Hand and distal joint tremor are most coherent with the activity of elbow flexors and wrist extensors in persons with essential tremor.

Essential tremor (ET) affects millions of people. Although frontline treatment options (medication, deep brain stimulation, and focused ultrasound ablation) have provided significant relief, many patients are unsatisfied with the outcomes. Peripheral suppression techniques, such as injections of botulinum toxin or sensory electrical stimulation of muscles, are gaining popularity, but could be optimized if the muscles most responsible for a patient's tremor were identified. The purpose of this study was to quantify the relationship between the activity in various upper limb muscles and the resulting tremor in patients with ET. Surface electromyogram (sEMG) from the 15 major superficial muscles of the upper limb and displacement of the hand and upper limb joints were recorded from 22 persons with ET while they performed kinetic and postural tasks representative of activities of daily living. We calculated the peak coherence (frequency-dependent correlation) in the tremor band (4-8 Hz) between the sEMG of each muscle and the displacement in each major degree of freedom (DOF). Averaged across subjects with ET, the highest coherence was found between elbow flexors (particularly biceps brachii and brachioradialis) and the distal DOF (forearm, wrist, and hand motion), and between wrist extensors (extensor carpi radialis and ulnaris) and the same distal DOF. These coherence values represent the upper bound on the proportion of the tremor caused by each muscle. We conclude that, without further information, elbow flexors and wrist extensors should be among the first muscles considered for peripheral suppression techniques in persons with ET.NEW & NOTEWORTHY We characterized the relationships between activity in upper limb muscles and tremor in persons with essential tremor using coherence, which provides an upper bound on the proportion of the tremor due to each muscle. Averaged across subjects and various tasks, tremor in the hand and distal joints was most coherent with elbow flexors and wrist extensors. We conclude that, without further information, these muscle groups should be among the first considered for peripheral suppression techniques.

Essential Tremor accentuates the pattern of tremor-band coherence between upper-limb muscles.

Although Essential Tremor is one of the most common movement disorders, current treatment options are relatively limited. Peripheral tremor suppression methods have shown potential, but we do not currently know which muscles are most responsible for patients' tremor, making it difficult to optimize suppression methods. The purpose of this study was to quantify the relationships between the tremorogenic activity in muscles throughout the upper limb. Muscle activity was recorded from the 15 major superficial upper-limb muscles in 24 subjects with Essential Tremor while they held various postures or made upper-limb movements. We calculated the coherence in the tremor band (4-12 Hz) between the activity of all muscle pairs and the time-varying phase difference between sufficiently coherent muscle pairs. Overall, the observed pattern somewhat mirrored functional relationships: agonistic muscle pairs were most coherent and in phase, whereas antagonist and unrelated muscle pairs exhibited less coherence and were either consistently in phase, consistently antiphase, consistently out of phase (unrelated pairs only), or else inconsistent. Patients exhibited significantly more coherence than control subjects (p<0.001) in the vast majority of muscle pairs (95 out of 105). Furthermore, differences between patients and controls were most pronounced among agonists; thus, the coherence pattern existing in control subjects was accentuated in patients with ET. We conclude that tremor-band activity is broadly distributed among the muscles of the upper limb, challenging efforts to determine which muscles are most responsible for a patient's tremor.

Distribution of tremorogenic activity among the major superficial muscles of the upper limb in persons with Essential tremor.

OBJECTIVE: Peripheral tremor suppression has the potential to reduce tremor, but we do not currently know where best to intervene. The purpose of this study was to characterize the distribution of tremorogenic activity among upper-limb muscles. METHODS: Surface electromyography was recorded from the 15 major superficial muscles of the upper limb while 25 patients with Essential Tremor performed postural and kinetic tasks. We defined tremorogenic activity as power in the tremor band (4-8 Hz) and determined the distribution of this power among muscles. RESULTS: The distribution varied considerably between patients (mean r = 0.58), but on average, the greatest power was found in the anterior deltoid and extensor carpi ulnaris muscles. Other muscles with high power included the extensor carpi radialis, pectoralis major, lateral deltoid, and brachialis muscles. This distribution was similar (mean r ≥ 0.88) for postural and kinetic tremor, various limb configurations, repetitions, and patient characteristics (sex, tremor severity, age of onset, and duration). CONCLUSIONS: We identified a rough pattern in which muscles opposing gravity appeared to have the highest tremor-band power; we hypothesize that the distribution of tremorogenic muscle activity depends in part on the distribution of voluntary activity required by the task. SIGNIFICANCE: Understanding which muscles exhibit the most tremorogenic activity is one of the steps in the pursuit of optimizing peripheral tremor suppression.

Distribution of tremor among the major degrees of freedom of the upper limb in subjects with Essential Tremor.

OBJECTIVE: Although Essential Tremor is one of the most common movement disorders, we do not currently know which muscles are most responsible for tremor. Determining this requires multiple steps, one of which is characterizing the distribution of tremor among the degrees of freedom (DOF) of the upper limb. METHODS: Upper-limb motion was recorded while 22 subjects with ET performed postural and kinetic tasks involving a variety of limb configurations. We calculated the mean distribution of tremor among the seven DOF from the shoulder to the wrist, as well as the effect of limb configuration, repetition, and subject characteristics (sex, tremor onset, duration, and severity) on the distribution. RESULTS: On average, kinetic tremor was greatest in forearm pronation-supination and wrist flexion-extension, intermediate in shoulder internal-external rotation and wrist radial-ulnar deviation and then shoulder flexion-extension and elbow flexion-extension, and least in shoulder abduction-adduction. The average distribution of postural tremor was similar except for forearm pronation-supination, which played a smaller role than in kinetic tremor. Limb configuration and subject characteristics did significantly affect tremor, but practically only in forearm pronation-supination and wrist flexion-extension. There were no significant differences between repetitions, indicating that the distribution was consistent over the duration of the experiment. CONCLUSIONS: This paper presents a thorough characterization of tremor distribution from the shoulder to the wrist. SIGNIFICANCE: Understanding which DOF exhibit the most tremor may lead to more targeted peripheral tremor suppression.

Toward quantitative characterization of essential tremor for future tremor suppression.

Tremor is the most common movement deficit and manifests in a variety of disorders, including Essential Tremor, Parkinson's Disease, Dystonia, and Cerebellar Ataxia. Although medication and surgical interventions have significantly reduced patient suffering, they are only partially effective and can carry undesired side effects, leaving many patients without satisfactory treatment options. Wearable tremor-suppressing devices could provide an alternative to medication and surgery. Multiple research groups have developed orthotic prototypes to low-pass filter tremor, but these devices have not yet been optimized for in-vivo use. Optimizing non-invasive tremor suppression requires an understanding of where the tremor originates mechanically (which muscles) and how it propagates to the hand (where it matters most). Here we present on the beginnings of our multi-pronged work to determine the origin, propagation, and distribution of Essential Tremor, and we provide preliminary results.