: Initiatives to reduce office-workplace sitting are proliferating, but the impact of pre-existing musculoskeletal symptoms on their effectiveness has not been determined. We assessed the influence of musculoskeletal symptoms on the outcomes of a workplace sitting intervention. METHODS: Baseline and 3-month data from a cluster-randomized controlled trial of a workplace sitting intervention (Stand Up Victoria; trial registration number ACTRN12611000742976) were used. Office workers (n = 231) from 14 work teams within one organisation were randomised (by worksite) to a multicomponent program with individual-, organisational-, and environmental-level (sit-stand workstations) change strategies; or, to a control condition (no intervention). Musculoskeletal symptoms in the low-back, upper and lower extremities (present/absent) were assessed through self-report. Linear regression models tested the moderation by baseline musculoskeletal symptoms of intervention effects on workplace sitting and standing time and on sitting and standing bout durations, assessed by the activPAL3™ activity monitor. RESULTS: There were significant reductions in sitting and increased standing at work (p < 0.05). However, effects varied significantly by the presence of pre-existing low-back (but not other) symptoms, with greater benefit being seen in those without symptoms. Effects on sitting time and sitting bout duration were weaker in those with low-back symptoms compared to those without by 34.6 [95% CI (0.9; 68.3)] min/8-h workday and 5.1 [95% CI (0.2; 9.9)] min, respectively. Comparable effects were seen for standing. CONCLUSION: Low-back symptoms may impact on the extent to which office workers change their workplace sitting and standing time. A prudent next step to improve the effectiveness of workplace sitting-reduction initiatives such as Stand Up Victoria may be to assess and address the needs of those who displayed comparatively limited behaviour change, namely those with pre-existing low-back discomfort.

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