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Effect of glucose and sodium chloride mouth rinses on neuromuscular fatigue: a preliminary study

Khong TK, Selvanayagam V, Yusof A

Eur J Sport Sci, 2021 Feb;21(2):224-230.
PMID: 32056510 DOI: 10.1080/17461391.2020.1730980

Carbohydrate (CHO) mouth rinse has been shown to improve endurance performance and maintain the central drive of contracting muscles. Salt (NaCl) mouth rinse solution, often used in dentistry to desensitise the oral cavity to pain, could also activate cortical areas of the brain. Hence, the objective of this preliminary study was to investigate whether CHO (glucose) and NaCl mouth rinses could attenuate the reduction in maximum voluntary contraction (MVC) and sustained MVC (sMVC) following an endurance exercise (30-minute cycling at 70% VO2max). Ten subjects (male, age: 22 ± 1 years, weight: 65.3 ± 12.4 kg, height: 164.5 ± 7.5 cm, VO2max: 48.3 ± 6.1 mL kg-1 min-1) completed three trials of 30-minute cycling exercise. In a randomised cross-over study, in each trial, the participants rinsed using either water, 6% glucose, or 6% NaCl solution for 5 s immediately prior to and every 10 min during the cycling exercise. The MVC and sMVC were measured pre and post cycling. Analysis of variance showed significant interaction and time effects for MVC, while for sMVC there was a significant interaction with time and group effects. Both MVC and sMVC were higher post cycling in the glucose and NaCl groups compared to the water group, which suggests that activation of glucose and NaCl oral receptors could better preserve post-exercise force production. This is the first study to show that NaCl mouth rinse can produce a comparable effect on glucose. Hence, mouth rinses may be able to activate other distinct pathways that could attenuate fatigue.
Fulltext Effect of Exercise on Arterial Stiffness in Healthy Young, Middle-Aged and Older Women: A Systematic Review

Lan YS, Khong TK, Yusof A

Nutrients, 2023 Jan 07;15(2).
PMID: 36678179 DOI: 10.3390/nu15020308

Arterial stiffness, an age-dependent phenomenon, is improved with exercise, which in turn may prevent cardiovascular diseases in women. However, there is a lack of consolidated information on the impact of exercise on arterial stiffness among healthy women. The aim of this review was to (i) analyse the effect of exercise on arterial stiffness in healthy young, middle-aged, and older women, and (ii) recommend types, intensity, and frequency for each age group. Database searches on PubMed, ScienceDirect, Web of Science, and Scopus were conducted using PRISMA guidelines until September 2022. The keywords were: exercise, women/female, and arterial stiffness. The inclusion criteria were: healthy women, supervised exercise, and arterial stiffness measures. Study quality and bias were assessed using the PEDro scale. Fifty-one papers were classified into young (n = 15), middle-aged (n = 14), and older (n = 22) women. Improvements in arterial stiffness were observed among: young women (Pulse Wave Velocity, PWV: 4.9-6.6 m/s), following an 8-week high-intensity aerobic (3 days/week) or hypoxic high-intensity interval training; middle-aged women (PWV: 5.1-7.9 m/s), aerobic exercise with moderate intensity or stretching exercise at "moderate to heavy" (Borg Scale), 20-30 s per site, 10 s of rest interval for 30 min; and for older women (PWV: 7.9-15.6 m/s), resistance training at light intensity, aerobic exercise at any intensity, or a combination of the two exercises. This review shows that arterial stiffness increases with age in healthy women and has an inverse relationship with exercise intensity. Therefore, when prescribing exercise to improve arterial stiffness, age and arterial stiffness measures should be accounted for.
Fulltext Effect of quantity and quality of pre-exercise carbohydrate meals on central fatigue

Khong TK, Selvanayagam VS, Hamzah SH, Yusof A

J Appl Physiol (1985), 2018 10 01;125(4):1021-1029.
PMID: 29975601 DOI: 10.1152/japplphysiol.00221.2018

Both the quantity and quality of pre-exercise carbohydrate (CHO) meals have been shown to improve endurance performance. However, their role in attenuating central fatigue (CF) is inconclusive. The use of neurophysiological techniques, such as voluntary activation (VA) and the central activation ratio (CAR), alongside maximum voluntary contraction (MVC) and sustained MVC (sMVC) can provide information on CF. Hence, the objective of this study was to investigate the effects of isocaloric pre-exercise meals: 1) a high versus low quantity of CHO and 2) a high quantity of CHO with a high versus low glycemic index (GI) on MVC, VA, and CAR following a 90-min run. The high and low quantity of CHO was 1.5 and 0.8 g/kg body wt, respectively, and high and low GI was ~75 and ~40, respectively. Blood insulin, serotonin, tryptophan, and gaseous exchange were also measured. High CHO preserved sMVC, VA, CAR, and serotonin postrunning with greater CHO oxidation and insulin response, whereas in low CHO, greater reductions in sMVC, VA, and CAR were accompanied by higher serotonin and fat oxidation with lower insulin response. These observations indicate central involvements. Meanwhile, high GI CHO better preserved force (sMVC), CAR, and tryptophan with greater CHO oxidation and insulin response compared with low GI. The findings of this study suggest that pre-exercise meals with varying quantity and quality of CHO can have an effect on CF, where greater CHO oxidation and insulin response found in both high CHO and high GI lead to attenuation of CF. NEW & NOTEWORTHY This paper examined the effects of carbohydrate interventions (high and low: quantity and quality wise) on central activity during prolonged exercise using mainly neurophysiological techniques along with gaseous exchange and blood insulin, serotonin, and tryptophan data.
Role of carbohydrate in central fatigue: a systematic review

Khong TK, Selvanayagam VS, Sidhu SK, Yusof A

Scand J Med Sci Sports, 2017 Apr;27(4):376-384.
PMID: 27714855 DOI: 10.1111/sms.12754

Carbohydrate (CHO) depletion is linked to neuromuscular fatigue during exercise. While its role at peripheral level is relatively well understood, less is known about its impact centrally. The aim of this systematic review was to critically analyze the effects of CHO on central fatigue (CF) assessed by various neurophysiological techniques. Four databases were searched using PRISMA guidelines through February 2016. The inclusion criteria were: CHO as intervention against a placebo control, fatigue induced by prolonged exercise and assessed using neurophysiological measures [voluntary activation (VA), superimposed twitch (SIT), M-wave, electromyography], alongside maximal voluntary contraction (MVC). Seven papers were reviewed, where exercise duration lasted between 115 and 180 min. CHO improved exercise performance in three studies, whereby two of them attributed it to CF via attenuation of VA and SIT reductions, while the other indicated peripheral involvement via attenuation of M-wave reduction. Although a few studies suggest that CHO attenuates CF, data on its direct effects on neurophysiological outcome measures are limited and mixed. Generally, measures employed in these studies were inadequate to conclude central contribution to fatigue. Factors including the techniques used and the lack of controls render additional confounding factors to make definitive deductions. Future studies should employ consistent techniques and appropriate neurophysiological controls to distinguish CHO effect at central level. The use of pharmacological intervention should be incorporated to elucidate involvement of central mechanisms.
Relationship between objective and subjective hydration measures on sprint performance among soccer players during actual matches in hot and humid environment

Lee MY, Khong TK, Ramliy N, Yusof A

J Sports Med Phys Fitness, 2021 Dec 21.
PMID: 34931790 DOI: 10.23736/S0022-4707.21.13413-9

BACKGROUND: Studies that investigate the effect of hydration on soccer performance in the heat are mostly carried out in a laboratory-controlled environment or simulated setting. Generally, on site, hydration is measured subjectively. Hence, the relationships between objective and subjective hydration measures during actual soccer matches in natural hot and humid environment with performance remain unclear.
METHODS: Forty-two soccer players (age: 27.34 ± 3.62 years; BMI: 23.80 ± 2.70 kg / m2; temperature: 30.8 ± 1.92 º C; humidity: 82 ± 1.4 %) hydration status were assessed using urine specific gravity (USG) and bodyweight (BW) as the objective measures, meanwhile, thirst was measured subjectively using a thirst scale. For performance evaluation, 20 and 50 m sprint time were measured before and after match.
RESULTS: The results showed a significant relationship between USG and sprint time (r = 1.00 (CI 0.98 - 0.99); p < 0.05), and as expected, USG at post-match was significantly higher than pre-match (p < 0.05) indicating dehydration. In line, changes (between post and pre-match) in BW also showed positive relationship with changes in sprint time (r = 0.99 (CI 0.98 - 0.99); p < 0.05). Not to our expectation, the self-reported thirst level was not found to be correlated with USG and sprint time.
CONCLUSIONS: Objective measures better reflect hydration status and predict sprint performance compared to subjective measure when playing in hot and humid environment. Players need to monitor their hydration status to maintain their sprint performances.
Central effects of mouth rinses on endurance and strength performance

Tan SH, Khong TK, Selvanayagam VS, Yusof A

Eur J Appl Physiol, 2024 Feb;124(2):403-415.
PMID: 38038740 DOI: 10.1007/s00421-023-05350-w

Rinsing the mouth with a carbohydrate (CHO) solution has been shown to enhance exercise performance while reducing neuromuscular fatigue. This effect is thought to be mediated through the stimulation of oral receptors, which activate brain areas associated with reward, motivation, and motor control. Consequently, corticomotor responsiveness is increased, leading to sustained levels of neuromuscular activity prior to fatigue. In the context of endurance performance, the evidence regarding the central involvement of mouth rinse (MR) in performance improvement is not conclusive. Peripheral mechanisms should not be disregarded, particularly considering factors such as low exercise volume, the participant's fasting state, and the frequency of rinsing. These factors may influence central activations. On the other hand, for strength-related activities, changes in motor evoked potential (MEP) and electromyography (EMG) have been observed, indicating increased corticospinal responsiveness and neuromuscular drive during isometric and isokinetic contractions in both fresh and fatigued muscles. However, it is important to note that in many studies, MEP data were not normalised, making it difficult to exclude peripheral contributions. Voluntary activation (VA), another central measure, often exhibits a lack of changes, mainly due to its high variability, particularly in fatigued muscles. Based on the evidence, MR can attenuate neuromuscular fatigue and improve endurance and strength performance via similar underlying mechanisms. However, the evidence supporting central contribution is weak due to the lack of neurophysiological measures, inaccurate data treatment (normalisation), limited generalisation between exercise modes, methodological biases (ignoring peripheral contribution), and high measurement variability.Trial registration: PROSPERO ID: CRD42021261714.