Rotational Power: A New Accelerometer-Derived Metric to Assess Functional Impairment in Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease that affects sensorimotor function, particularly in the lower limbs, leading to increased fatigue and impaired mobility. The 30-second chair stand test (30CST) is a functional assessment that captures muscular strength and endurance in people with MS (PwMS). This study introduces Rotational Power (RP), which is a new metric derived from a single thigh-worn accelerometer, that can serve as a surrogate for body-mass normalized mechanical power during sit-to-stand (SI-ST) and stand-to-sit (ST-SI) transitions. We validate the metric both analytically and clinically in seventeen PwMS who performed a 30CST while instrumented with an accelerometer and a motion-capture system for reference. Analytical validation demonstrated a strong correlation with peak mechanical power for both SI-ST (r = 0.79) and ST-SI (r = 0.60). Clinical validation revealed strong-to-moderate significant associations between the $95^{\text {th}}$ percentile of peak RP computed across the 30CST and established clinical measures, including the number of repetitions, activity specific balance confidence scale, and modified fatigue impact scale. The same analysis performed on the motion-capture mechanical power showed similar results and concordance in correlation direction. Analysis of acute fatigue during the 30CST showed a significant decline in RP during the task, particularly for concentric transitions. Findings suggest that RP is a valid and practical metric for monitoring functional performance and acute fatigue in PwMS using a single wearable sensor, calling for its future use in free-living settings.