The Influence of Unloading Reflexes on Arm Position: Considerations for the Control of Robotic Assistive Devices

P.S. Archambault (Canada)


Arm kinematics, reflexes, electromyography, unloading, assistive technology.


Reflexes are an integral part of voluntary movement. Their activity becomes obvious in situations were movement does not unfold as planned, for example after an unexpected change in external load. However, the factors influencing reflexive modifications in joint forces and kinematics are not clearly understood. This is of importance for the control of robotic devices such as exoskeletons, which rely on force or muscle activity feedback to assist movement. In this study, the effects on hand position following a decrease in load were examined. Subjects maintained an initial load produced by a two-joint manipulandum. The load was suddenly decreased, either in one step or in two successive steps with different time intervals, resulting in a rapid reflex change in arm position. It was found that the final hand position differed for single versus double-step unloading. Further, this difference was inversely proportional to the hand velocity at the time of the second change in load. Thus, both the change in load and the velocity at the time of the change may influence the magnitude of the unloading reflex. It may be possible to predict this behaviour using linear models based on force and velocity, for the control of robotic assistive devices.

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