Modifying Trunk Inclination and Low Back Curvature Inconsistently Affects Isometric Multijoint Lifting Strength Across Individuals

Movement assessment and coaching within health and human performance settings has targeted trunk inclination and low back curvature to reduce low back injury risk. However, it remains unclear how modifying these postural characteristics may also impact the ability to exert force in multijoint tasks to influence physical performance. This study investigated the independent and interacting effects of trunk inclination and low back curvature on maximal isometric lifting strength. Forty participants (50% female) exerted maximal isometric lifting force in 4 experimentally controlled postural conditions consisting of 2 trunk inclinations (vertical and horizontal), each performed with 2 low back curvatures (neutral and flexed). A linear mixed-effects model revealed a significant group level interaction between trunk inclination and low back curvature (β = 0.13, P = .002), but heterogeneity was high and indicated that some individual responses opposed the estimated group level response (random effect SD = 0.12). Individual responses substantially varied in magnitude (up to 620 N) and direction (increase/decrease). Modifying trunk inclination and low back curvature each have a similar potential to influence strength in multijoint tasks, but the response varies across individuals. Strength in multijoint tasks cannot be inferred solely from posture, and a single postural profile cannot be generalized as the strongest for all individuals.