Vector coding reveals the underlying balance control strategies used by humans during translational perturbation

Abstract Postural control research has mainly focused on standing balance experiments on a platform moving with relatively large amplitudes (0.1-0.2 m). To broaden knowledge about postural coordination, this study investigated balance strategies while standing on a platform moving in the anterior-posterior direction and whose frequency was scaled linearly from 0.4-6 Hz at a small amplitude of 2 mm peak. The coordination between hip, knee, and ankle joint torque, and centre of mass (COM) and centre of pressure (COP) motion was quantified by vector coding (VC) method to define the local and global strategy, respectively. VC revealed that at lower frequencies, an ankle strategy was recruited which induced an in-phase COPCOM motion with COP dominancy. As platform frequency reached a critical value, the transition frequency, COM dominated over COP and an anti-phase torque occurred between the knee and ankle, called a knee strategy. The knee strategy related to an anti-phase motion between the COP and COM and allowed the COP to regain its dominance over COM. Collectively, this work revealed the knee strategy as a new balance control strategy relevant in real-life settings, and a transition between ankle and knee strategies that underpinned a transition between COP-COM relative motion.