Effects of trunk exertion force and direction on postural control of the trunk during unstable sitting

Background. Pushing and pulling exertions have been implicated as risk factors of low-back disorders. In an attempt to investigate the mechanisms by which pushing and pulling influence risk for low-back disorders, the goal of this study was to investigate the effects of trunk exertion force and exertion direction on postural control of the trunk during unstable sitting. Methods. Seat movements were recorded while subjects maintained a seated posture on a wobbly chair against different exertion forces (0N, 40N, and 80N) and exertion directions (trunk flexion and extension). Postural control of the trunk was assessed from kinematic variability (root-mean-squared amplitude and 95% ellipse area) and non-linear stability analyses (stability diffusion exponent and maximum finite-time Lyapunov exponent). Findings. Kinematic variability and non-linear stability estimates increased as exertion force increased including root-mean-squared amplitude (P < 0.001), 95% ellipse area (P < 0.001), stability diffusion exponent (P = 0.042), and maximum finite-time Lyapunov exponent (P < 0.001). A subset of measures indicated postural control of the trunk was poorer during flexion exertions compared to extension exertions including root-mean-squared amplitude (P < 0.001), 95% ellipse area (P = 0.046), and maximum finite-time Lyapunov exponent (P = 0.002). Interpretation. Trunk exertion force and exertion direction affect postural control of the trunk. This study may aid in understanding how pushing and pulling exertions can potentially contribute to low-back disorders.