Local Dynamic Stability of the Lower Limb Kinematic Chain during Symmetric Lifting

During goal-directed tasks, such as lifting low-lying objects, the human motor system regulates the body’s centre of mass (CoM) position to maintain equilibrium and prevent falling [1]. During lifting, any voluntary trunk movement will perturb that equilibrium, which will elicit an automatic postural response requiring the coordinated control of several joints [2]. To actively counter perturbations and smoothly incorporate them into movement repertoires while walking, the central nervous system prioritizes the stability of the superior over the inferior segments [3]; however, this relationship is unknown for lifting. Therefore, we examined ankle, knee, and hip joint stability during repetitive lifting under two load conditions. Because the active control of trunk motion is crucial in lifting as in walking, we hypothesized that local dynamic stability would increase when moving up the kinematic chain [3]. Furthermore, because local dynamic spinal stability increases with an increase in load lifted [4], we hypothesized that stability would increase at all joints with increased load.