## 1 Introduction  
Human movement has been described as a complex activity with a multifactorial nature given its diverse involvement with the context [1]. The interaction between different sensorimotor, perceptual, and cognitive systems plays an important role in functional movement [2]. This interconnection, coordinated by the central nervous system (CNS), ensures motor responses with adequate stability and mobility. The nature of the interactions between posture and movement is an area of interest in movement neuroscience [1]. To ensure efficient movement, it is a prerequisite that the underlying postural control components, namely muscle tone, are intact [1; 3]. Traditionally, muscle tone has been defined as \"the tension in the relaxed muscle\" [4; 5] and the resistance to passive stretching [4; 6]. It has therefore been defined as the resistance felt by the examiner when passively mobilizing a body segment [7], which is why it is commonly assessed, in a clinical context, in the extremities as a rapid and short-term response [2]. However, this definition refers to the phasic classification of muscle tone [8]. It should be considered that the postural classification of muscle tone is often associated with antigravity support through tonic activation, which provides a specific postural attitude and generates force against gravity [1; 4]. This tonic activation, together with continuous minimal adjustments, allows the maintenance of a postural set such as sitting or standing [1; 2; 9] and facilitates the preparation and execution of movements, such as sit-to-stand or stand-to-sit, with adequate stability and motor control [10; 11; 12]. Without adequate muscle tone, motor functions and the ability to change position in space can be severely compromised [11; 12]. Therefore, this assumption should also be considered when evaluating tone. Thus, the assessment of muscle tone is a crucial aspect in the rehabilitation of adults with CNS disorders, as these conditions frequently result in significant alterations in muscle tone, which may manifest as muscle hypertonicity, rigidity, and dystonia [13]. These alterations may negatively impact on the ability to perform functional activities with repercussions on daily life functional tasks, patient quality of life, and health-related quality of life [14]. Therefore, accurate assessment of muscle tone is of paramount importance to set appropriate treatment goals for the diagnosis of the extent of motor impairment, the monitoring of the progression of neurological conditions, and the evaluation of the efficacy of therapeutic interventions [15; 16]. Nevertheless, during the physical examination, despite its contribution to clinically relevant information, the concept of what is being assessed may not correspond unambiguously to what is intended [2; 8]. It has been demonstrated that traditional clinical measures, such as the Modified Ashworth Scale (MAS), are not sufficiently reliable or valid for use in clinical practice, highlighting the need for more advanced and objective assessment instruments [2; 17]. Furthermore, the use of different nomenclature for the purpose of assessment, such as rigidity, hypertonia, or spasticity, reflects a lack of consensus regarding the presumed underlying pathophysiology [15; 16; 18]. While traditional methods of tone assessment are widely used, they often rely on subjective measures and manual examination, which can lead to inconsistencies in inter-rater and intra-rater reliability [19]. Instruments such as the MAS and the Tardieu Scale provide a semi-quantitative measure of resistance to passive movement, but they fail to capture the complex nature of muscle tone alterations comprehensively [19; 20]. These limitations highlight the pressing need for more sophisticated and standardised clinical measures that can offer precise, reliable, and objective data as well as comparable evaluations of different CNS clinical conditions across rehabilitation.