## 1 Introduction  
Postural control is considered a complex motor skill that integrates postural equilibrium and postural orientation. It results from the interaction of multiple and dynamic sensorimotor processes, somatosensory, vestibular, visual, and neuro-musculoskeletal systems, necessary to maintain an appropriate balance and perform different tasks [1]. Evidence suggests that postural control depends on attentional resources beyond automatic processes; these attentional requirements can depend on age, postural task, nature of the cognitive task, and balance skills [2]. In daily life, we are constantly performing several tasks simultaneously. When performing two or more tasks, the attention is divided between both tasks, which results in performance declines in one or both tasks (dual-task interference). The dual-task paradigm is an approach used in various studies to assess the relationship between attention and postural control [2]. There are three theories commonly used to explain the dual-task interference or the limitation of attentional resources while performing a dual-task: capacity sharing, bottlenecks or task switching, and cross-talk [3]; however, there is no consensus about the underlying mechanisms of the dual-task interference [4]. Postural control has been evaluated under the dual-task paradigm to understand the role of cognitive processes; most studies assessed static or dynamic postural control as a primary task and cognitive task as a secondary task [2; 5; 6; 7]. Keeping balance during a static standing posture while performing other tasks is practiced regularly on a daily basis. Maintaining an upright stance appears to be practically automatic without requiring attention; however, prior studies showed that postural control during standing posture is influenced by cognitive tasks performed simultaneously [8; 9]. Previous studies reported divergent results in the influence of cognitive tasks on postural control while performing a static standing posture in young adults. For example, some studies revealed that the postural sway was reduced in dual-task conditions compared to the standing postural (single-task) [10; 11; 12]. Conversely, others indicated greater postural sway in dual-task conditions compared to the single-task [13]. As available evidence suggests that postural control depends on attentional resources beyond automatic processes to complement and help understand these controversial results about the influence of the dual-task on postural control, it becomes relevant to analyze the cortical activation during the execution of postural motor tasks. Brain activity analysis during static and dynamic postural control tasks has emerged from neuroimaging studies using functional magnetic resonance imaging (fMRI) [14], electroencephalography (EEG) [15], positron-emission tomography (PET) [16; 17], and functional near-infrared spectroscopy (fNIRS) [18]. The fMIR, fNIR, and PET are neuroimaging techniques that depend on neurovascular coupling, while EEG detects the brain's electrical activity [19]. The fMIR and PET record brain activity in all cerebral regions while performing motor imagery or virtual reality tasks [18; 20], so they cannot be used during natural tasks, such as walking or standing while performing other tasks in real-time. The fNIRS has some advantages compared to other neuroimaging techniques, such as portability, motion tolerance, and low cost [19]. It is an optical neuroimaging technique that is based on hemodynamic responses of neuronal cortical tissues, measuring changes in oxygenated and deoxygenated hemoglobin concentrations in the active brain regions [21; 22]. The prefrontal cortex has an essential role in various brain functions, such as memory, attention, executive function, other cognitive functions [23], and cognitive postural dual-task performance [24]. Previous studies that assessed prefrontal cortex activation during dual-task conditions found an increase in brain activity in the prefrontal cortex during dual-task compared to postural single-task [25].