Inaccurate load compensation force during fast goal-directed elbow flexion

Inaccurate load compensation force during fast goal-directed elbow flexion

Everling, M.

Abbink, D.A. (mentor)
Schorsch, J.F. (mentor)

Mechanical, Maritime and Materials Engineering

BioMechanical Design

2016-12-22

Wearable robotic lifting aids compensate a proportion of the gravity during heavy lifting and can thereby reduce the burden on the operator’s musculoskeletal system. This study investigates the effect of errors in the compensation force of an inaccurate load compensation system on fast reaching movements of an operator. In an experimental study, subjects (n=18) held a 2.7 [kg] object while connected to a haptic manipulator that largely compensated the load by a constant force of 23 [N]. Subjects were are asked to perform elbow-flexion movements against gravity towards a target, either 30 or 50° away in a Fitts’ like task. The experiment consisted of two parts: In Part A the independent measure was a noise over the compensation force (a filtered Gaussian white noise with variances of either 1 or 1.5 [N], over a set of trials). In Part B an estimation error was added as a second independent measure (a sudden overestimate or underestimate of the compensation force of 5 [N] added as a step in the middle of catch trials). Dependent metrics used are the rise time, settling time, first peak error and the agonist-antagonist muscle co-activation. It was hypothesized that subjects would show increased settling times for both error types. In Part A it was further hypothesized that subjects would react to the noise by intentionally moving slower (increased rise time) and by stiffening their limb (increased agonist-antagonist muscle co-activation). As hypothesized, the results showed that the settling time significantly increased for both the added noise and the sudden estimation error. Interestingly, contrary to what was hypothesized, the results of Part A generally did not show an increase in rise time and overall a lower muscle activity with noise of both muscles of the agonist-antagonist pair. From these results, it can be concluded that noise and estimation errors lead to longer movement times during fast goal-directed elbow flexion. Contrary to what current literature suggests, a reasonable amount of noise over the compensation force even seemed to cause subjects reducing the amount of co-activation. Apparently unpredictabilities like noise not always cause an increase of co-activation, and it can be concluded that more research is needed on what exactly makes humans choose their limb-impedance strategy in practical situations.

Load compensation
human movement control
force noise
estimation error

http://resolver.tudelft.nl/uuid:88787580-3307-4924-a031-2b45b4a36a3c

2016-12-22

Student theses

master thesis

(c) 2016 Everling, M.