Fundamental Principles of Tremor Propagation in the Upper Limb

Abstract

Although tremor is the most common movement disorder, there exist few effective tremor-suppressing devices, in part because the characteristics of tremor throughout the upper limb are unknown. To clarify, optimally suppressing tremor requires a knowledge of the mechanical origin, propagation, and distribution of tremor throughout the upper limb. Here we present the first systematic investigation of how tremor propagates between the shoulder, elbow, forearm, and wrist. We simulated tremor propagation using a linear, time-invariant, lumped-parameter model relating joint torques and the resulting joint displacements. The model focused on the seven main degrees of freedom from the shoulder to the wrist and included coupled joint inertia, damping, and stiffness. We deliberately implemented a simple model to focus first on the most basic effects. Simulating tremorogenic joint torque as a sinusoidal input, we used the model to establish fundamental principles describing how input parameters (torque location and frequency) and joint impedance (inertia, damping, and stiffness) affect tremor propagation. We expect that the methods and principles presented here will serve as the groundwork for future refining studies to understand the origin, propagation, and distribution of tremor throughout the upper limb in order to enable the future development of optimal tremor-suppressing devices.

Physiological tremor increases when skeletal muscle is shortened: implications for fusimotor control

“…). Thus, a reduction of muscle stiffness with muscle shortening during plantarflexion might explain our results (Davidson & Charles, ).…”

Simulated Tremor Propagation in the Upper Limb: From Muscle Activity to Joint Displacement

“…Thus, the entire model transforms the tremorogenic muscle activity in the 15 major superficial muscles from the shoulder to the wrist into tremulous joint displacement in the 7 major DOF actuated by those muscles. It expands the previous investigation of tremor propagation [41], which focused only on the propagation from joint torques to joint displacements (the third sub-model). Note that the second and third sub-models mix the muscle forces into the various joint torques and joint displacements, thereby propagating the tremor,…”

“…Matrices I, D, and K are 7-by-7 impedance matrices representing inertia, damping, and stiffness, respectively. The default values of I, D, and K were obtained from the literature [50][51][52][53] and are described in detail in [41]. Summarizing, I was calculated from the inertia values of individual limb segments for an average young adult male [53] using the Robotics, Vision and Control (RVC) toolbox in Matlab [54].…”

“…Most of our investigation, however, focused on the steady-state response, which we analyzed as follows. The steady-state response of a linear system to a periodic input is characterized by the frequency response of the system at the frequencies of the input [41,60]. The input, approximated as a train of triangular pulses, contains power at the frequency at which the pulses repeat (i.e.…”

“…We built on prior research by Davidson and Charles that modeled tremor in the upper limb from joint torques to joint displacement [41]. The current work expanded the model to begin with muscle activity, which was required to understand how tremor moves from muscle signals to torques at each DOF.…”