The different effect of tongue motor task training (TTT) and strength training (ST) on the modulation of genioglossus corticomotor excitability and upper airway stability in rats

Abstract

Study objectives: The mechanical efficiency of upper airway (UA) muscles are pivotal in maintaining UA stability. We aimed to investigate if different tongue training approaches could differently induce signs of neuroplastic in the corticomotor pathways and upper airway stability changes.

Methods: 36 Sprague-Dawley rats were trained daily for eight weeks to lick an isotonic force-sensing disc at targeting forces using 30%-50% of maximal achieved lick force (MALF) for tongue task training (TTT) or targeting force set above 50%, 60%, and 70% of MALF progressively for tongue strength training (TST). Corticomotor excitability was dynamically assessed by GG response to transcortical magnetic stimulation (TMS) at different sessions. GG EMG activity, GG ultrastructure and myosin heavy chain (MHC), UA dynamics were assessed after eight weeks.

Results: After 4 weeks, GG TMS latencies decreased in both tongue training groups when compared with the control group (p < .05) and this excitability was more stable in TTT group. After 8 weeks, both GG TMS response and EMG activity revealed increased excitability in TTT and TST groups. The apoptotic pathological morphology changes of GG ultrastructure were observed in TST group, but not TTT. Percentage of GG MHC type I fibers in TST group was higher than the control and TTT groups (p < .05). The UA Pcrit decreased significantly in TTT group (p < .05) and tend to decrease in TST group (p = .09).

Conclusion: TTT could improve the UA stability and induce the neuroplastic changes more efficiently without training-induced muscle injury, while TST revealed a fatigue-resistance change in GG.

Keywords: corticomotor excitability; genioglossus activity; tongue strength training; tongue task training; upper airway stability.