Role of Rate of Force Development in Mediating the Relationship Between Task-Specific Fear and Chronic Low Back Pain Among Japanese Caregivers: A Pilot Cross-Sectional Mediation Study

Background Chronic low back pain (CLBP) is prevalent among caregivers; however, the mechanisms linking psychological factors like fear of movement to pain are unclear. This study investigated whether the rate of force development (RFD), a measure of explosive strength, mediates the relationship between task-specific fear and CLBP intensity in this high-risk population. Methods Thirty-two Japanese care workers (13 CLBP, 19 healthy) performed sit-to-stand and stand-to-sit tasks on a force plate after rating their task-specific fear. Vertical ground reaction force was used to calculate early-phase RFD (0-100 ms) during stand-to-sit, the prespecified mediator. CLBP was defined as pain persisting for >3 months. Associations were analyzed using Spearman's correlation and a 1,000-sample bootstrap mediation model. Results Early-phase RFD (0-100 ms) during stand-to-sit correlated negatively with both task-specific fear (ρ = -0.43) and CLBP intensity (ρ = -0.51). The mediation analysis confirmed a significant indirect effect of fear on CLBP through RFD (B = 0.39, 95% CI (0.06, 1.30)). The total effect of fear on pain was also significant (B = 5.58, 95% CI (0.39, 18.61)). Conclusion In caregivers, higher task-specific fear is associated with suppressed RFD during the stand-to-sit task, reflecting a force-control deficit linked to CLBP. Early-phase RFD is a promising biomarker for identifying fear-driven motor inhibition. These findings suggest that workplace interventions that combine graded exposure with explosive-strength training may effectively reduce the risk of chronic pain.