Background: Treatment options for essential (ET) and Parkinson disease (PD) tremor are suboptimal, with significant side effects. Botulinum toxin type A (BoNT A) is successfully used in management of various focal movement disorders but is not widely used for tremor. Method: This study examines complexity of wrist tremor in terms of involvement of its three anatomical degrees of freedom (DOF) in two common situations of rest and posture. The study examines tremor in 11 ET and 17 PD participants by kinematic decomposition of motion in 3-DOF. Results: Tremor decomposition showed the motion involved more than one DOF (<70% contribution in one DOF) in most ET (rest: 100%, posture: 64%) and PD (rest: 77%, posture: 77%) patients. Task variation resulted in change in both amplitude and composition in ET, but not in PD. Amplitude significantly increased from rest to posture in ET. Directional bias was observed at the wrist for ET (pronation), and PD (extension, ulnar deviation, pronation). Average agreement between clinical visual and kinematic selection of muscles was 55% across all subjects. Conclusion: This study shows the complexity of tremor and the difficulty in visual judgment of tremor, which may be key to the success of targeted focal treatments such as BoNT A
