The effect of menthol application to the skin on sweating rate response during exercise in swimmers and controls

Influence of topical menthol gel on thermoregulation and perception while walking in the heat

PURPOSE

Menthol is known to elicit opposing thermoregulatory and perceptual alterations during intense exercise. The current purpose was to determine the thermoregulatory and perceptual effects of topical menthol application prior to walking in the heat.

METHODS

Twelve participants walked (1.6 m s-1, 5% grade) for 30 min in the heat (38 °C, 60% relative humidity) with either a 4% menthol or control gel on the upper (shoulder to wrist) and lower (mid-thigh to ankle) limbs. Skin blood flow (SkBF), sweat (rate, composition), skin conductivity, heart rate, temperature (skin, core), and thermal perception were measured prior to and during exercise.

RESULTS

Skin conductivity expressed as time to 10, 20, 30, and 40 µS was delayed due to menthol (559 ± 251, 770 ± 292, 1109 ± 301, 1299 ± 335 s, respectively) compared to the control (515 ± 260, 735 ± 256, 935 ± 300, 1148 ± 298 s, respectively, p = 0.048). Sweat rate relative to body surface area was lower due to menthol (0.55 ± 0.16 L h-1 m(2)-1) than the control (0.64 ± 0.16 L h-1 m(2)-1, p = 0.049). Core temperature did not differ at baseline between the menthol (37.4 ± 0.3 °C) and control (37.3 ± 0.4 °C, p = 0.298) but was higher at 10, 20, and 30 min due to menthol (37.5 ± 0.3, 37.7 ± 0.2, 38.1 ± 0.3 °C, respectively) compared to the control (37.3 ± 0.4, 37.4 ± 0.3, 37.7 ± 0.3 °C, respectively, p < 0.05). The largest rise in core temperature from baseline was at 30 min during menthol (0.7 ± 0.3 °C) compared to the control (0.4 ± 0.2 °C, p = 0.004). Overall, the menthol treatment was perceived cooler, reaching "slightly warm" whereas the control treatment reached "warm" (p < 0.001).

CONCLUSION

Menthol application to the limbs impairs whole-body thermoregulation while walking in the heat despite perceiving the environment as cooler.

Topical application of L-Menthol - Physiological and genetic considerations to assist in developing female athlete research: A narrative review

L-menthol is a cyclic monoterpene derived from aromatic plants, which gives a cooling sensation upon application. With this in mind, L-menthol is beginning to be considered as a potential ergogenic aid for exercise and sporting competitions, particularly in hot environments, however female-specific research is lacking. The aim of this narrative review is to summarize available literature relating to topical application of L-menthol and provide commentary on avenues of consideration relating to future research developments of topical L-menthol in female athletes. From available studies in male participants, L-menthol topical application results in no endurance exercise performance improvements, however decreases in thermal sensation are observed. Mixed results are observed within strength performance parameters. Several genetic variations and single nucleotide polymorphisms have been identified in relation to sweat production, fluid loss and body mass changes - factors which may influence topical application of L-menthol. More specifically to female athletes, genetic variations relating to sweat responses and skin thickness, phases of the menstrual cycle, and body composition indices may affect the ergogenic effects of L-menthol topical application, via alterations in thermogenic responses, along with differing tissue distribution compared to their male counterparts. This narrative review concludes that further development of female athlete research and protocols for topical application of L-menthol is warranted due to physiological and genetic variations. Such developments would benefit research and practitioners alike with further personalized sport science strategies around phases of the menstrual cycle and body composition indices, with a view to optimize ergogenic effects of L-menthol.

Topical application of isolated menthol and combined menthol-capsaicin creams: Exercise tolerance, thermal perception, pain, attentional focus and thermoregulation in the heat

We determined the effects of topically applied (i) isolated menthol cream, (ii) menthol and capsaicin co-application or (iii) placebo cream on exercise tolerance, thermal perception, pain, attentional focus and thermoregulation during exercise in the heat. Ten participants cycled at 70% maximal power output until exhaustion in 35°C and 20% relative humidity after application of (i) 5% isolated menthol, (ii) 5% menthol and 0.025% capsaicin co-application or (iii) placebo cream. Thermo-physiological responses were measured during exercise, with attentional focus and pain determined post-exercise on a 0-to-10 scale. Across the three conditions, time to exhaustion was 13.4 ± 4.8 min, mean ± SD infrared tympanic and skin temperature was 37.2 ± 0.6°C and 35.1 ± 1.2°C, respectively, and heart rate was 152 ± 47 bpm, with no changes between conditions (p > 0.05). Perceived exertion was lower in the isolated menthol vs. all other conditions (p < 0.05, ηp2 = 0.44). Thermal sensation was higher in menthol-capsaicin co-application vs. isolated menthol (p < 0.05, d = 1.1), while sweat rate was higher for capsaicin and menthol co-application compared to menthol (p < 0.05, d = 0.85). The median and interquartile range scores for pain were lower (p < 0.05) in the menthol condition (8, 7-8) compared to both menthol and capsaicin (10, 9-10) and placebo (9, 9-10), which was coupled with a greater distraction (p < 0.05) in the menthol condition (9, 7-10) compared to placebo (6, 5-7). Despite no performance effects for any topical cream application condition, these data reiterate the advantageous perceptual and analgesic role of menthol application and demonstrate no advantage of co-application with capsaicin.HighlightsTopical application of isolated menthol cream to cold-sensitive areas of the body during exhaustive exercise in the heat, elicited reduced perception of pain and enhanced sensation of cooling.While this reduction in generally unpleasant feelings (i.e. pain and heat) were coupled with lower RPE scores in the menthol condition and could be considered beneficial, there was no apparent ergogenic effect in an exercise tolerance test.Co-application of capsaicin and menthol appeared to inhibit the positive sensory effects elicited by menthol.Isolated menthol can induce changes in cognitive processes related to pain and exertion, while also reducing thermal sensation; however, the decision to use menthol creams must be balanced with the limited performance or thermoregulatory effects reported herein during exercise in hot environments.

Effects of cutaneous administration of an over-the-counter menthol cream during temperate-water immersion for exercise-induced hyperthermia in men

Introduction: Hyperthermia impairs various physiological functions and physical performance. We examined the effects of cutaneous administration with an over-the-counter (OTC) analgesic cream containing 20% methyl salicylate and 6% L-menthol during temperate-water immersion (TWI) for exercise-induced hyperthermia. Methods: In a randomized crossover design, twelve healthy males participated in both of two experiments. Firstly, participants underwent a 15-min TWI at 20°C with (CREAM) or without (CON) cutaneous application of an analgesic cream. Cutaneous vascular conductance (CVC) was measured using laser doppler flowmetry during TWI. In a subsequent experiment, same participants performed a 30-min strenuous interval exercise in a heated (35°C) environment to induce hyperthermia (~39°C), which was followed by 15 min of TWI. Results: Core body temperature, as measured by an ingestible telemetry sensor, and mean arterial pressure (MAP) were measured. CVC and %CVC (% baseline) were higher during TWI in CREAM than in CON (Condition effect: p = 0.0053 and p = 0.0010). An additional experiment revealed that core body heat loss during TWI was greater in CREAM than in CON (Cooling rate: CON 0.070 ± 0.020 vs. CREAM 0.084°C ± 0.026°C/min, p = 0.0039). A more attenuated MAP response was observed during TWI in CREAM than in CON (Condition effect: p = 0.0007). Conclusion: An OTC analgesic cream containing L-menthol and MS augmented cooling effects when cutaneously applied during TWI in exercise-induced hyperthermia. This was, at least in part, due to the counteractive vasodilatory effect of the analgesic cream. The cutaneous application of OTC analgesic cream may therefore provide a safe, accessible, and affordable means of enhancing the cooling effects of TWI.

Sex differences in thermal sensitivity and perception: Implications for behavioral and autonomic thermoregulation

Temperature sensitive receptors in the skin and deep body enable the detection of the external and internal environment, including the perception of thermal stimuli. Changes in heat balance require autonomic (e.g., sweating) and behavioral (e.g., seeking shade) thermoeffector initiation to maintain thermal homeostasis. Sex differences in body morphology can largely, but not entirely, account for divergent responses in thermoeffector and perceptual responses to environmental stress between men and women. Thus, it has been suggested that innate differences in thermosensation may exist between men and women. Our goal in this review is to summarize the existing literature that investigates localized and whole-body cold and heat exposure pertaining to sex differences in thermal sensitivity and perception, and the interplay between autonomic and behavioral thermoeffector responses. Overall, it appears that local differences in thermal sensitivity and perception are minimized, yet still apparent, when morphological characteristics are well-controlled. Sex differences in the early vasomotor response to environmental stress and subsequent changes in blood flow likely contribute to the heightened thermal awareness observed in women. However, the contribution of thermoreceptors to observed sex differences in thermal perception and thermoeffector function is unclear, as human studies investigating these questions have not been performed.

Topical Analgesic Containing Methyl Salicylate and L-Menthol Accelerates Heat Loss During Skin Cooling for Exercise-Induced Hyperthermia

Hyperthermia impairs physical performance and, when prolonged, results in heat stroke or other illnesses. While extensive research has investigated the effectiveness of various cooling strategies, including cold water immersion and ice-suit, there has been little work focused on overcoming the cutaneous vasoconstriction response to external cold stimulation, which can reduce the effectiveness of these treatments. Over-the-counter (OTC) topical analgesics have been utilized for the treatment of muscle pain for decades; however, to date no research has examined the possibility of taking advantage of their vasodilatory functions in the context of skin cooling. We tested whether an OTC analgesic cream containing 20% methyl salicylate and 6% L-menthol, known cutaneous vasodilators, applied to the skin during skin cooling accelerates heat loss in exercise-induced hyperthermia. Firstly, we found that cutaneous application of OTC topical analgesic cream can attenuate cold-induced vasoconstriction and enhance heat loss during local skin cooling. We also revealed that core body heat loss, as measured by an ingestible telemetry sensor, could be accelerated by cutaneous application of analgesic cream during ice-suit cooling in exercise-induced hyperthermia. A blunted blood pressure response was observed during cooling with the analgesic cream application. Given the safety profile and affordability of topical cutaneous analgesics containing vasodilatory agents, our results suggest that they can be an effective and practical tool for enhancing the cooling effects of skin cooling for hyperthermia.

(-)-Menthol-β-cyclodextrin inclusion complex production and characterization

(-)-Menthol has been widely used in clinical medicine, flavor, and fragrance. However, high volatility, short retention time, low solubility in water, and whisker growth of menthol are crucial problems for its application. In this paper, (-)-menthol-β-cyclodextrin inclusion complex was fabricated to solve these problems. The product was characterized by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. The results showed that menthol was successfully encapsulated in the cavity of β-cyclodextrin. Menthol itself vaporized almost completely at around 120 oC, while the maximum menthol release rate occurred at 267.5 oC after the formation of the inclusion complex. The stability and retention time were improved. The menthol release reaction order, apparent activation energy and the pre-exponential factor were obtained and their values were 0, 142.9 kJ/mol and 1.6 × 1013 respectively. The structure of menthol-β-cyclodextrin inclusion complex was investigated by molecular simulation and the minimum energy, –116.7 kJ/mol, was obtained at –0.8 × 10–10 m.

The effect of body surface area exposure to menthol on temperature regulation and perception in men

INTRODUCTION

In warm conditions topical application of menthol increases cool sensations and influences deep body temperature. The purpose of this experiment was to explore whether different body surface areas (BSA) exposed to menthol influence these responses. It was hypothesized that the forcing function exerted by menthol will be proportionally related to BSA.

METHOD

Using a within-participant design, 13 participants underwent three BSA exposures (Small [S; finger]; Medium [M; arm]; Large [L; upper/lower body]) to 4.13% menthol, and one Placebo exposure. During each exposure participants rested supine in a tent (30 °C, 50%rh) for 30-min before their intervention and 30-min thereafter. Measures included thermal sensation, thermal comfort, irritation, skin blood flow (finger SkBF; laser Doppler flowmetry), rectal temperature (Tre), and skin temperature (chest, forearm, thigh, calf). The Area Under the Curve from minute 30 to 60 was calculated and analyzed using a one-way ANOVA or Friedman's test with post-hoc testing (0.05 alpha level).

RESULTS

There was no significant difference in any measure of thermometry (p > 0.05), while SKBF was significantly lowered in L, M, and S vs. P respectively (p < 0.05). Participants in L felt cooler vs. P and S (p < 0.05). Losses in thermal comfort were noted in L and M vs. P and S (p < 0.05), along with increased irritation in L vs. S (p < 0.05).

CONCLUSIONS

Despite similar skin temperatures, larger BSA's exposed to menthol caused cooler sensations, likely due to the activation of a larger pool of menthol-sensitive neurons. This occurred in the absence of thermal discomfort and without perceptions of irritation exceeding 'weak'. Larger BSA's also exhibited greater alterations in Tre, likely driven by a reduction in SkBF, but despite this mean body temperature was regulated suggesting the thermoregulatory system can cope with the range of BSA exposures studied herein.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Menthol as an Ergogenic Aid for the Tokyo 2021 Olympic Games: An Expert-Led Consensus Statement Using the Modified Delphi Method

INTRODUCTION

Menthol topical application and mouth rinsing are ergogenic in hot environments, improving performance and perception, with differing effects on body temperature regulation. Consequently, athletes and federations are beginning to explore the possible benefits to elite sport performance for the Tokyo 2021 Olympics, which will take place in hot (~ 31 °C), humid (70% RH) conditions. There is no clear consensus on safe and effective menthol use for athletes, practitioners, or researchers. The present study addressed this shortfall by producing expert-led consensus recommendations.

METHOD

Fourteen contributors were recruited following ethical approval. A three-step modified Delphi method was used for voting on 96 statements generated following literature consultation; 192 statements total (96/96 topical application/mouth rinsing). Round 1 contributors voted to "agree" or "disagree" with statements; 80% agreement was required to accept statements. In round 2, contributors voted to "support" or "change" their round 1 unaccepted statements, with knowledge of the extant voting from round 1. Round 3 contributors met to discuss voting against key remaining statements.

RESULTS

Forty-seven statements reached consensus in round 1 (30/17 topical application/rinsing); 14 proved redundant. Six statements reached consensus in round 2 (2/4 topical application/rinsing); 116 statements proved redundant. Nine further statements were agreed in round 3 (6/3 topical application/rinsing) with caveats.

DISCUSSION

Consensus was reached on 62 statements in total (38/24 topical application/rinsing), enabling the development of guidance on safe menthol administration, with a view to enhancing performance and perception in the heat without impairing body temperature regulation.

Menthol can be safely applied to improve thermal perception during physical exercise: a meta-analysis of randomized controlled trials

Menthol is often used as a cold-mimicking substance to allegedly enhance performance during physical activity, however menthol-induced activation of cold-defence responses during exercise can intensify heat accumulation in the body. This meta-analysis aimed at studying the effects of menthol on thermal perception and thermophysiological homeostasis during exercise. PubMed, EMBASE, Cochrane Library, and Google Scholar databases were searched until May 2020. Menthol caused cooler thermal sensation by weighted mean difference (WMD) of - 1.65 (95% CI, - 2.96 to - 0.33) and tended to improve thermal comfort (WMD = 1.42; 95% CI, - 0.13 to 2.96) during physical exercise. However, there was no meaningful difference in sweat production (WMD = - 24.10 ml; 95% CI, - 139.59 to 91.39 ml), deep body temperature (WMD = 0.02 °C; 95% CI, - 0.11 to 0.15 °C), and heart rate (WMD = 2.67 bpm; 95% CI - 0.74 to 6.09 bpm) between the treatment groups. Menthol improved the performance time in certain subgroups, which are discussed. Our findings suggest that different factors, viz., external application, warmer environment, and higher body mass index can improve menthol's effects on endurance performance, however menthol does not compromise warmth-defence responses during exercise, thus it can be safely applied by athletes from the thermoregulation point of view.

Current Knowledge on the Vascular Effects of Menthol

Menthol is a monoterpene alcohol, widely used in several food and healthcare products for its particular odor and flavor. For some decades, menthol has been known to act on the vasculature directly in the endothelium and vascular smooth muscle, with recent studies showing that it also evokes an indirect vascular response via sensory fibers. The mechanisms underlying menthol's vascular action are complex due to the diversity of cellular targets, to the interplay between signaling pathways and to the variability in terms of response. Menthol can evoke either a perfusion increase or decrease in vivo in different vascular territories, an observation that warrants a critical discussion. Menthol vascular actions in vivo seem to depend on whether the vascular territory under analysis has been directly provoked with menthol or is located deep/distant to the application site. Menthol increases perfusion of directly provoked skin regions due to a complex interplay of increased nitric oxide (NO), endothelium-derived hyperpolarization factors (EDHFs) and sensory nerve responses. In non-provoked vascular beds menthol decreases perfusion which might be attributed to heat-conservation sympathetically-mediated vasoconstriction, although an increase in tissue evaporative heat loss due the formulation ethanol may also play a role. There is increasing evidence that several of menthol's cellular targets are involved in cardiovascular diseases, such as hypertension. Thus menthol and pharmacologically-similar drugs can play important preventive and therapeutic roles, which merits further investigation.

Thermoregulatory and cardiovasculareffects of capsaicin application on human skin during dynamic exercise to temperate and warm conditions

Thermoregulatory and cardiovascular responses during cycling in temperate and warm environments without and with application of capsaicin on the skin were investigated. We hypothesized that regardless of environmental temperature, capsaicin application would activate heat loss mechanisms attenuating exercise‐induced rectal temperature (Tre) and blood pressure increase. Eight males cycled at 55% of their maximal aerobic power so long as to reach 38.2°C Tre at 20.8 ± 1.0°C and at 30.6 ± 1.1°C ambient temperatures twice: without (NCA) and with (CA) application of capsaicin patches (12 × 18 cm, 4.8 mg). Patches were applied on pectoralis major, trapezius and vastus lateralis muscles. Thermoregulatory (Tre, proximal‐distal skin temperature gradient, sweating rate), cardiovascular variables and oxygen uptake were continuously recorded. In both ambient conditions, during the first 14 min of exercise, the local vasoconstrictive tone as a function of the relative change in Tre was lower in CA than NCA (p < .05, d = 0.84–1.15). Further, sweating rate was higher and occurred at a lower Tre increase in CA compared to NCA (p = .03, d = 0.6) resulting in extended time to reach 38.2°C Tre (p = .03, d = 0.9). Moreover, oxygen consumption was higher in CA than in NCA (p < .001, d = 0.8). Mean arterial pressure was lower during cycling in warm compared to temperate environment, but was unaffected by capsaicin. We conclude that activation of thermal sensors by capsaicin results in lower Tre rise during exercise, which is mediated through greater skin vasodilation along with higher rate and earlier onset of sweating. Nonetheless, capsaicin application has no extra effect on exercise cardiovascular responses.

Enhancement of Exercise Capacity in the Heat With Repeated Menthol-Spray Application

Purpose: Exercise performance is impaired in the heat, and a contributing factor to this decrement is thermal discomfort. Menthol spraying of skin is one means of alleviating thermal discomfort but has yet to be shown to be ergogenic using single-spray applications. The authors examined whether repeated menthol spraying could relieve thermal discomfort, reduce perception of exertion, and improve exercise performance in hot (35°C), dry (22% relative humidity) conditions, hypothesizing that it would. Methods: A total of 8 trained cyclists completed 2 separate conditions of fixed-intensity cycling (50% maximal power output) for 45 min before a test to exhaustion (TTE; 70% maximal power output) with 100 mL of menthol spray (0.20% menthol) or control spray applied to the torso after 20 and 40 min. Perceptual (thermal sensation, thermal comfort, and rating of perceived exertion) performance (TTE duration), thermal variables (skin temperature, rectal temperature, and cardiac frequency), and sweating were measured. Data were compared using analysis of variance to .05 alpha level. Results: Menthol spray improved thermal sensation (cold sensation cf warm/hot after first spraying; P = .008) but only descriptively altered thermal comfort (comfortable cf uncomfortable; P = .173). Sweat production (994 [380] mL cf 1180 [380] mL; P = .020) and sweat rate (827 [327] mL·h^-1 cf 941 [319] mL·h^-1; P = .048) lowered. TTE performance improved (4.6 [1.74] cf 2.4 [1.55] min; P = .004). Menthol-spray effects diminished despite repeated applications, indicating increased contribution of visceral thermoreceptors to thermal perception. Conclusion: Repeated menthol spraying improves exercise capacity but alters thermoregulation, potentially conflicting behavioral and thermoregulatory drivers; care should be taken with its use. Carrying and deploying menthol spray would impose a logistical burden that needs consideration against performance benefit.

Endurance Performance is Influenced by Perceptions of Pain and Temperature: Theory, Applications and Safety Considerations

Models of endurance performance now recognise input from the brain, including an athlete's ability to cope with various non-pleasurable perceptions during exercise, such as pain and temperature. Exercise training can reduce perceptions of both pain and temperature over time, partly explaining why athletes generally have a higher pain tolerance, despite a similar pain threshold, compared with active controls. Several strategies with varying efficacy may ameliorate the perceptions of pain (e.g. acetaminophen, transcranial direct current stimulation and transcutaneous electrical stimulation) and temperature (e.g. menthol beverages, topical menthol products and other cooling strategies, especially those targeting the head) during exercise to improve athletic performance. This review describes both the theory and practical applications of these interventions in the endurance sport setting, as well as the potentially harmful health consequences of their use.

Effects of capsaicin application on the skin during resting exposure to temperate and warm conditions

We investigated thermoregulatory and cardiovascular responses at rest in a temperate (20°C) and in a warm (30°C) environment (40% RH) without and with the application of capsaicin on the skin. We hypothesized that regardless of environmental temperature, capsaicin application would stimulate heat loss and concomitantly deactivate heat conservation mechanisms, thus resulting in rectal temperature (Tre) and mean blood pressure decline due to excitation of heat-sensitive TRPV1. Ten male subjects were exposed, while seated, for 30 minutes to 20.8 ± 1.0°C or to 30.6 ± 1.1°C: without (NCA) and with (CA) application of capsaicin patches on the skin. Thermoregulatory (Tre, proximal-distal skin temperature gradient) and cardiovascular variables (modelflow technique) as well as oxygen uptake were continuously measured. The area under the curve for Tre decline at 20°C was smaller in CA (-2.1 ± 1.3 a.u.) than in NCA (-0.6 ± 1.1 a.u., P < 0.01, r = 0.8). Likewise, at 30°C it was smaller in CA (-2.2 ± 2.1 a.u.) compared to NCA (-0.8 ± 2.0 a.u., P = 0.02, r = 0.7). Local vasomotor tone and oxygen uptake, were significantly lower by 36.7% ± 94.2% and 12.3% ± 12.3%, respectively, with capsaicin compared to NCA (P = 0.05 and P < 0.01, respectively). Additionally, in 30°C CA mean arterial pressure was lower by 10.7% ± 5.9%, 8.9% ± 5.9%, and 10.6% ± 7.0% compared to 30°C NCA, 20°C NCA, and 20°C CA, respectively (P < 0.01, P = 0.02, and P < 0.01, respectively, d = 1.4-1.8). In conclusion, capsaicin application on the skin induced vasodilation and Tre decline. At 30°C CA, thermal responses were accompanied by arterial hypotension most likely due to the interactive effects of both stressors (warm environment and capsaicin) on cutaneous vascular regulation.

Immersion with menthol improves recovery between 2 cycling exercises in hot and humid environment

Endurance exercise performance is impaired in a hot and humid environment. This study compared the effects of cold water immersion, with (CMWI) and without (CWI) menthol, on the recovery of cycling performance. Eight heat-acclimatized cyclists (age, 24.1 ± 4.4 years; mass, 65.3 ± 5.2 kg) performed 2 randomized sessions, each consisting of a 20-min cycling trial (T1) followed by 10 min of immersion during recovery and then a second 20-min cycling trial (T2). Mean power output and perceived exertion (RPE) were recorded for both trials. Rectal (Trec) and skin temperatures were measured before and immediately after T1, immersion, and T2. Perceived thermal sensation (TS) and comfort were measured immediately after T1 and T2. Power output was significantly improved in T2 compared with T1 in the CMWI condition (+15.6%). Performance did not change in the CWI condition. After immersion, Trec was lower in CWI (–1.17 °C) than in CMWI (–0.6 °C). TS decreased significantly after immersion in both conditions. This decline was significantly more pronounced in CMWI (5.9 ± 1 to 3.6 ± 0.5) than in CWI (5.6 ± 0.9 to 4.4 ± 1.2). In CMWI, RPE was significantly higher in T1 (6.57 ± 0.9) than in T2 (5.14 ± 1.25). However, there was no difference in TC. This study suggests that menthol immersion probably (i) improves the performance of a repeated 20-min cycling bout, (ii) decreases TS, and (iii) impairs thermoregulation processes.

Menthol: A Fresh Ergogenic Aid for Athletic Performance

The application of menthol has recently been researched as a performance-enhancing aid for various aspects of athletic performance including endurance, speed, strength and joint range of motion. A range of application methods has been used including a mouth rinse, ingestion of a beverage containing menthol or external application to the skin or clothing via a gel or spray. The majority of research has focussed on the use of menthol to impart a cooling sensation on athletes performing endurance exercise in the heat. In this situation, menthol appears to have the greatest beneficial effect on performance when applied internally. In contrast, the majority of investigations into the external application of menthol demonstrated no performance benefit. While studies are limited in number, menthol has not yet proven to be beneficial for speed or strength, and only effective at increasing joint range of motion following exercise that induced delayed-onset muscle soreness. Internal application of menthol may provoke such performance-enhancing effects via mechanisms related to its thermal, ventilatory, analgesic and arousing properties. Future research should focus on well-trained subjects and investigate the addition of menthol to nutritional sports products.

Effects of menthol application on the skin during prolonged immersion in cool and cold water

The aim of the study was to compare the effect of skin surface menthol application on rectal temperature (Tre) during prolonged immersion in cool and cold water. We hypothesized that menthol application would lead to a slower Tre decline due to the reduced heat loss as a consequence of the menthol-induced vasoconstriction and that this effect would be attenuated during cold-water immersion. Six male subjects were immersed for 55 minutes in stirred cool (24°C) or cold (14°C) water immediately after attaining a Tre of 38°C by cycling at 60% of maximum heart rate on two occasions: without (ΝM) and with (M) whole-body skin application of menthol cream. Tre, the proximal-distal skin temperature gradient, and oxygen uptake were continuously measured. ANOVA with repeated measures was employed to detect differences among variables. Significance level was set at 0.05. The area under the curve for Tre was calculated and was greater in 24°C M (-1.81 ± 8.22 a.u) compared to 24°C NM (-27.09 ± 19.09 a.u., P = .03, r = .90), 14°C NM (-18.08 ± 10.85 a.u., P = .03, r = .90), and 14°C M (-11.71 ± 12.58 a.u, P = .05, r = .81). In cool water, oxygen uptake and local vasoconstriction were increased (P ≤ .05) by 39 ± 25% and 56 ± 37%, respectively, with menthol compared to ΝM, while no differences were observed in cold water. Menthol application on the skin before prolonged immersion reduces heat loss resulting in a blunted Tre decline. However, such a response is less obvious at 14°C water immersion, possibly because high-threshold cold-sensitive fibers are already maximally recruited and the majority of cold receptors saturated.

Oral L-menthol reduces thermal sensation, increases work-rate and extends time to exhaustion, in the heat at a fixed rating of perceived exertion

PURPOSE

The study investigated the effect of a non-thermal cooling agent, L-menthol, on exercise at a fixed subjective rating of perceived exertion (RPE) in a hot environment.

METHOD

Eight male participants completed two trials at an exercise intensity between 'hard' and 'very hard', equating to 16 on the RPE scale at ~35 °C. Participants were instructed to continually adjust their power output to maintain an RPE of 16 throughout the exercise trial, stopping once power output had fallen by 30%. In a randomized crossover design, either L-menthol or placebo mouthwash was administered prior to exercise and at 10 min intervals. Power output, [Formula: see text]O₂, heart rate, core and skin temperature was monitored, alongside thermal sensation and thermal comfort. Isokinetic peak power sprints were conducted prior to and immediately after the fixed RPE trial.

RESULTS

Exercise time was greater (23:23 ± 3:36 vs. 21:44 ± 2:32 min; P = 0.049) and average power output increased (173 ± 24 vs. 167 ± 24 W; P = 0.044) in the L-menthol condition. Peak isokinetic sprint power declined from pre-post trial in the L-menthol l (9.0%; P = 0.015) but not in the placebo condition (3.4%; P = 0.275). Thermal sensation was lower in the L-menthol condition (P = 0.036), despite no changes in skin or core temperature (P > 0.05).

CONCLUSION

These results indicate that a non-thermal cooling mouth rinse lowered thermal sensation, resulting in an elevated work rate, which extended exercise time in the heat at a fixed RPE.

Effects of menthol application on the skin during prolonged immersion in swimmers and controls

We hypothesized that menthol application on the skin would enhance vasoconstriction of subjects immersed in cool water, which would reduce heat loss and rectal temperature (Tre) cooling rate. Furthermore, it was hypothesized that this effect would be greater in individuals acclimatized to immersion in 24 °C water, such as swimmers. Seven swimmers (SW) and seven physical education students (CON) cycled at 60% VO₂ max until Tre attained 38 °C, and were then immediately immersed in stirred water maintained at 24 °C on two occasions: without (NM) and with (M; 4.6 g per 100 mL of water) whole-body skin application of menthol cream. Heart rate, Tre, proximal-distal skin temperature gradient, oxygen uptake (VO₂ ), electromyographic activity (EMG), and thermal sensation were measured. Tre reduction was similar among SW and CON in NM and CON in M (-0.71±0.31 °C in average), while it was smaller for SW in M (-0.37±0.18 °C, P < 0.01). VO₂ and heart rate were greater in M compared with NM condition (P = 0.01). SW in M exhibited a shift of the threshold for shivering, as reflected in increased VO₂ and EMG activity, toward a higher Tre compared with the other trials. Menthol application on the skin before immersion reduces heat loss, but defends Tre decline more effectively in swimmers than in non-swimmers.

The influence of a menthol and ethanol soaked garment on human temperature regulation and perception during exercise and rest in warm, humid conditions

This study assessed whether donning a garment saturated with menthol and ethanol (M/E) can improve evaporative cooling and thermal perceptions versus water (W) or nothing (CON) during low intensity exercise and rest in warm, humid conditions often encountered in recreational/occupational settings. It was hypothesised there would be no difference in rectal (Tre) and skin (Tsk) temperature, infra-red thermal imagery of the chest/back, thermal comfort (TC) and rating of perceived exertion (RPE) between M/E, W and CON, but participants would feel cooler in M/E versus W or CON.

METHODS

Six volunteers (mean [SD] 22 [4] years, 72.4 [7.4] kg and 173.6 [3.7] cm) completed (separate days) three, 60-min tests in 30°C, 70%rh, in a balanced order. After 15-min of seated rest participants donned a dry (CON) or 80mL soaked (M/E, W) long sleeve shirt appropriate to their intervention. They then undertook 30-min of low intensity stepping at a rate of 12steps/min on a 22.5cm box, followed by 15-min of seated rest. Measurements included heart rate (HR), Tre, Tsk (chest/back/forearm), thermal imaging (back/chest), thermal sensation (TS), TC and RPE. Data were reported every fifth minute as they changed from baseline and the area under the curves were compared by condition using one-way repeated measures ANOVA, with an alpha level of 0.05.

RESULTS

Tre differed by condition, with the largest heat storage response observed in M/E (p<0.05). Skin temperature at the chest/back/forearm, and thermal imaging of the chest all differed by condition, with the greatest rate of heat loss observed in W and M/E respectively (p<0.01). Thermal sensation differed by condition, with the coolest sensations observed in M/E (p<0.001). No other differences were observed.

CONCLUSIONS

Both M/E and W enhanced evaporative cooling compared CON, but M/E causes cooler sensations and a heat storage response, both of which are likely mediated by menthol.

Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat

L-menthol stimulates cutaneous thermoreceptors and induces cool sensations improving thermal comfort, but has been linked to heat storage responses; this could increase risk of heat illness during self-paced exercise in the heat. Therefore, L-menthol application could lead to a discrepancy between behavioral and autonomic thermoregulatory drivers. Eight male participants volunteered. They were familiarized and then completed two trials in hot conditions (33.5 °C, 33% relative humidity) where their t-shirt was sprayed with CONTROL-SPRAY or MENTHOL-SPRAY after 10 km (i.e., when they were hot and uncomfortable) of a 16.1-km cycling time trial (TT). Thermal perception [thermal sensation (TS) and comfort (TC)], thermal responses [rectal temperature (Trec ), skin temperature (Tskin )], perceived exertion (RPE), heart rate, pacing (power output), and TT completion time were measured. MENTHOL-SPRAY made participants feel cooler and more comfortable and resulted in lower RPE (i.e., less exertion) yet performance was unchanged [TT completion: CONTROL-SPRAY 32.4 (2.9) and MENTHOL-SPRAY 32.7 (3.0) min]. Trec rate of increase was 1.40 (0.60) and 1.45 (0.40) °C/h after CONTROL-SPRAY and MENTHOL-SPRAY application, which were not different. Spraying L-menthol toward the end of self-paced exercise in the heat improved perception, but did not alter performance and did not increase heat illness risk.

The absorption and metabolism of a single L-menthol oral versus skin administration: Effects on thermogenesis and metabolic rate

We investigated the absorption and metabolism pharmacokinetics of a single L-menthol oral versus skin administration and the effects on human thermogenesis and metabolic rate. Twenty healthy adults were randomly distributed into oral (capsule) and skin (gel) groups and treated with 10 mg kg(-1) L-menthol (ORALMENT; SKINMENT) or control (lactose capsule: ORALCON; water application: SKINCON) in a random order on two different days. Levels of serum L-menthol increased similarly in ORALMENT and SKINMENT (p > 0.05). L-menthol glucuronidation was greater in ORALMENT than SKINMENT (p < 0.05). Cutaneous vasoconstriction, rectal temperature and body heat storage showed greater increase following SKINMENT compared to ORALMENT and control conditions (p < 0.05). Metabolic rate increased from baseline by 18% in SKINMENT and 10% in ORALMENT and respiratory exchange ratio decreased more in ORALMENT (5.4%) than SKINMENT (4.8%) compared to control conditions (p < 0.05). Levels of plasma adiponectin and leptin as well as heart rate variability were similar to control following either treatment (p > 0.05). Participants reported no cold, shivering, discomfort, stress or skin irritation. We conclude that a single L-menthol skin administration increased thermogenesis and metabolic rate in humans. These effects are minor following L-menthol oral administration probably due to faster glucuronidation and greater blood menthol glucuronide levels.

Quantification of continual anthropogenic pollutants released in swimming pools

Disinfection in swimming pools is often performed by chlorination, However, anthropogenic pollutants from swimmers will react with chlorine and form disinfection by-products (DBPs). DBPs are unwanted from a health point of view, because some are irritating, while others might be carcinogenic. The reduction of anthropogenic pollutants will lead to a reduction in DBPs. This paper investigates the continual release of anthropogenic pollutants by means of controlled sweat experiments in a pool tank during laboratory time-series experiments (LTS experiments) and also during on-site experiments (OS experiments) in a swimming pool. The sweat released during the OS and LTS experiments was very similar. The sweat rate found was 0.1-0.2 L/m(2)/h at water temperatures below 29 °C and increased linearly with increasing water temperatures to 0.8 L/m(2)/h at 35 °C. The continual anthropogenic pollutant release (CAPR) not only consisted of sweat, particles (mainly skin fragments and hair) and micro-organisms, but also sebum (skin lipids) has to be considered. The release of most components can be explained by the composition of sweat. The average release during 30 min of exercise is 250 mg/bather non-purgeable organic carbon (NPOC), 77.3 mg/bather total nitrogen (TN), 37.1 mg/bather urea and 10.1 mg/bather ammonium. The release of NPOC cannot be explained by the composition of sweat and is most probably a result of sebum release. The average release of other components was 1.31 × 10(9) # particles/bather (2-50 μm), 5.2 μg/bather intracellular adenosine triphosphate (cATP) and 9.3 × 10(6) intact cell count/bather (iCC). The pool water temperature was the main parameter to restrain the CAPR. This study showed that a significant amount of the total anthropogenic pollutants release is due to unhygienic behaviour of bathers.

Body regional influences of L-menthol application on the alleviation of heat strain while wearing firefighter's protective clothing

The purpose of this study was to investigate the influences of menthol application according to the amount of surface area on physiological and psychological heat strains, along with body regional influences. Male students underwent two stages of experiments: [Experiment 1] Cutaneous thermal threshold test at rest on eight body regions with/without a 0.8% menthol application at T (a) 28°C and 50% RH; [Experiment 2] Six exercise tests with/without a 0.8% menthol spray at T (a) 28°C and 40% RH, while wearing firefighter's protective clothing (No menthol, PC(NO); Face and neck menthol, PC(FN); Upper body menthol, PC(UP); Whole body menthol application, PC(WB)) or wearing normal clothing (No menthol, NC(NO); Upper body menthol, NC(UP)). Experiment 1 showed that menthol caused no significant influence on cutaneous warm thresholds, while menthol applications evoked earlier detection of cool sensations, especially on the chest (P = 0.043). Experiment 2 revealed that NC(UP), PC(UP) and PC(WB) caused lower mean skin temperature, especially with higher peripheral vasoconstrictions on the extremities at rest. During exercise, NC(UP), PC(UP) and PC(WB) induced greater and earlier increases in rectal temperatures (T (re)) and a delayed sweat response, but lessened psychological burdens (P < 0.05). Both physiological and psychological effects of PC(FN) were insignificant. For a composite analysis, individual Menthol Sensitivity Index at cooling in Experiment 1 had significant relationships with the threshold for T (re) increase and changes in heart rate in NC(UP) of Experiment 2 (P < 0.05). Our results indicate that menthol's topical influence is body region-dependent, as well as depending on the exposed body surface area.