Inertial measurement unit (IMU) sensors are valid and reliable in the assessment of spinal mobility in patients with axial spondyloarthritis (axSpA), according to study results published in Rheumatology.

Investigators conducted a repeated measurement study with a convenience sample of adult patients with axSpA who met the Assessment of SpondyloArthritis International Society classification criteria. Patients were approached for participation during routine clinic visits or physiotherapy sessions, with data from those patients with a history of spinal/hip surgery, spinal fracture, or major scoliosis deformity excluded from the study. Demographic and clinical factors were extracted at baseline. Measurements were performed on separate days, 1 to 2 weeks apart. At each timepoint, 3 measurement trials were performed: 2 by the same physiotherapist and 1 by a physiotherapist blinded to the results of prior measurements. Pairs of IMU sensors were placed at the head, neck, and lumbar spine; the maximum range of motion (ROM) was assessed over a series of movements. The ROM was expressed as the maximum angle of sensor movement. A composite IMU score was calculated by normalizing ROM angles to global indices of movement and then calculating the mean of normalized scores for each region. Patients also received conventional motion testing, according to the Bath Ankylosing Spondylitis Metrology Index (BASMI). The agreement of IMU measures with conventional BASMI tests was assessed. Intra-rater, inter-rater, and test-retest intra-class correlation coefficients (ICCs) for reliability were calculated for each IMU sensor measurement score.

The study cohort included 40 participants, among whom 29 (72.5%) were men. Mean age at enrollment was 48 years, with mean disease duration of 13 years. The composite IMU score was closely correlated with the BASMI (r=0.88). Results from conventional cervical rotation and lateral flexion tests were also strongly correlated with their IMU equivalents (r=0.85 and 0.84, respectively). According to ICC calculations, the reliability of both conventional and IMU tests ranged from good to excellent. Test-retest ICCs for conventional movement vs IMU tests varied from 0.57 to 0.91 vs 0.74 to 0.98, respectively. Inter-rater and intra-rater ICCs were particularly high for cervical IMU measurements; ICCs for lumbar spine were also high. Each of the composite regional IMU scores had excellent test-retest reliability (ICCs, 0.94-0.97) and were comparable to the reliability of the BASMI (ICC, 0.96). The conversion of raw movement angles to normalized scales did not appear to affect test-retest reliability.

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Overall, the wearable IMU sensors displayed excellent validity and reliability in the measurement of cervical and lumbar spine mobility. As such, a composite IMU-based metrology index appears to be a valid outcome measure for axSpA spinal mobility.

Study limitations included the small sample size and the large age range of participants.

The normalized IMU scores may be more informative if derived from a “range of movements [in] a larger, age-adjusted…population,” the investigators wrote. Even so, these preliminary data support the larger application of IMU-based metrology in clinical practice.

Disclosure: Study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.


Gardiner PV, Small D, Muñoz-Esquivel K, Condell J, Cuesta-Vargas A, Williams J, et al. Validity and reliability of a sensor-based electronic spinal mobility index for axial spondyloarthritis [published online April 28, 2020. Rheumatology (Oxford). doi:10.1093/rheumatology/keaa122