http://www.jneuroengrehab.com/content/10/1/104/abstract
Journal of NeuroEngineering and Rehabilitation 2013, 10:104 doi:10.1186/1743-0003-10-104
Published: 9 October 2013
Published: 9 October 2013
Abstract (provisional)
Background
Stroke survivors often have difficulties in manipulating objects with their affected
hand. Thumb control plays an important role in object manipulation. Surface functional
electrical stimulation (FES) can assist movement. We aim to control the 2D thumb force
by predicting the sum of individual muscle forces, described by a sigmoidal muscle
recruitment curve and a single force direction.
Methods
Five able bodied subjects and five stroke subjects were strapped in a custom built
setup. The forces perpendicular to the thumb in response to FES applied to three thumb
muscles were measured. We evaluated the feasibility of using recruitment curve based
force vector maps in predicting output forces. In addition, we developed a closed
loop force controller. Load sharing between the three muscles was used to solve the
redundancy problem having three actuators to control forces in two dimensions. The
thumb force was controlled towards target forces of 0.5 N and 1.0 N in multiple directions
within the individual's thumb work space. Hereby, the possibilities to use these force
vector maps and the load sharing approach in feed forward and feedback force control
were explored.
Results
The force vector prediction of the obtained model had small RMS errors with respect
to the actual measured force vectors (0.22?0.17 N for the healthy subjects; 0.17?0.13
N for the stroke subjects). The stroke subjects showed a limited work range due to
limited force production of the individual muscles. Performance of feed forward control
without feedback, was better in healthy subjects than in stroke subjects. However,
when feedback control was added performances were similar between the two groups.
Feedback force control lead, especially for the stroke subjects, to a reduction in
stationary errors, which improved performance.
Conclusions
Thumb muscle responses to FES can be described by a single force direction and a sigmoidal
recruitment curve. Force in desired direction can be generated through load sharing
among redundant muscles. The force vector maps are subject specific and also suitable
in feedforward and feedback control taking the individual's available workspace into
account. With feedback, more accurate control of muscle force can be achieved.
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