A fall is defined as an event in which a person unintentionally comes to a stop on a lower-level surface, most likely the floor [1]. The World Health Organization (WHO) reports that falls are a global health problem since they are common worldwide and often result in major morbidities and/or mortality [1]. According to the WHO, each year 684.000 fatal falls occur and particularly older adults who are over 60 are at an increased risk of falling due to several reasons including impaired balance [1], [2]. Furthermore, falls resulting in major injuries increase healthcare costs substantially. It was found that a single intervention to prevent falls could decrease the number of falls by 45.000 and the medical costs by $442 million annually [3]. Despite the utmost importance of falls, the underlying mechanisms of balance are still poorly understood and the current literature does not yet possess a tool or measure that can completely link balance impairments and fall risk in older adults to underlying mechanisms [4]. Therefore, there is a need for further research on the development of reliable assessment tools and a deeper understanding of the factors that contribute to balance impairment in older adults to effectively reduce fall-related injuries and associated healthcare costs.

In literature, balance is often subdivided into static and dynamic balance. Static balance is defined as the ability to maintain the line of gravity within the base of support against gravity and perturbations [5]. During a dynamic task, for instance, during walking, balance is defined differently, as mobility in a specific direction may require the loss of static balance in that direction. Balance control can be, on the other hand, defined as the mechanism used to maintain balance by moving the center of pressure (CoP) with respect to the vertical projection of the center of mass (CoM), counter-rotating segments around the CoM, and/or applying an external force [6].

Control of both static and dynamic balance requires the integration of inputs from the visual, somatosensory, and vestibular systems in the central nervous system- a process known as sensory integration [7]. The sensory integration process is critical, as the sensory inputs are the essential components that the central nervous system relies on to regulate balance control [8]. Any disruption or alteration in sensory input from these systems can lead to compensatory changes in movement patterns to maintain balance and subsequently to falls [9]. For instance, both younger and older adults present a more cautious pattern with shorter step length and stance phase when walking with eyes closed compared to eyes open walking, though this cautious pattern is more pronounced in older adults, which is linked to increased fall risk [10]. In a similar manner, mastoid vibration (inducing altered vestibular input) caused an increase in the amount of sway variability in walking as well as an increase in rigidity of movement in older adults [11], which highlights the critical role of accurate sensory integration in maintaining balance control, also during dynamic tasks such as walking.

Some tests have been developed to evaluate sensory integration processes (and individual sensory systems) such as the Sensory Organization Test (SOT) and the Clinical Test of Sensory Interaction and Balance (CTSIB), which have been developed with the same principle [12], [13]. The CTSIB is particularly considered for its clinical use to evaluate sensory integration, as it evaluates a person's reliance on the visual, somatosensory and vestibular systems during standing conditions by manipulating the sensory systems [12], [14]. It has six conditions that require a shift of reliance from one sensory system (i.e., modality) to another [12]. However, both CTSIB and SOT only provide information on sensory integration in standing. Given the fact that falls in daily life mostly occur during gait and other dynamic conditions [15], there is an immense need for an additional sensory integration test for balance during locomotion in particular. Although an earlier attempt was made to develop a test to comprehensively assess sensory integration during gait, this effort encountered certain limitations that impacted its overall effectiveness [16]. The Locomotor Sensory Organization Test (LSOT), designed by Chien et al. [16], involved the same number and concept of the conditions as developed for the CTSIB and SOT based on combinations of stable/sway-referenced surface, vision available/unavailable, and visual surrounding stationary/sway-referenced, but transferred these to walking conditions. Chien et al. [16] implemented changes in treadmill speed during walking as a substitute for the sway-referenced surface (i.e., sway-referenced surface in SOT or the compliant foam surface in CTSIB). However, as the authors themselves acknowledged, altering gait speed could influence not only the somatosensory system but also the vestibular system[16]. Additionally, for the sway-referenced visual surrounding condition, they used pseudo-randomized manipulations for the optic flow as a substitute for the sway-referenced visual surrounding in SOT or the visual conflict dome in CTSIB. The aim of the sway-referenced visual surround and the visual dome is to render the visual input unreliable (as the visual input is actually the same despite the postural sway), which is not the same as pseudo-randomized manipulations of optic flow used in the LSOT. As a result, the current LSOT may have some limitations in its ability to provide reliable insights into the functioning of individual sensory systems or the overall sensory integration process.

Therefore, the primary aim of this study is to develop a novel sensory integration test for use during locomotion, named the “Sensory Integration in Walking (SensIWalk) Test”, and to evaluate its validity and reliability in both young and older adults.