BACKGROUND: The high prevalence of low back pain (LBP) in today's working population and its substantial impact on quality of life call for preventive and sustainable strategies. Physical activity (PA), especially during leisure-time (LTPA), has been proposed as one of the few promising, active preventive measures against the onset of LBP. This is the protocol for the prospective observational study workHealth that aims to describe the patterns of PA among workers (including back posture and movement), examine the link between PA, LBP and back health and identify theory-based psychological determinants of LTPA. METHODS: The proposed study is a longitudinal observational study taking place over 2 months with an intensive-longitudinal ecological momentary assessment (EMA) phase. A sample of 252 adults will be recruited from two working populations, sedentary workers, and manually working physiotherapists. At baseline, participants complete a self-report questionnaire and undergo an objective measurement of their back posture and mobility. At 2, 4, and 8 weeks after baseline, participants fill in the follow-up questionnaires. Starting at baseline, participants will also wear an accelerometer and will complete three daily questionnaires for the following 14 days. In addition to the main study, a sub-sample of 8 observational N-of-1 studies will have an extended EMA phase of 68 days and a data-driven exit interview. The primary outcome is moderate-to-vigorous LTPA. Data will be analyzed using regression and multi-level models. For the N-of-1 studies, a mixed-methods approach will be used including dynamic regressions. DISCUSSION: Rather than solely examining LBP from a pathological perspective, workHealth is one of the first studies to investigate psychological, behavioral and biomechanical risk factors and protective resources against LBP. The study will offer insight into theory-based, psychological determinants of LTPA and its relationship to both low back pain and back health. Between-person and within-person level analyses will provide insight on group comparisons of average effects and individual patterns of physical activity in daily life, respectively. Understanding these relationships can inform future behavioral interventions and ultimately contribute to prevention efforts against LBP and the promotion of back health. TRIAL REGISTRATION: German Clinical Trials Register DRKS00025296|| https://drks.de/ || Registration date 28/06/2021.