## 1 Introduction  
Postural transitions (PTs) are the movements that occur during the progression from one posture to another, such as from sitting to standing. Considering a posture as a dynamically stable condition, PTs appear critical since they describe the evolution between two different postures, and therefore involve important kinematic and dynamic transitions [1], unlike postural shifts, which happen as movements or postural adjustments around what could be considered the same stability condition [2]. PTs are one of the essential conditions for performing activities of daily living. Their assessment enables gathering valuable functional information, especially about the state of the movement effector system [3], whether musculoskeletal, nervous, or cognitive. In particular, two aspects are fundamental during the functional assessment of patients with complex rehabilitative needs in a clinical setting: the ability of the subject to perform these movements safely without supervision or with help, and in a time valuable for the performance of the task. Many clinical outcome scales and questionnaires contain the assessment of a single or, at most, a couple of PTs among their items [4; 5; 6]. Among these, those of main rehabilitation interest, according to our clinical expertise and preliminary data, and also the most widely used for the functional assessment of patients in clinical practice, are sit to stand (SiSt), stand to sit (SiSi), sit to supine (SiSu), supine to sit (SuSi), and roll (Roll). These five PTsrepresent excellent simulation of challenging activities for static and dynamic balance, as well as the possible postural variations required during the performance of activities of daily living (ADLs). These five PTs can be considered fundamental transitions that often occur, including in combination, in more complex tasks. Therefore, the main advantage is that with these five PTs, we can assess a patient's ability to perform complex movements in their environment. Limitations in accomplishing or the inability to accomplish one or more of these PTs could severely affect the lives of patients in all ICF (International Classification of Functioning, Disability and Health) care settings [7]. Different technologies have been developed and applied to the assessment of movement [8; 9; 10; 11; 12] and thus also to postural transitions [13; 14]. These include the use of accelerometry, particularly that implemented in IMUs (Inertial Measurement Units), which enables reliable analysis and simple, non-intrusive, on-subject application with a relatively low cost of production and use. These types of sensors are widely found in the literature, with the set-ups differing by the number of devices used, with single [15] or multiple sensor applications [16], and the different positioning strategies and locations on the body of the analyzed subjects (e.g., head, sternum, spine, or lower limbs) depending on the aim of the analysis and in general on the assessment conditions [17; 18; 19]. Due to its ease of use and impedance in providing result data, IMU-based technology lends itself to motion analysis for multiple purposes, such as gait analysis [20], the instrumentation of field tests [21; 22], and monitoring ADLs or subject performance [23]. However, the sensor location strongly affects the quality of the raw signal and may alter the natural movement performed by the subject. Therefore, different acquisition protocols are typically designed to evaluate different tasks, optimizing the sensor location to minimize its possible drawbacks [24]. While this approach surely improves the quality of the signals, it is also poorly suitable for those applications oriented toward continuous monitoring or multi-task assessment. In these cases, the protocol should foresee a fixed positioning of the sensor, which favors the possibility of the subject performing all the required movements over the data quality. Important interventions may be required in the data post-processing to overcome the consequently introduced technical limitations.