Parkinson's disease (PD) is a neurodegenerative brain disorder that results in a broad range of disabling motor signs, including akinesia, bradykinesia, tremor, and muscle rigidity. Typically, a clinician will quantify the severity of these motor signs from 0 (normal) to 4 (severe) with a unified Parkinson's disease rating scale (UPDRS, Sec. 3). These subjective assessments, while useful, often vary among clinicians [1], making it challenging to evaluate medication and deep brain stimulation (DBS) therapies in multicenter trials.

Several previous studies have developed biomechanical devices to measure muscle rigidity in human [1–3] and nonhuman primates [4] using motorized actuators that can be selectively programmed to articulate the elbow joint across a range of angles and frequencies. However, correlating the results from these studies with clinical assessments of rigidity prove inconsistent [5]. In addition, force measurements are highly sensitive to the location of...

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