Mouth rinsing and ingesting salty or bitter solutions does not influence corticomotor excitability or neuromuscular function

Abstract

Purpose To explore the effect of tasting unpleasant salty or bitter solutions on lower limb corticomotor excitability and neuromuscular function. Methods Nine females and eleven males participated (age: 27 ± 7 years, BMI: 25.3 ± 4.0 kg m−2). Unpleasant salty (1 M) and bitter (2 mM quinine) solutions were compared to water, sweetened water, and no solution, which functioned as control conditions. In a non-blinded randomized cross-over order, each solution was mouth rinsed (10 s) and ingested before perceptual responses, instantaneous heart rate (a marker of autonomic nervous system activation), quadricep corticomotor excitability (motor-evoked potential amplitude) and neuromuscular function during a maximal voluntary contraction (maximum voluntary force, resting twitch force, voluntary activation, 0–50 ms impulse, 0–100 impulse, 100–200 ms impulse) were measured. Results Hedonic value (water: 47 ± 8%, sweet: 23 ± 17%, salt: 71 ± 8%, bitter: 80 ± 10%), taste intensity, unpleasantness and increases in heart rate (no solution: 14 ± 5 bpm, water: 18 ± 5 bpm, sweet: 20 ± 5 bpm, salt: 24 ± 7 bpm, bitter: 23 ± 6 bpm) were significantly higher in the salty and bitter conditions compared to control conditions. Nausea was low in all conditions (< 15%) but was significantly higher in salty and bitter conditions compared to water (water: 3 ± 5%, sweet: 6 ± 13%, salt: 7 ± 9%, bitter: 14 ± 16%). There was no significant difference between conditions in neuromuscular function or corticomotor excitability variables. Conclusion At rest, unpleasant tastes appear to have no influence on quadricep corticomotor excitability or neuromuscular function. These data question the mechanisms via which unpleasant tastes are proposed to influence exercise performance.

Mouth Rinsing and Ingestion of Unpleasant Salty or Bitter Solutions Does Not Improve Cycling Sprint Performance in Trained Cyclists

“…Therefore, it is possible that the effect of tasting quinine on corticomotor excitability that was previously observed in the upper limb would be reduced if measured in the lower limb muscles, the latter being more relevant to most exercise contexts. Indeed, we recently found that unpleasant salty or bitter tastes have no influence on knee extensor corticomotor excitability or neuromuscular function in a nonfatigued state (Gray et al, 2023a). Mouth rinsing a salty solution has been shown to ameliorate neuromuscular fatigue (Khong et al, 2020); however, we have found unpleasant salty and bitter tastes to have no influence on knee extensor force characteristics in a fatigued state (Gray et al, 2023b).…”

“…However, different from the previous work, we found that the quinine solution had no influence on mean power (Figure 1; −0.1% vs. water and −0.4% vs. control) or peak power (0.4% vs. water and −0.2% vs. control) during the cycling sprint. Furthermore, the salt solution had no effect on mean or peak power, despite a similar unpleasantness and autonomic nervous system response to the bitter condition (Gray et al, 2023a).…”

“…The mechanisms that may explain the ergogenic effect of tasting quinine are suggested to be due to increases in autonomic nervous system activation (Gam et al, 2014(Gam et al, , 2016, driven by the compounds' unpleasantness, and/or increases in corticomotor excitability facilitating enhanced motor output . However, the proposed neural mechanisms are ambiguous as we recently found that mouth rinsing and ingesting unpleasant bitter or salt solutions had no effect on knee extensor neuromuscular function or corticomotor excitability (Gray et al, 2023a). Furthermore, multiple studies have found tasting quinine to have no influence on short-duration exercise performance (Etxebarria et al, 2019(Etxebarria et al, , 2021.…”

“…All 25-ml solutions were served at room temperature in identical containers. The salt condition was a 1 M (5.8%) salt solution (Table Salt; Coles), a concentration determined from a pilot study from our previous research (Gray et al, 2023a). The bitter condition was a 2 mM quinine hydrochloride dihydrate solution (Sigma Aldrich) to replicate the concentration used in prior research (Etxebarria et al, 2019(Etxebarria et al, , 2021Gam et al, 2014;.…”

“…However, the motor-evoked potential changes were observed in the upper limb, which may not be relevant to whole-body exercise. Indeed, we recently found that tasting quinine, or a similarly unpleasant salt solution, did not influence knee extensor corticomotor excitability or neuromuscular function (Gray et al, 2023a).…”