Event Title

Dynamics of Wrist and Forearm Rotation in 3DOF

Presenter Information

Allan PeadenFollow

Description

Healthy upper limb movement generally involves multiple degrees of freedom moving in synchrony. Although these movements often appear simple, there are complex underlying forces required to give them their grace. Understanding these forces will both aid in developing our understanding of human motor control, and provide insight for the design of ergonomic and therapeutic devices. One cause for the complexity of these forces is coupling between the various degrees of freedom (DOF), where a displacement or motion in one DOF will induce a force in another. Coupling therefore requires applied compensatory forces in joints not directly involved in an intended movement. The wrist joint has been shown to be coupled primarily due to its stiffness, although this has only been considered for a two DOF scenario involving flexion-extension (FE) and radial-ulnar deviation (RUD). Building off of this foundation, the wrist and forearm were considered together in a three DOF model to determine the presence and significance of interaction torques on a broader scale which included pronation-supination (PS). First, a dynamic model of the wrist and forearm was derived in which they were modeled as a universal joint with intersecting axis. Model parameters were acquired from published literature except in the case of stiffness. Stiffness parameters were obtained experimentally by moving the wrist and forearm through a set of equally spaced points in FE-RUD-PS space while measuring the passive force applied. Kinematic data were then acquired from all subjects as they moved their wrist and forearm in pure PS, FE, and RUD, as well as every possible combination of these DOFs combined. When the kinematic data was applied to the model, the following observations were made: 1) that stiffness and gravitational forces dominate wrist movement and 2) that there is a substantial amount of coupling, particularly due to joint stiffness and damping. It is also noteworthy that the model may be drastically simplified while incurring only minor reductions in accuracy.

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Dynamics of Wrist and Forearm Rotation in 3DOF

Healthy upper limb movement generally involves multiple degrees of freedom moving in synchrony. Although these movements often appear simple, there are complex underlying forces required to give them their grace. Understanding these forces will both aid in developing our understanding of human motor control, and provide insight for the design of ergonomic and therapeutic devices. One cause for the complexity of these forces is coupling between the various degrees of freedom (DOF), where a displacement or motion in one DOF will induce a force in another. Coupling therefore requires applied compensatory forces in joints not directly involved in an intended movement. The wrist joint has been shown to be coupled primarily due to its stiffness, although this has only been considered for a two DOF scenario involving flexion-extension (FE) and radial-ulnar deviation (RUD). Building off of this foundation, the wrist and forearm were considered together in a three DOF model to determine the presence and significance of interaction torques on a broader scale which included pronation-supination (PS). First, a dynamic model of the wrist and forearm was derived in which they were modeled as a universal joint with intersecting axis. Model parameters were acquired from published literature except in the case of stiffness. Stiffness parameters were obtained experimentally by moving the wrist and forearm through a set of equally spaced points in FE-RUD-PS space while measuring the passive force applied. Kinematic data were then acquired from all subjects as they moved their wrist and forearm in pure PS, FE, and RUD, as well as every possible combination of these DOFs combined. When the kinematic data was applied to the model, the following observations were made: 1) that stiffness and gravitational forces dominate wrist movement and 2) that there is a substantial amount of coupling, particularly due to joint stiffness and damping. It is also noteworthy that the model may be drastically simplified while incurring only minor reductions in accuracy.