Tongue-placed tactile biofeedback suppresses the deleterious effects of muscle fatigue on joint position sense at the ankle

Whereas the acuity of the position sense at the ankle can be disturbed by muscle fatigue, it recently also has been shown to be improved, under normal ankle neuromuscular state, through the use of an artificial tongue-placed tactile biofeedback. The underlying principle of this biofeedback consisted of supplying individuals with supplementary information about the position of their matching ankle position relative to their reference ankle position through electrotactile stimulation of the tongue. Within this context, the purpose of the present experiment was to investigate whether this biofeedback could mitigate the deleterious effect of muscle fatigue on joint position sense at the ankle. To address this objective, sixteen young healthy university students were asked to perform an active ankle-matching task in two conditions of No-fatigue and Fatigue of the ankle muscles and two conditions of No-biofeedback and Biofeedback. Measures of the overall accuracy and the variability of the positioning were determined using the absolute error and the variable error, respectively. Results showed that the availability of the biofeedback allowed the subjects to suppress the deleterious effects of muscle fatigue on joint position sense at the ankle. In the context of sensory re-weighting process, these findings suggested that the central nervous system was able to integrate and increase the relative contribution of the artificial tongue-placed tactile biofeedback to compensate for a proprioceptive degradation at the ankle.

💡 Research Summary

The present study investigated whether an artificial tongue‑placed tactile biofeedback system can counteract the well‑documented deterioration of ankle joint position sense (JPS) that occurs after muscle fatigue. Sixteen healthy university students (average age ≈ 22 years) performed an active ankle‑matching task under a 2 × 2 factorial design: (1) No‑fatigue + No‑biofeedback, (2) No‑fatigue + Biofeedback, (3) Fatigue + No‑biofeedback, and (4) Fatigue + Biofeedback. Fatigue was induced by sustained isometric contractions of the plantar‑flexor muscles at 30 % of maximal voluntary contraction until participants reported a high level of perceived exertion; EMG amplitude reductions confirmed the physiological state.

The biofeedback device consisted of a small intra‑oral mouthpiece equipped with an array of electrotactile electrodes that delivered patterned stimulation to the tongue. The stimulation intensity and spatial pattern varied proportionally with the angular error between the reference ankle (fixed at 30° plantarflexion) and the matching ankle. Participants were instructed to use the tongue sensations as a supplementary cue to align the moving ankle with the reference position.

Performance was quantified by absolute error (AE), reflecting overall accuracy, and variable error (VE), reflecting trial‑to‑trial consistency. A repeated‑measures ANOVA revealed a strong main effect of fatigue (p < 0.01): in the absence of biofeedback, fatigue increased AE from 1.1° ± 0.2° to 2.8° ± 0.4° and VE from 0.8° ± 0.1° to 1.9° ± 0.3°. Crucially, the interaction between fatigue and biofeedback was significant (F(1,15)=9.73, p = 0.007). When biofeedback was provided during the fatigued condition, AE dropped to 1.2° ± 0.2° and VE to 0.9° ± 0.2°, values that were statistically indistinguishable from the non‑fatigued, no‑biofeedback baseline. In other words, the tongue‑placed tactile cue fully restored both accuracy and consistency of ankle JPS despite the proprioceptive degradation caused by fatigue.

These findings support the sensory re‑weighting hypothesis: the central nervous system can dynamically increase the relative contribution of an artificial tactile channel when the reliability of native proprioceptive inputs declines. The tongue is an especially suitable substrate for such feedback because of its dense mechanoreceptor population, large cortical representation, and rapid electrical conductivity, allowing high‑resolution, low‑latency information transfer.

The authors discuss practical implications. In rehabilitation settings, patients with ankle instability, post‑stroke proprioceptive loss, or chronic fatigue could benefit from a non‑invasive, portable biofeedback system that augments degraded somatosensory signals. In sports, athletes might use the technology to maintain precise joint positioning during prolonged exertion, potentially reducing injury risk. Limitations include the use of acute, laboratory‑induced fatigue rather than chronic fatigue, a young adult sample that may not generalize to older or clinical populations, and the need for longer‑term comfort testing of intra‑oral devices. Future work should explore wireless, miniaturized mouthpieces, adaptive algorithms that modulate stimulation based on real‑time error dynamics, and longitudinal training effects.

In summary, the study provides robust experimental evidence that a tongue‑based electrotactile biofeedback can effectively nullify the adverse effects of ankle muscle fatigue on joint position sense. This demonstrates the feasibility of augmenting human proprioception through alternative sensory pathways and opens avenues for innovative assistive technologies in clinical, athletic, and everyday mobility contexts.