OBJECTIVE Imbalance between work demands and individual resources can lead to musculoskeletal disorders and reduced work ability. The aim of this study was to evaluate the effect of two contrasting interventions on work ability among slaughterhouse workers with chronic pain and work disability. METHODS Sixty-six slaughterhouse workers with upper-limb chronic pain and work disability were randomly allocated to 10 weeks of either strength training for the shoulder, arm, and hand muscles (3 times per week, 10 minutes per session) or ergonomic training (usual care control group) from September to December 2012. The outcome measure was the change from baseline to 10-week follow-up in the work ability index (WAI). RESULTS A priori hypothesis testing showed a group×time interaction for WAI (P<0.05). Compared with the ergonomic training group, WAI increased 2.3 [95% confidence interval (95% CI) 0.9-3.7] in the strength training group corresponding to a moderate effect size (Cohen's d 0.52). Within-group changes indicated that between-group differences were mainly caused by a reduction in WAI in the ergonomic group. Of the 7 items of WAI, item 2 (work ability in relation to the demands of the job) and item 7 (mental resources) increased following strength training compared with ergonomic training (P<0.05). CONCLUSIONS Implementation of strength training at the workplace prevents deterioration of work ability among manual workers with chronic pain and disability exposed to forceful and repetitive job tasks. Thus, strength training performed at the workplace may in fact be regarded as a complex biopsychosocial intervention modality that reaches further than the specific physiological benefits of training per se.