With recent advances in the accuracy of ‘wearable sensors’ they are now widely used in mobile phones, watches and other wearable devices. This technology has been tried in patients with low back pain but has not yet been used in patients with AS. The standard method of measuring spinal mobility using a tape measure is known as the Bath AS Metrology Index (BASMI). Unfortunately, it is not accurate enough to evaluate new treatments for AS and it cannot be used in the home setting. A more accurate ‘motion-tracking’ method uses a set of cameras to measure movement in a ‘gait laboratory’. Tests with one of these setups (UCOTrack) showed that it was more accurate and reliable than BASMI, and it was also better able to show changes with treatment. MRI scans of the spine can detect changes in inflammation before and after treatment but again this is too expensive to be widely used. Previous studies showed that changes in the BASMI didn’t match the changes seen on MRI, so part of our study will be to compare the MRI changes with changes in the ‘sensor mobility index’ (we’re calling it IMU-ASMI for now).