Abstract
BACKGROUND: This study examined whether the effects of a sit-stand desk (SSD) intervention on employees' musculoskeletal complaints (i.e., intensity and prevalence) and activation (i.e., vigilance and vitality) persist or fade out and whether velocity and acceleration of health improvements can predict medium-term (six-month) and long-term (24-month) improvements. Drawing from dynamic models of self-regulation, as well as the psychological momentum theory, we hypothesized that velocity and acceleration of health improvements in the early stages of the intervention would predict medium-term health improvements, which sustain long-term. METHODS: We used data from a six-month seven-wave randomized controlled trial with employees in mostly sedentary occupations and supplemented this by follow-up data from the same participants 18 months later, resulting in eight waves. RESULTS: Bayesian structural equational modeling revealed no significant intervention effect after 24 months implying a fade-out. But more importantly, velocity and, partially, acceleration of health improvements at earlier stages predicted medium-term improvements in musculoskeletal complaints and long-term improvements in vigilance. CONCLUSION: The findings of this study suggest that positive intervention effects fade out over time and health effects benefit from prompt progresses at the beginning of the intervention, warranting exploration in prolonged longitudinal studies.