Objective: To quantitatively characterize the anatomical distribution of tremor power in patients with essential tremor (ET) and determine which joint-specific contributions most strongly correlate with Fahn-Tolosa-Marin (FTM) clinical severity scores.

Background: ET is a common movement disorder characterized by a 4 to 11 Hz action tremor. While clinically assessed using the FTM scale, kinematic analysis may objectively differentiate proximal (shoulder and elbow) from distal (wrist) dominance, which is critical for guiding individualized local therapies like botulinum toxin or assistive devices.

Method: Kinematic data were collected from 20 subjects with clinically diagnosed ET. Tremor was recorded using multi-axis torsiometers at the shoulder (2 axes), elbow (1 axis), and wrist (3 axes) in FTM tasks and rated by an expert neurologist. Angular Root-Mean Squared (RMS) values were converted to approximate linear displacement at the hand by multiplying by corresponding lever arms (arc length) from each joint assuming a linear anatomical model. (Gordon et al., 1989)

Results: Across nine static tasks, the highest proximal-to-distal (P/D) ratios, reflecting greater relative involvement of the shoulder and elbow, occurred in flexed positions: “full cup, cup-to-mouth” (ratio: 5.26) and “empty cup, cup-to-mouth” (4.29). The lowest ratios, indicating distal (wrist-dominant) tremor, were found during “pen hovering” (2.96) and “empty cup, arms out-stretched” (2.97). Clinical FTM scores demonstrated the strongest correlations with wrist tremor (Kendall’s τ=0.66, p<0.001) and elbow tremor (τ=0.64, p<0.001), followed by the shoulder (τ=0.56, p<0.001).

In the three dynamic tasks (writing and two spiral drawings), P/D ratios were lower and more consistent (range: 2.13-2.48), suggesting uniform wrist dominance during movement. Correlations with clinical scores were much weaker in the dynamic tasks. The wrist remained the strongest correlate (τ=0.34, p=0.001), followed by the shoulder (τ=0.21, p=0.04), while the elbow’s correlation was weak (τ=0.14, p=0.17).

Conclusion: Wrist tremor is the primary driver of subjective clinical severity ratings. However, specific postures like elbow flexion appear to significantly increase proximal contribution, though calculations in this study may be limited by the simple anatomical modelling assumptions. We suggest that ET management should be tailored to task-specific joint dominance.

References: Gordon CC, Churchill T, Clauser CE, Bradtmiller B, McConville JT, Tebbetts I, Walker RA. Anthropometric survey of US Army personnel: Summary statistics, interim report for 1988. 1989 Mar 1. Excerpts retrieved March 15, 2026, from: https://multisite.eos.ncsu.edu/www-ergocenter-ncsu-edu/wp-content/uploads/sites/18/2016/06/Anthropometric-Detailed-Data-Tables.pdf

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MDS Abstracts - https://www.mdsabstracts.org/abstract/kinematic-characterization-of-upper-limb-tremor-distribution-in-essential-tremor/