Validation of In-built Muscle Activation Feedback Sensor for a Closed Loop Functional Electrical Stimulator Development

In this paper, we presented the first stage of our closed loop functional electrical stimulator (FES) development work in reducing errors occurred at sensory feedback during FES-evoked contraction. This preliminary experimental study aims to verify that the measured voltage signal evoked by functional electrical stimulator across healthy human muscles satisfies the ohm's law and is independent of electromyography signal, electrostatic charges and earthing effect which has potential to affect the sensory feedback of the stimulator. Four able-bodied subjects were recruited to undergo evoked muscle contraction with voltage measured across their electrically stimulated tibialis anterior and brachialis muscles. The tolerated stimulation current amplitude used in this experiment was 20mA to 43mA. The stimulation current pulse width and frequency were fixed at 200us and 35Hz respectively, as the parameters evoked the most consistent response. Results showed that the range of measured voltage that is least affected by noise is above 0.1V at room temperature. The measured voltage satisfied ohm's law with linear voltage-current relationship (r = 0.98). A trend of increased resistance over time may was observed, which may suggest change in muscle behavior throughout the evoked contraction period. These preliminary results suggested strong relationship between muscle activation with measured voltage and resistance. This warrants further exploration study on spinal cord injured subjects with respect to force production to verify the efficacy of the sensory feedback. The acquisition region range setting will then be used on spinal cord injured subject to validate effectiveness of muscle activation feedback sensor during functional electrical stimulation assisted exercise