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Many balance assist or training systems have been developed to improve postural control, however these have limitations such as being bulky or unable to provide 2-dimensional feedback. Furthermore, efficiency of the commonly used vibro-tactile feedback modality can be reduced due to muscle fatigue. Therefore, we suggest a transcutaneous electrical nerve stimulation (TENS) based electro-tactile biofeedback system to provide efficient balance biofeedback even under muscle fatigue condition. A feedback scheme based on Stevens' power law is devised to provide 2-dimensional feedback using only four electrodes attached to the user's leg. A pilot test involving the tracking of directions indicated by stimuli was done to select the algorithm appropriate for use in the directional perception test and the balance study. The perception, in 8 directions, of electro-tactile cues delivered using the developed scheme is compared with that of vibro-tactile cues under both normal and fatigued conditions. In addition, we have done standing balance experiments to verify the effectiveness of the developed balance biofeedback scheme. 20 healthy participants took part in these experiments that were conducted under two conditions. The perception results show that under the fatigued condition, the electro-tactile modality is more perceptible than the vibro-tactile modality. Results of the balance biofeedback experiment with no feedback, continuous stimulation with and without dead-zone and directional feedback conditions show that directional electro-tactile feedback helped the most in improving the users' balance under normal and fatigued conditions. These promising results pave the way for future studies on the applications of this scheme for balance rehabilitation.INDEX TERMS Electro-tactile feedback, biofeedback, postural control, phantom sensation, transcutaneous electrical nerve stimulation (TENS).

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