# Balance Control in Obese Subjects during Quiet Stance: A State-of-the Art
## 1 Introduction  
Postural stability can be explained as the ability of one's motor control to maintain the standing posture, even during external perturbations. It is often described as static (quiet standing) or dynamic (maintaining a stable position while undertake a prescribed movement) [1]. Postural stability is achieved when a subject can regain equilibrium after being disturbed. Postural control requires the integration of inputs from various sensory systems, which are: (a) the proprioceptive system (receptors in joints, muscles, tendons, skin) [2], (b) the visual system [3] and (c) the vestibular system (semicircular canals and macular otoliths) [4]. Sensorimotor integration and balance regulation rely on the cerebellum; most of the evidence of the function of the cerebellum in humans comes from patients with cerebellar damage who display balance abnormalities and gait ataxia [5]. This multimodal (integrating visual, vestibular, and proprioceptive inputs) and redundant control causes chaotic center of mass (CoM) oscillations known as postural sway [6; 7; 8]. In clinics the quality and integrity of balance control is generally assessed using various measures of postural sway [6]. Instrumental evaluation with platform stabilometry or static posturography is a technique where CoM fluctuations are represented by the center of foot pressure (CoP) displacements [9] and it consists in the measurement of forces exerted against a force platform during quiet stance. This method is widely used in clinical settings to evaluate the integrity of the postural control system and to obtain functional markers on fine competencies and their development in different testing conditions (i.e., eyes open vs. eyes closed, feet position, and presence of external stimuli) [10]. Commonly, the study of properties of the CoP trajectory in clinics is performed using traditional time series, in particular analyzing length, displacement excursion, velocity and frequency analysis. The main advantages of these analysis techniques are the simplicity of the experimental set-ups and the safety for the subjects evaluated, aparticularly important consideration in pathological conditions [11]. However, some limitations are present: (a) the lack of a normal pattern due to intra- and inter-individuals variability; (b) the difficulties inherent to standardizing the measurement experimental set-up (reproducibility of the experimental protocol, environmental conditions, random errors, signal processing); and (c) the presence of highly coupled parameters measured by means of the force platform [12; 13]. Due to the presence of these limitations, it is necessary to have reliable methods available to extract physiologically significant information from the platform data [11; 14]. Alongside the study of properties of the CoP trajectory using traditional time series, some advanced mathematical methods have been developed applying a dynamic approach, such as entropy and fractal dimension (FD) analysis [11; 15; 16]. Even if previous studies [14; 17; 18] suggested that these methods could represent reliable techniques complementary to the analysis in time and frequency domain in order to evaluate postural control, their use in clinical setting is not yet widespread. Balance is a fundamental element in carrying out daily life activities. Aging and various pathologies increase the postural instability which can lead to falls [19]. In particular, some anthropometric measurements (height and weight) are demonstrated to significantly influence both postural sway and postural stability [12; 20; 21]. The excessive presence of fat adds mass to different regions and modifies the body geometry [22]: in this way the altered CoM position leads to balance impairments [23]. It is estimated that in 2016, more than 1.9 billion adults aged 18 years and older were overweight; of these over 650 million were obese [24]. A recent paper suggests that by 2030 nearly one in two adults in the United States will be obese and nearly one in four adults will have severe obesity [25].