Axial spondyloarthritis (axSpA) is a chronic inflammatory disease mainly affecting the sacroiliac joints and spine, characterized by inflammation and subsequent bone formation [1,2]. This condition is categorized into two subgroups: radiographic axSpA, in which plain sacroiliac X-rays show structural changes, and non-radiographic axSpA, where these structural alterations are absent [3,4].

The predominant clinical manifestations of axSpA are inflammatory back pain, morning stiffness, fatigue [5], and reduced mobility, particularly in the late stages of the disease, all being indicative of spine inflammation and consequent structural damage [6,7]. These symptoms affect multiple aspects of patients’ lives, impacting their psychological well-being, occupational productivity [8], and overall quality of life [9].

Magnetic Resonance Imaging (MRI) is an essential tool for detecting inflammation in the sacroiliac joints and is part of the ASAS classification criteria for axSpA. However, MRI is not routinely used to assess structural damage in the spine [10]. Evaluating cervical and lumbar mobility is important for identifying patients who may benefit from additional imaging or personalized rehabilitation programs to improve their quality of life.

Monitoring spinal mobility in axSpA patients is important and involves measures such as modified Schober's test and lateral spinal flexion, which have been shown to correlate well with both radiographic damage [11], and disease activity [12].

Recent advancements in the use of Inertial Measurement Unit (IMU) sensor systems have enabled precise quantification of spine mobility, providing objective data on the range of motion in both cervical and lumbar regions [13,14]. IMU system has been validated in axSpA, demonstrating high reliability with strong inter- and intraobserver consistency and significant correlation with patient-reported outcomes (PROs), such as the Bath Ankylosing Spondylitis Functional Index (BASFI) and with the Bath Ankylosing Metrology Index (BASMI) [15,16]. The BASFI, a validated tool for assessing functional limitations in axSpA, consists of ten questions that evaluate the patient's ability to perform specific tasks, with higher scores indicating greater impairment [17].

Most studies assessing mobility in axSpA rely on the BASMI. While BASMI is a valuable tool, it doesn't distinguish between cervical and lumbar mobility, potentially masking critical differences in how these spinal regions affect patients’ quality of life. This can lead to an underestimation of the cervical spine involvement, despite its significant impact on patient functionality. This study aims to fill this gap by hypothesizing that cervical mobility is more vital for axSpA patients than lumbar mobility. Difficulties in moving the neck can significantly impact everyday activities like driving, reading, and social interactions, thereby having a more substantial effect on functional ability [18].

The primary objective of this study was to evaluate how cervical and lumbar mobility, measured by IMUs, influence functional outcomes measured by BASFI in patients with axSpA. Additionally, it aimed to identify which specific spinal movements are linked to function and to determine if these relationships vary between patients with longstanding and non-longstanding diseases.