A Novel Method for Quantifying Contributions from Individual Muscles to Tremor: A Methodological Feasibility Study of Wrist Tremor.

[PURPOSE] Tremor is the most common movement disorder, and many individuals remain unsatisfied with outcomes from standard treatments, which intervene at the level of the central nervous system. Peripheral suppression methods, including botulinum toxin A injections, show promise but could be optimized by identifying the muscles most responsible for tremor. This study aimed to demonstrate the feasibility of a novel method for quantifying the contribution of individual wrist muscles to hand tremor.

[METHODS] Surface electromyography from the four prime wrist muscles and motion capture of hand translation were recorded in 14 individuals with Essential Tremor. Subjects attempted to maintain a neutral wrist posture while the upper limb was constrained to isolate the wrist joint. Contributions of individual muscles to hand tremor were determined using excluded coherence (a novel measure capable of estimating input contributions in multi-input systems with correlated inputs) between muscle activity and hand motion.

[RESULTS] We found many statistically significant differences in tremor contributions between muscles. Across subjects, the flexor carpi radialis contributed the most to constrained wrist tremor, followed by the flexor carpi ulnaris and extensor carpi radialis. However, the dominant contributing muscles varied between individuals. Most tremor power was caused by direct contributions from individual muscles, not constructive interference between muscles. Additionally, we found that although suppressing certain muscles could theoretically increase tremor (because they interfere destructively with other muscles), this is unlikely to occur.

[CONCLUSION] This is the first quantification of muscle-specific contributions to tremor in multiple subjects. Excluded coherence analysis provides an effective tool for calculating these contributions.