## 1 Introduction  
Postural stability deterioration is a risk factor in older people falling [1; 2; 3; 4]. Because falling is a major cause of morbidity and mortality in -older people [4], improving postural stability is important. Therefore, simple and effective postural stability assessment methods are highly valuable and should be developed.  
Postural stability is closely related to the interaction between weight force and ground reaction force (GRF). The weight force acts at the center of mass (COM) of the human body, and the GRF acts at the center of pressure (COP), which is the point at which the resultant GRF is applied below the body. To maintain postural stability, the vertical projection of the COM must remain within the polygon of support. To this end, the postural control system must generate stabilizing moments to regulate the COM by adaptively changing the position of the COP [5; 6]. Consequently, many postural stability measures have been developed using the trajectories of the COM and COP [7; 8; 9; 10; 11; 12; 13]. Such development has been particularly active for the COP because it can be easily measured using force plates. In fact, review papers have indicated that COP features have been employed in approximately 60% of published studies on postural control [14; 15]. In addition, COP features have also been extensively employed to assess the effects on posture stability from various health-related conditions, such as aging [13; 16], peripheral neuropathy [17], musculoskeletal disorders [18], stroke [19; 20], spinal cord injury [21], concussion [22], cancer [23], frailty syndrome [24], symptomatic degenerative lumbar disease [25], Parkinson's disease [26; 27], multiple sclerosis [28; 29], and high fall risk [30]. Interested readers are referred to the review papers by [PERSON] et al. [31] and [PERSON] et al. [32] for the background and utility of posturography.  
As detailed in the Methods section, the number of those GRF components (measured using force plates) that are associated with the COP is higher than the degrees of freedom (DOFs) of the COP. Therefore, there necessarily exists an infinite number of GRF component combinations that can satisfy a given COP position requirement. This phenomenon is similar to the motor redundancy problem formulated by [PERSON] [33]. As an important problem in motor control, motor redundancy occurs when the number of motor control task constraints is lower than the number of control variables of the motor control system. Therefore, the motor control system must incorporate strategies for coordinating these redundant DOFs. Possible strategies include: first, simplifying the complexity of the motor control task by synchronizing the DOFs of the motor control system [33; 34]; second, using the redundant DOFs to optimize the performance of the control task [35], and; third, assisting the operation of secondary tasks [36]. However, to the best of our knowledge, the redundancy problem has never been systematically studied for the COP.  
To investigate the potential utility of such redundancy to improve postural stability, instead of relying only on information extracted from the COP, we develop new postural stability features by quantifying the coupling strengths between the COP and its GRF components. In addition to developing new postural stability measures, we hope that the experimental results will elucidate how our postural control system utilizes the redundant GRF components to assist balance control.