Eric Hermand
email: eric.hermand17@gmail.com

Johan Cassirame

Gaël Ennequin

Olivier Hue

Validation of a photoplethysmographic heart rate monitor: Polar OH1

Keywords:

published or not. The documents may come     Validation of a photoplethysmographic heart rate monitor: Polar OH1

INTRODUCTION

Over the past decades, heart Rate Monitor (HRMs) have become popular tools for training guidance and physical activity monitoring [START_REF] Achten | Heart rate monitoring: applications and limitations[END_REF][START_REF] Schneider | Heart Rate Monitoring in Team Sports-A Conceptual Framework for Contextualizing Heart Rate Measures for Training and Recovery Prescription[END_REF] . These devices are regularly upgraded with improving technology (GPS integration, inertial sensor, memory) to respond to user requirements [START_REF] Cardinale | Wearable Training-Monitoring Technology: Applications, Challenges, and Opportunities[END_REF]. The first commercial HRMs based heart rate (HR) measurement on thoracic electrical measurement using chest belts to obtain ECG signal transduced in bursts when R peaks occur. More recently, manufacturers have equipped their devices with a photoplethysmography (PPG) system behind the wrist clock, as chest belts are less convenient for women and less hygienic.

Photoplethysmography was first used in the late 1930s [START_REF] Hertzman | The blood supply of various skin areas as estimated by the photoelectric plethysmograph[END_REF] especially in the medical field for measuring HR, O 2 saturation and cardiac output [START_REF] Allen | Photoplethysmography and its application in clinical physiological measurement[END_REF]. In brief, a diode emits a single wavelength light which is propagated through the tissue of interest and then is captured by a distant detector. The analysis of the temporal difference between the signal source and collection provides the required information.

PPG is mainly used with two light wavelengths. Red light is common in medical and hospital technologies, whereas green LEDs have gained popularity for consumer products. Compared to red light, they show higher sensitive to skin melanin and shallower data acquisition but much greater affinity to oxyhemoglobin and deoxyhemoglobin [START_REF] Van Kampen | Determination of hemoglobin and its derivatives[END_REF], which means better accuracy in HR monitoring [START_REF] Lee | Comparison between red, green and blue light reflection photoplethysmography for heart rate monitoring during motion[END_REF]. Green light monitors are therefore less subject to movement artifacts than red light monitors [START_REF] Maeda | Relationship between measurement site and motion artifacts in wearable reflected photoplethysmography[END_REF], and the low cost of green light technology has gradually made them available to the general public, especially for HR-monitored activities, like exercise and sports activities. Major brands have developed wrist-worn watches designed for both recreational and trained athletes, integrating an optical HR (OHR) monitor in the watch case, to replace the traditional chest belt. The HR values of OHR are insufficiently accurate [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF][START_REF] Gorny | Fitbit Charge HR Wireless Heart Rate Monitor: Validation Study Conducted Under Free-Living Conditions[END_REF][START_REF] Parak | Evaluation of wearable consumer heart rate monitors based on photopletysmography[END_REF][START_REF] Gillinov | Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise[END_REF][START_REF] Boudreaux | Validity of Wearable Activity Monitors during Cycling and Resistance Exercise[END_REF], despite recent improvements in the number of usable light emitters and the built-in algorithms [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF]. Among the many reasons are mainly low skin temperature [16], motion artifacts [16,[START_REF] Lin | Heart rate estimation using wrist-acquired photoplethysmography under different types of daily life motion artifact[END_REF], inadequate attachment or positioning [START_REF] Boudreaux | Validity of Wearable Activity Monitors during Cycling and Resistance Exercise[END_REF]16,[START_REF] Stahl | How accurate are the wrist-based heart rate monitors during walking and running activities? Are they accurate enough?[END_REF], skin type [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF], activity type [19] and intensity of exercise [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF]19,[START_REF] Dooley | Estimating Accuracy at Exercise Intensities: A Comparative Study of Self-Monitoring Heart Rate and Physical Activity Wearable Devices[END_REF]. Another reason is the number of LED sources: most of brands use two or three light sources, which may be insufficient to retrieve blood flow data, especially through the bony wrist region, more sensitive to motion artifacts [START_REF] Lin | Heart rate estimation using wrist-acquired photoplethysmography under different types of daily life motion artifact[END_REF].

Most of the aforementioned studies validating OHR monitors were performed in laboratory or controlled environments like sports gyms [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF][START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF][START_REF] Schubert | The Polar® OH1 Optical Heart Rate Sensor is Valid during Moderate-Vigorous Exercise[END_REF][START_REF] Abt | The validity and inter-device variability of the Apple Watch TM for measuring maximal heart rate[END_REF], using standardized protocols of low-to-medium intensity and limited duration (typically <1 hour) [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF][START_REF] Schubert | The Polar® OH1 Optical Heart Rate Sensor is Valid during Moderate-Vigorous Exercise[END_REF][START_REF] Abt | The validity and inter-device variability of the Apple Watch TM for measuring maximal heart rate[END_REF][START_REF] Claes | Validity of heart rate measurements by the Garmin Forerunner 225 at different walking intensities[END_REF]. From a more training-centered perspective, these systematic protocols might not be valid for use in ecological conditions, i.e., outdoor or with varying intensities and durations, for team sports (e.g., soccer) or endurance sports (e.g., running and cycling) at regional or national levels requiring daily training load monitoring [16].

The accuracy and reactivity of these devices are essential to HR measurement. Currently, HR measures are mainly used to calibrate training intensity with real-time HR values and calculate training load [START_REF] Achten | Heart rate monitoring: applications and limitations[END_REF][START_REF] Schneider | Heart Rate Monitoring in Team Sports-A Conceptual Framework for Contextualizing Heart Rate Measures for Training and Recovery Prescription[END_REF]. In the first case, athletes use the HR on the wrist display to adjust speed or other mechanical variables. Hence, accuracy and reactivity are both important for proper intensity calibration. In the second case, collected HR values are computed with various methods to obtain a global training load, such as TRIMP by Banister et al. [START_REF] Banister | Modeling elite athletic performance[END_REF][START_REF] Banister | Planning for future performance: implications for long term training[END_REF], which remains the gold standard. Inaccurate HR measurement yields incorrect TRIMP calculation [START_REF] Banister | Modeling elite athletic performance[END_REF][START_REF] Manzi | Relation between individualized training impulses and performance in distance runners[END_REF], which means that race day fitness level and fatigue are also likely to be incorrectly assessed [START_REF] Banister | Modeling elite athletic performance[END_REF][START_REF] Busso | Variable dose-response relationship between exercise training and performance[END_REF].

Recently, Polar Electro Oy (Kempele, Finland) developed a 6-LED wrist clock (M600) producing a valid HR signal in most (controlled) conditions [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]. Nevertheless, Horton et al.

point out that activities like weight lifting, muscle and ligament tension in the wrist may interfere with HR detection from capillary blood flow [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]. More recently, the same manufacturer designed an autonomous 6-LED OHR sensor, the OH1, that can be strapped around the arm or forearm and is potentially less subject to the motion artifacts of wrist-worn watches. Its use was validated for moderate-intensity sports activities [START_REF] Schubert | The Polar® OH1 Optical Heart Rate Sensor is Valid during Moderate-Vigorous Exercise[END_REF].

The aim of this study is [START_REF] Achten | Heart rate monitoring: applications and limitations[END_REF] to assess the accuracy of the HR signal in trained athletes wearing Polar OH1 monitor in ecological conditions during outdoor activities, including team and endurance sports, compared to traditional chest belt (Polar H7, Polar, Electro Oy, Kempele, Finland), and (2) to compare training load obtained from the HR calculation provided by the two devices.

METHODS

Subjects. 70 subjects, 56 males and 14 females, participated in this study. All were in good physical condition and exercised regularly, from 5 to 20 hours per week, during personal free time or with a club. Mean (± SD) age, height and body mass of all participants were 19.7±5.8 yrs, 174.4±10.5 cm, and 66.9±12.1 kg, respectively. The skin types of athletes living in Burgundy (France, 23 participants) and the West Indies (47) were assessed with the Fitzpatrick skin scale [START_REF] Fitzpatrick | The validity and practicality of sun-reactive skin types I through VI[END_REF], from 1 (lightest tone) to 6 (darkest).

Experimental protocol and data collection.

Participants followed their usual training routine following coaches' instructions. Running, biking and walking were performed on various terrains, switching between flats, hills and downhills, which induced a wide HR spectrum from low to high. Tennis, crossfit and soccer were performed on flat ground or in water, but also showed low and high HRs due to successive pauses and sudden accelerations or repetitions.

For each session, participants wore two HR monitors: the Polar OH1 monitor and the Polar H7 belt (Polar Electro Oy, Kempele, Finland) as the criterion measurement [START_REF] Gillinov | Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise[END_REF][START_REF] Wang | Accuracy of Wrist-Worn Heart Rate Monitors[END_REF], paired with a Polar M400 watch. Following the manufacturer's instructions, the OH1 device was strapped around the upper arm, firmly enough to remain in place but not enough to obstruct blood flow. Recordings for both were started at rest before the exercise start and terminated after a short recovery time. Procedures were conformed ethics in sport and exercise science [START_REF] Harriss | Standards for Ethics in Sport and Exercise Science Research: 2018 Update[END_REF].

Data processing and statistical analyses.

HR data from both M400-H7 and OH1 were retrieved from the Polar Flow web service, visually inspected for criterion dysfunction, discarded when necessary, and exported as spreadsheets. For comparisons of each measurement session, signals were synchronized with the least square method and smoothed on a 10-s window.

First, OHR quality was defined as the percentage of data within ±20 bpm of the Polar H7 signal (OH1-IN), with data outside this range labeled OH1-OUT. Second, OHR accuracy was assessed with Bland-Altman analysis [31] to test agreement between OH1 and H7 data: bias (mean difference, MD), standard deviation (SD) and upper and lower limits of agreement (LOA, defined as MD±SD) were calculated. Last, minimal, maximal and average HRs were calculated and training load values were computed following Banister's modified TRIMP [START_REF] Manzi | Relation between individualized training impulses and performance in distance runners[END_REF], using theoretical or measured rest and maximal HRs. Intraclass correlation coefficients (ICCs) [START_REF] Bartko | The Intraclass Correlation Coefficient as a Measure of Reliability[END_REF] were computed using OH1 and H7 data for each sport: their value indicates the reliability of the OH1 measures vs. criterion (<0.5: poor, 0.5-0.75: moderate, 0.75-0.9: good, >0.9: excellent) [START_REF] Koo | A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research[END_REF].

For the skin tone effect on HR accuracy, linear regressions established potential correlations between these discrete quantitative values and biases. All parameters including the training loads obtained from the OH1 and H7 sensors from each session were compared using Student tests. A p-value <0.05 was considered significant.

RESULTS

Examples of the simultaneous HR recordings from the H7 and OH1 are presented in Figure 1; the right figure illustrates the dropout phenomenon outside the 20-bpm criterion zone.

In all, 390 hours and 38 minutes of recordings were analyzed (cycling: 113hr,49min; running:102hr,26min; soccer: 47hr,45min; kayaking: 50hr,41min; walking: 37hr,30min; tennis: 18hr,35min; and fitness: 19hr,53min), distributed across 233 sessions. Differences in sports durations were mainly due to the much longer duration of walking and cycling sessions (up to several hours). Results are compiled in Table 1.

Sports involving more vigorous upper limbs movements (kayak, tennis and fitness) exhibited wider LOAs than endurance sports (cycling and running), whereas biases were not impacted by activity type (Fig. 2 and Table 1). ICCs for all sports were above 0.99 (Table 1), indicating the excellent reliability of OH1 data vs. criterion.

The mean value of skin type was 3.4±2.2 and it affected bias and LOAs. The correlation between skin tone and bias was positive (p<0.001), i.e., accuracy (bias) was decreased with darker skin.

Training loads were not different in walking and soccer, but they were systemically higher with the criterion H7 monitor for other sports (Table 2).

DISCUSSION

Although most studies have sought to validate similar HR devices in a laboratory environment, our research was based on data collected in real-life situations by athletes following their training routines in various type of activities.

Our first result was the difference in the proportion of data outside the threshold limitsi.e., 20 bpm from criterion value (OH1-OUT)between the arm-driven sports (tennis, kayaking and fitness) and non-arm-driven sports. The latter showed a very high percentage of useful values in endurance sports like walking, running and cycling, whereas the upper-body based sports presented a much higher error rate (Fig. 1B and1C, Table 1). In the absence of dedicated tools to assess this imprecision, we can only assume that this was due to motion artifacts from the arm and chest movements, as reported by others [START_REF] Gillinov | Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise[END_REF][START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]16]. As a corollary, these three sports also provided the widest LOAs, yet still less than those of the same brand wrist-worn model [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]. Nevertheless, the OH1 provided less than 1% data out of threshold in the traditional endurance sports (cycling, running, walking) and soccer (Table 1), which represents a very decent number for athletes and coaches relying on HR data. Although the algorithm to extrapolate HR remains Polar-proprietary, we can nevertheless assume that the 6-LED system provides reliability superior to that of the traditional 2-or 3-LED devices [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF]: subcutaneous blood information transmitted and collected through six light sources and captors is centrally analyzed, and therefore erroneous data is better detected and discarded. From the synchronized HR signals, we also note that the OH1 HR values were regularly slow to increase or decrease during intensity variations (Fig. 1C). Technically, this phenomenon can be related to delays in microvascular blood flow increases or the smoothing filters integrated into the OH1 device to avoid large errors in HR measurement. Hence, in intermittent exercise, this phenomenon can enlarge LOAs because of the shift in instantaneous HR values collected by the two systems.

In addition, as noted above, the sports showed discrepancies. Sports involving active use of arms (tennis, fitness, kayaking) led to decoupling mainly in the transitions from low to high HR, for example during acceleration (Fig. 1B), and several minutes might have been necessary for the system to readjust properly. A similar phenomenon was recently observed in more controlled conditions with another wristwatch from the same brand [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]. On a side note, a non-negligible number of recordings could not be analyzed due to criterion dysfunction, mostly in kayaking, as vigorous chest movements impaired HR detection by the H7 belt. As the standardized positioning of the OH1 on the upper arm also presented motion artifacts, it might be better to strap it to body limb less subject to data collection failure.

Similarly, biases were systematically greater in arm-driven sports, though they remained under 1 bpm and negative, implying that the OH1 measures a (very slightly) lower HR than the criterion (Table 1), as noted by others [START_REF] Horton | Comparison of Polar M600 Optical Heart Rate and ECG Heart Rate during Exercise[END_REF]19,[START_REF] Schubert | The Polar® OH1 Optical Heart Rate Sensor is Valid during Moderate-Vigorous Exercise[END_REF][START_REF] Parak | Evaluation of the beat-to-beat detection accuracy of PulseOn wearable optical heart rate monitor[END_REF] or not [16,[START_REF] Dondzila | Congruent Accuracy of Wrist-worn Activity Trackers during Controlled and Free-living Conditions[END_REF]. They also had larger Bland-Altman agreements, although just above 5% for tennis and kayaking. Mean HRs measured by the OH1 were lower for all sports except walking and soccer, but the difference was limited to a gap less than 1 bpm (Table 1), as observed for other devices [START_REF] Stahl | How accurate are the wrist-based heart rate monitors during walking and running activities? Are they accurate enough?[END_REF]19]. Yet, the minimal and maximal measured HR values showed no difference, and these undervalued data are still reliable markers in detecting overtraining risks [START_REF] Achten | Heart rate monitoring: applications and limitations[END_REF]. Therefore, differences in mean HR could lead to errors in the computation of training load, as athletes and coaches collect and analyze these valuable data to assess fitness peaks and tapering periods [START_REF] Meur | Tapering for competition: A review[END_REF]. Indeed, according to the HR data, the TRIMP values extrapolated from the OH1 device were lower than those of the criterion (Table 2) except for walking and soccer, but the difference remained small, from 3 % for kayaking to 0.7 % for cycling. Therefore, OH1 TRIMP may be used for training endurance athletes (cyclists and runners), who mostly use it in their training program [START_REF] Manzi | Relation between individualized training impulses and performance in distance runners[END_REF]. To our knowledge, no studies on OHR have focused on training loads for confirmed athletes. Overall, the ICCs confirmed the excellent correlations between OH1 and criterion HR for all sports studied here (Table 1). Interestingly, at times the OH1 provided apparently trustworthy when criterion failed. For example, several kayaking recordings were discarded because of Polar H7 dysfunction underwater (rolling) or during higher exercise intensities (maximal or submaximal intervals) when the H7 belt did not remain properly strapped around the chest [START_REF] Terbizan | Validity of Seven Commercially Available Heart Rate Monitors[END_REF]. The same observation were made in swimming, and OH1 accuracy should be evaluated in aquatic sports against a validated criterion [START_REF] Sarwe | Heart Rate and Motion Detection on Swimmers Simultaneous monitoring of heart rate and motion of swimmers[END_REF]. Similarly, when soccer players chested the ball, it occasionally displaced the HR belt and disrupted HR data, whereas the OH1 provided accurate HR values (Table 1).

Our large and various sample from the West Indies and Burgundy enabled us to compare the effect of skin tone on HR accuracy. As have other studies, our study confirms that biases become larger with darker skins [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF], though this was negligible. Nevertheless, the biases were notably very low in soccer, where more than 90% of players (Table 1) from the West Indies exhibited values on Fitzpatrick skin scale equal to or above 5, which does not agree with previous conclusions [START_REF] Spierer | Validation of photoplethysmography as a method to detect heart rate during rest and exercise[END_REF][START_REF] Shcherbina | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort[END_REF]. A plausible explanation might be West Indies' hot and humid climate, which could have induced greater peripheral vasodilation [START_REF] Johnson | Exercise in a hot environment: the skin circulation[END_REF], thereby making the blood signal more accessible to the OH1 sensor, regardless of skin tone. This device is also useful for daily heart rate monitoring in patients to assess, for example, daily energy expenditure [40], particularly in overweight and pregnant women, whose tolerance and acceptability of chest strap constriction are lower [START_REF] Andre | Recent Advances in Free-Living Physical Activity Monitoring: A Review[END_REF]. An OHR device strapped around the arm or forearm would supposedly be better tolerated.

In conclusion, the Polar OH1 was worn by athletes and studied in field conditions. It showed good overall reliability for all activities, especially traditional endurance sports like running and cycling. It might therefore be a reliable alternative to constrictive chest strap for regular and intensive training. In that matter, multiple emitters (6 in the Polar OH1) certainly play a key role. However, sports implying chest and arm movements induce a higher rate of errors and heart rate dropouts. In the future, studies will be needed to assess its accuracy for water sports, and interesting potentialities should be explored in health tracking.

Figure 1

 1 Figure 1 Examples of HR recordings of Polar H7 (dashed black line) and OH1 (solid grey line) in cyclic (left) and non-cyclic (right). Whereas both signals are mostly identical in running (panel A), Polar OH1 may show a decoupling signal out of the threshold limits (HR dropout, arrow, panel B), in non-cyclic sports (here, kayaking). Panel C illustrates another minor decoupling phenomenon (arrow), potentially impacting values of minimum, maximum and mean HR and therefore TRIMPs.

Figure 2

 2 Figure 2Bland-Altman plots of HR OH1 and HR H7 signals for cycling (left) and tennis (right), with bias (thick-dashed black line) and lower and higher values of agreement (thin-dashed black lines).

Table 1

 1 Session durations, percentage of values out of the 20bpm threshold zone (OH1-OUT), minimum / maximum / mean HR values, bias and LOAs,

	Bland & Altman (B&A) agreements, intraclass correlation coefficients (ICC) of Polar OH1 and H7 data.
	Difference H7 vs. OH1: *P < 0.05, **P < 0.01, ***P < 0.001.
	Analyzed duration	OH1 -OUT		H7	HR (bpm)		OH1		Bias	LOA (bpm)	B&A
	Activity	(min)	(%)	Min	Max	Mean	Min	Max	Mean	(bpm)	Lower Upper	agreement	ICC
	Cycling	3355	0.31%	81.3 ± 12.9	162.3 ± 18.1	132.2 ± 25.8	81.6 ± 13.6	162.2 ± 18.2	132.3 ± 25.6*	-0.08	-3.74	3.59	4.63%	0.999
	Running	2972	0.93%	94.1 ± 16.3	180.2 ± 15.2	152.1 ± 28.3	94.8 ± 16.9	180.0 ± 15.2 151.8 ± 13.7*** -0.37	-4.96	4.21	4.88%	0.998
	Walking	1124	0.07%	64.2 ± 5.3	109.6 ± 19.4	80.6 ± 9.2	63.7 ± 5.8	110.1 ± 19.0	80.4 ± 9.2	-0.18	-3.67	3.32	4.84%	0.996
	Tennis	508	8.81%	92.1 ± 13.2	186.0 ± 13.1	152.3 ± 12.5	92.5 ± 13.4	184 ± 11.8**	151.3 ± 12.3*	-0.83	-7.53	5.87	5.79%	0.995
	Kayak	1450	4.65%	87.0 ± 22.9	171.1 ± 24.9	121.5 ± 25.5	86.8 ± 23.4	167.6 ± 26.8	120.6 ± 25.4** -0.68	-8.81	7.45	5.17%	0.995
	Fitness	545	8.64%	74.8 ± 11.3	172.9 ± 19.1	124.4 ± 15.9	77.1 ± 12.9	170.7 ± 18.5	123.6 ± 15.8** -0.74	-11.08	9.6	8.68%	0.992
	Soccer	1430	0.19%	83.2 ± 15.2	189.3 ± 19.5	132.6 ± 17.2	83.5 ± 15.5	189.3 ± 19.5	132.5 ± 17.3	-0.08	-4.56	4.39	4.90%	0.999
	Mean ± SD

Table 2

 2 TRIMP values for each sport, and their mean differences on each session.

	Difference H7 vs. OH1: *P < 0.05, **P < 0.01, ***P < 0.001.
		TRIMP
		(n.u.)
	Activity	H7	OH1	Difference (%)
	Cycling	44.55 ± 76.32	44.49 ± 76.25 *	0,67 ± 0,93
	Running	74.86 ± 42.69	74.38 ± 42.46 ***	0,71 ± 0,98
	Walking	18.53 ± 12.87	18.38 ± 12.88	---
	Tennis	65.61 ± 28.25	64.63 ± 28.72 *	1,90 ± 2,39
	Kayak	29.35 ± 12.79	28.88 ± 12.94 **	3,01 ± 3,17
	Fitness	42.21 ± 23.57	41.50 ± 23.49 *	1,93 ± 1,87
	Soccer	71.78 ± 42.00	71.68 ± 42.00	---
	Mean ± SD


Eric Hermand
email: eric.hermand17@gmail.com

Aurélie Collado

Olivier Hue

Skin application of 4% menthol enhances running performance in hot and humid climate Running head: Skin menthol and external cooling: similar effect on performance

Aerobic performance is negatively impacted by tropical climate due to impairment of thermoregulatory mechanisms. We tested the hypothesis that a torso application of a 4% menthol solution would have the same effect on a best performance 10-km run as an external use of cold water. Thirteen trained male athletes completed four outdoor 10-km runs (T=29.0±1.3°C, relative humidity 59.0±13.6%) wearing a tee-shirt soaked every 2-km either in a cold (~6°C) or warm/ambient (~28°C) solution, consisting in water or in a 4% menthol solution, (CTL, MENT-Amb, CLD and MENT-CLD). Run performances were improved from 4.8 to 6.1% in CLD (51.4±5.5min), MENT-Amb (52.2±5.9min) and MENT-CLD (51.4±5.1min) conditions (vs. CTL, 55.4±8.4min, P<0.05), without differences between these three conditions, whereas heart rate (177±13bpm), body temperature (38.7±0.6°C) and drink ingestion (356±170g) were not modified. Thermal sensation after running was lower in MENT-CLD (vs. CTL, P<0.01) and thermal acceptability was higher in CLD and MENT-Amb (vs. CTL, P<0.05), but thermal comfort, feeling scale and rate of perceived exertion remained unchanged. The use of menthol on skin enhances aerobic performance in a tropical climate, and no differences in performance were observed between menthol and traditional percooling strategies. However, combining both menthol and traditional percooling brought no further improvements.

INTRODUCTION

Hot and humid climate decreases physical performances, especially in aerobic sports [START_REF] Hue | The challenge of performing aerobic exercise in tropical environments applied knowledge and perspectives[END_REF].

Although the mechanisms leading to theses alterations are not fully understood, it is well established that thermoregulatory system dysfunction is involved into the loss of ability to dissipate excess metabolic heat production [START_REF] Kenefick | Thermoregulatory function during the marathon[END_REF]. These mechanisms, already impaired in hot environment are further altered in humid climate, which drastically limits evapotranspiration due to water-saturated atmosphere [START_REF] Brotherhood | Heat stress and strain in exercise and sport[END_REF]. Hence, running may be the most impacted sport by tropical climate as heat dissipation by circulating air convection around the body, as in cycling, is reduced [START_REF] Ely | Impact of weather on marathon-running performance[END_REF]. To limit its effect in endurance sports and to enhance thermoregulatory processes, several countermeasures have been studied with various outcomes on performance: acclimation, hydration, pacing and cooling strategies [START_REF] Hue | The challenge of performing aerobic exercise in tropical environments applied knowledge and perspectives[END_REF].

Among the latter, external cooling solutions exist in the form of pads containing lowtemperature inserts and may be applied on different body parts such as arms, legs, torso and neck [START_REF] Tyler | Heat-stress exercise and cooling[END_REF]. Used before (pre-) or during (per-) exercise, they allow better performance in hot and humid environment by limiting the raise of body core temperature [START_REF] Bongers | Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review[END_REF]. For example, performances from sprints to short distance running (until 5 km) are enhanced by wearing 'ice vests' during warm-up [START_REF] Arngrïmsson | Cooling vest worn during active warm-up improves 5-km run performance in the heat[END_REF], and the use of a cooled neck-collar augments the limit time to exhaustion at aerobic sub-maximal intensity [START_REF] Luomala | Adding a cooling vest during cycling improves performance in warm and humid conditions[END_REF]. Performances are furthered improved when methods are combined (pre-and per-) [START_REF] Hasegawa | Combined effects of pre-cooling and water ingestion on thermoregulation and physical capacity during exercise in a hot environment[END_REF] and/or used with more "aggressive" cooling temperature, around 0°C [START_REF] Bongers | Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review[END_REF]. Indeed, the disequilibrium between heat production and heat loss during a prolonged exercise, leading to hyperthermia, has a negative impact on physiological functions and exercise performance: heat-dissipating mechanisms (skin vasodilatation, sweating response) are not enough to maintain an optimal body core temperature. Strategies relying on pre-(before exercise) and percooling (during) then augment heat storage and diminish the thermoregulatory strain.

Beyond traditional approaches of body cooling, efficient in laboratory-controlled environment but difficult to use in ecological conditions, alternative strategies have been recently developed to maximize performance in tropical conditions. One of them is menthol, a compound largely used in food and tobacco industries, and popular in Asia for its therapeutic properties [START_REF] Eccles | Menthol and related cooling compounds[END_REF]. It mostly acts on TRPM8 and TRPA1 thermoreceptors [START_REF] Patel | Menthol: a refreshing look at this ancient compound[END_REF] on skin and internal mucous membranes: when activated by menthol on skin, they induce a local cold sensation, prone to modify thermal sensation, especially in hot environment [START_REF] Stevens | Cooling during exercise: an overlooked strategy for enhancing endurance performance in the heat[END_REF], leading to a reduced fatigue perception during maximal and submaximal exercise, and potentially to better performances [START_REF] Schlader | The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior[END_REF][START_REF] Stevens | Running performance and thermal sensation in the heat are improved with menthol mouth rinse but not ice slurry ingestion[END_REF]. These preliminary observations should have led to further comparisons between the respective uses of menthol and external proven cooling. However, to our knowledge, there is no study directly comparing the use of menthol at ambient temperature on skin and traditional cooling strategies, to test if menthol could allow the same level of performance in ecological hot and humid environment, without the equipment burden. In other conditions, in a laboratory-controlled environment (34°C, RH 30%), there was no difference of performance between the use of cold 4% menthol gel on skin and cold pads, both applied around the heat-sensitive neck compared to control condition [START_REF] Bright | Self-paced exercise performance in the heat with neck cooling, menthol application, and abdominal cooling[END_REF]. Barwood et al. showed no difference in running performance between sprayed 0.05% menthol and water at 20°C [START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF].

Hence, in the lack of data comparing skin-applied menthol and cold-water cooling, as aerobic performance is potentially reduced by the neurosensitive perception of environmental heat and humidity, we hypothesize that the use of menthol on skin would blunt these perception inhibition mechanisms and lead to enhanced performances, similar to the external use of cold water for body cooling.

SUBJECTS AND METHODS

Subjects.

Thirteen middle to well trained and heat-acclimatized male athletes, with a performance level from 3 to 4 out of 5 [START_REF] De Pauw | Guidelines to classify subject groups in sport-science research[END_REF], participated in this study (age: 21 ± 4 years, height: 176 ± 6 cm, body mass: 70 ± 9 kg, maximal aerobic speed 16.2 ± 1.3 km.h -1 ). All participants went under prior medical examination to check for any cardiopulmonary disease or previous heat stroke, and were tested for their maximal aerobic speed [START_REF] Léger | An indirect continuous running multistage field test: the Université de Montréal track test[END_REF]. The study was approved by the National Ethic Committee (CPP, registration number 2018-A00295-50) and subjects gave their informed consent. Procedures were conformed ethics in sport and exercise science [START_REF] Harriss | Standards for ethics in sport and exercise science research: 2018 update[END_REF].

Experimental design

The protocol was conducted in ecological conditions: participants in this field study ran outside, on a flat paved road, directly exposed to the tropical heat and humidity of French West Indies (WBGT: 29.0 ±1.3°C, relative humidity 59.0 ± 13.6 %), as any athletes would be confronted in endurance races held in the same environment.

Subjects completed four 10-km run tests as fast as they could on a flat course, 4 to 7 days apart, in a randomized cross-over design including four experimental conditions in which athlete's shirt would be soaked in four different solutions: (1) Control (CTL): water at ambient temperature (T = 28.7 ± 2.9°C); (2) Cold water (CLD) at low temperature (T = 6.0 ± 0.8°C); (3) Menthol at ambient temperature (MENT-Amb): 4% menthol solution at ambient temperature (T = 28.2 ± 2.3°C); and (4) Menthol at low temperature (MENT-CLD): 4% menthol solution at cold temperature (T = 6.1 ± 0.6°C). The menthol solutions were prepared from an 86% menthol-concentrated menthol solution (Robertet, Grasse, France) which was diluted to obtain a 4% solution. Athletes were asked to restrain from training the day before the tests and avoid caffeine on test day. For every athlete, trials were undertaken at the same time of the day to limit both variations of wet bulb globe temperature (WBGT) throughout the day and circadian variations of core temperature. Finally, athletes were asked to wear the same outfit at all sessions, except the white shirt provided by the experimenter.

Experimental procedure

Heart rate (HR), stride rate and run duration were recorded continuously (sampling frequency 1 sec) using a M400 Polar watch appaired with a H7 strap belt or a OH1 sensor [START_REF] Hermand | Validation of a photoplethysmographic heart rate monitor: Polar OH1[END_REF][START_REF] Plews | Comparison of heart-rate variability recording with smartphone photoplethysmography, Polar H7 chest strap, and electrocardiography[END_REF] (Polar Electro Oy, Kempele, Finland). WBGT was continuously measured during the trials with a HD32.2 device (Delta Ohm, Padoue, Italia), placed immediately next to the run course. Core temperature (T co ) was telemetrically measured via ingestible temperature measurement pills (BodyCap, Caen, France), with an embedded memory (sampling frequency 30 sec): athletes were instructed to ingest these pills 6 to 8 h before trials to ensure the pill was out of the stomach, thereby avoiding changes in T co due to fluid consumption. Each trial included a prior 15 to 20 minutes standardized warm-up. After 5 minutes of gear equipment and weighing, they started the 10-km run on the 1-km out-and-back course. At start, and every 2 km, athletes stopped for 30 to 40 seconds, during which: 1) athletes took their shirt off; 2) the experimenter soaked it in a solution corresponding to the tested condition (CTL, CLD, MENT-Amb or MENT-CLD), adding or renewing between 250 and 300 g of solution in teeshirt fabric; 3) athletes put back their shirt on; 4) athletes could hydrate ad libitum with water at ambient temperature (T = 26.9 ± 1.6°C). The 2-km stops were necessary due to the pronounced sweating rate in the hot and humid condition, which would drip the solution off the shirt and blunt its potential effects. Finally, psychological parameters were assessed via oral or written questionnaires immediately before start: feeling scale (FS, from -5 'Very bad to +5 'Very good) [START_REF] Hardy | Not what, but how one feels: the measurement of affect during exercise[END_REF], thermal comfort (TC, from -3 'Very uncomfortable' to +3 'Very comfortable') [START_REF] Fanger | Assessment of man's thermal comfort in practice[END_REF], thermal sensation (TS, from -3 'Very cold' to +3 'Very hot') [START_REF] Goto | Thermal sensation and thermophysiological responses to metabolic step-changes[END_REF], thermal acceptability (TA, from -1 'Clearly unacceptable' to +1 'Clearly acceptable') [START_REF] Zhang | Overall thermal sensation, acceptability and comfort[END_REF]; and after the race: FS, TC, TS, TA and rate of perceived exertion (RPE, from 6 'Very, very light' to 20 'Very, very hard') [START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Barwood | Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat[END_REF][START_REF] Xiong | Effects of temperature steps on human health and thermal comfort[END_REF].

Data collection and statistical analyses

Following the guidelines of the National Ethic Committee, the different run conditons were strictly randomized to avoid effects of trial order. Run duration, HR and stride rate data were measured and averaged on the successive 2-km intervals and on the whole run, and were retrieved from Polar platform website. T co values were telemetrically updated and downloaded at the end of each session from monitors. Using calibrated balances (Terraillon, Croissy-sur-Seine, France), athletes were weighed-in, dry, shoes and shirt off, immediately before and after each run and water consumption (g) was measured after each stop: water loss during the run was then calculated by adding the weight variation to the quantity of consumed water, and the percentage of weight loss was extracted using initial body weight. Finally, the effect size was assessed by computing the Hedge's g when applicable. Statistical significance was set at P < 0.05.

RESULTS

Results are presented in table 1 and in figures 1 and 2.

- ----------------------------------------------Insert Figure 1----------------------------------------------Environmental conditions WBGT and relative humidity are presented in table 1. There was no difference between conditions.

km performance (fig. 1A)

Athletes were faster in CLD (-6.1 %), MENT-Amb (-4.8%) and MENT-CLD (-6.1 %) when compared with control (P<0.05, g=0.55, 0.44, 0.57 respectively). However, no differences were observed between MENT-Amb, MENT-CLD and CLD (fig. 1A).

km intervals

Interval splits (i.e., running performance) increased for all conditions (P<0.01, fig. 1A), meaning that athletes were getting slower throughout the run.

There was a condition effect on 2-km splits (P<0.01, fig. 1A). No differences were observed between conditions on the first two intervals. From the [4-6 km] interval to the last, split durations were longer in CTL (vs CLD, Ment-Amb and Ment-CLD, P<0.05, fig. 1A and1B).

Heart Rate

No differences were observed between conditions (fig. 1C and1D).

- ----------------------------------------------Insert Figure 2-----------------------------------------------

Stride Rate

Stride rate was not modified by condition (fig. 2A and2B).

Core temperature

T co increased throughout the run (P<0.001), for each condition (fig. 2D,P<0.001). However, there was no difference between conditions (fig. 2C).

- ----------------------------------------------Insert Table 1-------------------------------------------------

Perceptual measures

At the end of the run, FS, TS, TC and TA were all lower (P<0.001 and g=1.12, P<0.05 and g=0.40, P<0.001 and g=0.71, and P<0.05 and g=0.44, respectively) compared to the trial start.

TS at the end of the run was lower in MENT-CLD vs. CTL, and vs. CLD (P<0.01, g=1.23 and 1.22 respectively). Similarly, TA was increased in CLD, MENT-Amb (vs. CTL, P<0.05, g=0.74 and 1.04 respectively) (table 1). Condition had an effect on TA (P<0.05, table 1).

RPE, FS and TC were not modified by conditions (table 1).

Drink ingestion (table 1)

Fluid ingestion increased throughout the run (P<0.01), although there was no difference in drink ingestion between immediate successive intervals. However, some differences could be observed between intervals [0-2 km] and [4-6 km] (CLD, P<0.01; Ment-CLD, P<0.05), and between [0-2 km] and [6-8 km] (Ment-CLD, P<0.001).

Condition had no effect on drink ingestion.

Weight loss (table 1)

As expected, participants were lighter at the end of the run (P<0.001), but conditions had no effect on weight loss nor on weight loss percentage.

DISCUSSION

Our main finding points out that the use of menthol on skin in men enhances aerobic performance in a hot and humid climate, and that there is no difference in performance gain between menthol and traditional percooling strategy; core temperature was not raised by a faster pace, and menthol brought a lower thermal sensation. However, combining both menthol and percooling did not bring any further improvements.

First, overall performance was positively impacted by cold and menthol conditions whereas Barwood reported no difference between menthol and water sprays (at ambient temperature) in run speed on a 5-km run held in hot and humid environment [START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF]. In a laboratorycontrolled study, an only acute applying of an 8% menthol gel on the face increased the time limit in a high intensity exercise by more than 20% in a time-to-exhaustion tests on ergometer [START_REF] Schlader | The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior[END_REF], demonstrating a positive effect of menthol on performance.

Second, run speed decreased continuously from the start in CTL condition (fig. 1A and1B), which did not happen in the other conditions. In the study of Barwood et al. [START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF], there was no difference in performance between conditions and athletes' speed remained steady during the 5-km run, whereas we found a negative effect of time on performance during our longer run. Moreover, HR (fig. 1C and1D) and core body temperature (fig. 2C and2D) remained stable between conditions (fig. 1C and1D), even if athletes were performing better, which was also reported in other works [START_REF] Stevens | Running performance and thermal sensation in the heat are improved with menthol mouth rinse but not ice slurry ingestion[END_REF][START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Mündel | The effects of swilling an L(-)-menthol solution during exercise in the heat[END_REF][START_REF] Gillis | The influence of menthol on thermoregulation and perception during exercise in warm, humid conditions[END_REF]. As for the heat-related perception measures, participants felt the same lower TS at the end of the run than reported in other works [START_REF] Schlader | The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior[END_REF][START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Barwood | Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat[END_REF][START_REF] Gillis | The influence of a menthol and ethanol soaked garment on human temperature regulation and perception during exercise and rest in warm, humid conditions[END_REF] while TC was also decreased [START_REF] Flood | Oral l-menthol reduces thermal sensation, increases work-rate and extends time to exhaustion, in the heat at a fixed rating of perceived exertion[END_REF] or maintained/augmented [START_REF] Schlader | The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior[END_REF][START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Barwood | Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat[END_REF] (table 1). It is confirmed here by a better acceptability of the heat both in cold and menthol conditions. RPE was not modified by menthol, meaning that subjects performed all the tests with the same maximal perceived intensity whereas run duration was better for some conditions: this therefore suggests an existing enhancing-performance effect in these conditions [START_REF] Gillis | The influence of a menthol and ethanol soaked garment on human temperature regulation and perception during exercise and rest in warm, humid conditions[END_REF]. The underlying physiological mechanisms are still to be fully understood, but the type of activity (running, cycling) or the environmental conditions (dry or humid heat) might play a role.

In our study, cold water cooling and 4% menthol produced a similar effect on performance: the first strategy is long-known to buffer and help dissipate the excess metabolic-heat [START_REF] Bongers | Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review[END_REF] whereas the second modifies the perception of the environmental heat and humidity [START_REF] Stevens | Menthol: A fresh ergogenic aid for athletic performance[END_REF]. Our study design also allowed us to study the effects of separate (CLD, Ment-Amb) and combined (Ment-CLD) conditions, but we found no further improvement of the latter compared to CLD or Ment-Amb taken separately, as an additive effect could have been expected. This underlines a limit in the combination of multiple strategies to maintain a thermal homeostasis when exercising in a tropical climate. If the effect of external cooling is well understood, mainly by augmenting the heat capacity, the mechanisms elicited by the internal or external use of menthol remain to be fully understood.

The absence of difference in T co between conditions (fig. 2C) implies that, at a faster pace, the thermoregulatory system would have to dissipate a greater amount of metabolic heat. If we assume that the low temperature solutions (~6°C) soaking the tee-shirt in CLD and MENT-CLD conditions were able to absorb a portion of the excess heat, this mechanism was not possible in the MENT-Amb condition, as the solution temperature remained above 28°C.

Regarding the non-modified T co in all conditions, this raises interrogations about the functioning of thermoregulatory mechanisms, already impaired by the humid atmosphere, and by the limited cooling by air convection during running [START_REF] Kenefick | Thermoregulatory function during the marathon[END_REF]. Moreover, this excess of heat could not be evacuated through the process of water ingested / loss of water (sweat), as there was no difference, on one hand, between environment and beverage temperatures (heat capacity), and on the other hand, between weight loss in the different conditions (physically evacuated heat through sweat). If aerobic performance strongly depends on core temperature and by its capacity to dissipate excess heat, an alternative mechanism may play a key role in thermoregulation under these harsh conditions, here potentially triggered by menthol applied on skin. This mechanism could be related to the effect of menthol on peripheral blood circulation. A first study reported a skin vasoconstriction when a 3.5% menthol gel was applied on skin [START_REF] Topp | Comparison of the effects of ice and 3.5% menthol gel on blood flow and muscle strength of the lower arm[END_REF], which was only noted only during the first 5 min after application. In more recent studies, a vasodilatation phenomenon was observed from 5 min after application and was maximal between 15 and 45 min; a dose-dependent activation was also recorded, optimal from 4 to 7% concentration [START_REF] Craighead | Mechanisms and time course of menthol-induced cutaneous vasodilation[END_REF], associated with a proportional related-cold sensation [START_REF] Green | The sensory effects of l-menthol on human skin[END_REF]. These menthol-related mechanisms in peripheral vasodilatation would activate, other than TRPM8 thermoreceptors [START_REF] Patel | Menthol: a refreshing look at this ancient compound[END_REF], multiple vasodilator pathways, such as nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF), hence increasing blood flow.

Although these studies were performed at rest in thermoneutral conditions, these recent advances could lead to further studies in order to assess these effects at exercise, in hot/humid environment. This opening could also bring a novel light to the central governor model [START_REF] Noakes | Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia[END_REF],

regulating afferent and efferent mechanisms to prevent the occurrence of bodily harm, for example by adjusting race pace under a high thermal stress. Whereas the cold sensation elicited by the use of menthol would act on the central governor through thermosensitive afferences and therefore "trick" it into adopting a higher exercise intensity [38], locally NOmediated and EDHF actions would promote a cutaneous vasodilatation to counteract a greater metabolic heat production. This would lead to a higher core temperature, and therefore maintain a system homeostasis as long as possible, or as long as the central command is under the influence of TRPM8 afferences. Regarding core temperature, it's noticeable that numerous previous studies using menthol reported no increase in T co [START_REF] Stevens | Running performance and thermal sensation in the heat are improved with menthol mouth rinse but not ice slurry ingestion[END_REF][START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Mündel | The effects of swilling an L(-)-menthol solution during exercise in the heat[END_REF][START_REF] Gillis | The influence of menthol on thermoregulation and perception during exercise in warm, humid conditions[END_REF]. This could be interpreted as a defense mechanism, peripherally triggered, and would compensate a dysfunction from the central command potentially leading to body harm. Moreover, although experimental conditions were designed to preserve a single-blind protocol, it was possible that the aroma from menthol impregnated in the tee-shirt fabric would reach the respiratory system through the nasal passages. As inhaled menthol provokes a large increase of ventilation at rest and exercise [START_REF] Meamarbashi | The effects of peppermint on exercise performance[END_REF], an additional heat loss could be made through the augmented respiratory process [START_REF] Cain | Respiratory heat loss during work at various ambient temperatures[END_REF].

Finally, as menthol promotes a higher exercise intensity through sensory nerve-dependent mechanisms [38], our TS data confirm a significant cooling sensation felt by athlete after the run, as observed in other works [START_REF] Schlader | The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior[END_REF][START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Barwood | Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat[END_REF][START_REF] Stevens | Menthol: A fresh ergogenic aid for athletic performance[END_REF][START_REF] Topp | Comparison of the effects of ice and 3.5% menthol gel on blood flow and muscle strength of the lower arm[END_REF], potentially acting on central command. This is also confirmed in our study by a higher TA in MENT-Amb condition (vs. CTL, P<0.05, table 1). However, we did not observe changes in TC (table 1), unlike other works [START_REF] Barwood | Spraying with 0.20% L-menthol does not enhance 5 km running performance in the heat in untrained runners[END_REF][START_REF] Barwood | Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat[END_REF][START_REF] Barwood | Early change in thermal perception is not a driver of anticipatory exercise pacing in the heat[END_REF][START_REF] Topp | Comparison of the effects of ice and 3.5% menthol gel on blood flow and muscle strength of the lower arm[END_REF]. To summarize, when using menthol in tropical climate, athletes ran faster and felt "comfortably fresher" than in CTL condition.

In conclusion, our works show improved 10-km performance in ecological conditions when using a 4% menthol skin application. This enhancement was the same as using cold water or cold menthol. The use of a higher menthol concentration (4%) than those used in most studies underlines a dose effect of menthol, while physiological parameters, such body core Mean values (± SD) of environmental condition (WBGT and relative humidity RH) rate of perceived exertion (RPE, from 6 'Very, very light' to 20 'Very, very hard'), feeling scale (FS, from -5 'Very bad to +5 'Very good), thermal sensation (TS, from -3 'Very cold' to +3 'Very hot'), thermal comfort (TC, from -3 'Very uncomfortable' to +3 'Very comfortable'), thermal acceptability (TA, from -

  Psychological parameters were compiled before and after each run. Normality of data on each condition was verified by a Kolmogorov-Smirnov test. Thus, repeated measures ANOVAs were conducted with Condition (i.e., CTL, CLD, MENT-Amb, MENT-CLD) as the betweenfactor and Time (i.e., 2-km intervals) as the within-factor. Posthoc Student t-tests were performed when applicable, to compare mean values between successive intervals ([0-2 km] vs. [2-4 km], [2-4 km] vs. [4-6 km], [4-6 km] vs. [6-8 km] and [6-8 km] vs. [8-10 km]) to assess the kinetics of the run pace throughout the 10-km course, and between conditions.

  Interval splits continuously increased from the start in CTL condition ([0-2 km] vs. [2-4 km], P<0.05; [2-4 km] vs. [4-6 km], P<0.001; [4-6 km] vs. [6-8 km], P<0.01) before stabilizing on the two last intervals. In CLD, Ment-Amb and Ment-CLD, after some pace variations (or not), speed was not modified on the last three intervals ([4-6 km], [6-8 km] and [8-10 km]).

  temperature and heart rate, did not show further impaired thermal stress compared to control condition. The underlying mechanisms are not fully understood yet, but recent findings on menthol-related cutaneous vasodilatation open new perspectives of research and bring a novel light on the central governor theory. Regarding the potential physiological mechanisms induced by the application of menthol on skin, future studies may focus on the effects of different concentration of menthol on aerobic performance, especially in long-duration

  1 'Clearly unacceptable' to +1 'Clearly acceptable'), ingested drink and percentage of weight loss.

	*P<0.05, **P<0.01: vs. CTL
	++P<0.01: vs. CLD
	†P<0.05: vs. MENT-AMB
			CTL	CLD	MENT-AMB	MENT-CLD
	Environmental	WBGT (°C)	29.1 ± 1.5	28.9 ± 1.2	28.8 ± 1.5	29.1 ± 1.2
	conditions	RH (%)	57.7 ± 12.9	60.0 ± 13.4	63.6 ± 14.5	54.6 ± 13.3
	RPE (n.u.)		15.8 ± 2.2	14.5 ± 3.1	14.6 ± 1.8	15.6 ± 1.7
		Pre	2.0 ± 2.0	2.3 ± 2.1	2.5 ± 2.1	1.9 ± 2.3
	FS (n.u.)
		Post	-0.8 ± 2.3	0.1 ± 2.3	0.7 ± 2.0 *	-0.9 ± 1.7 †
		Pre	1.2 ± 0.9	0.7 ± 0.9	1.0 ± 0.8	1.1 ± 1.0
	TS (n.u.)
		Post	1.2 ± 1.4	1.1 ± 0.9	0.0 ± 1.7	-0.4 ± 1.2 ** ++
		Pre	0.5 ± 0.7	0.6 ± 0.6	0.8 ± 0.7	0.6 ± 0.8
	TC (n.u.)
		Post	-0.5 ± 1.2	-0.1 ± 1.2	0.3 ± 0.9	0.2 ± 0.6
		Pre	0.3 ± 0.4	0.3 ± 0.3	0.5 ± 0.3	0.3 ± 0.4
	TA (n.u.)
		Post	-0.1 ± 0.5	0.2 ± 0.4 *	0.4 ± 0.5 *	0.2 ± 0.5
	Drink ingested (g)		363 ± 173	367 ± 162	318 ± 164	379 ± 198
	% weight loss		-1.3 ± 0.8	-1.0 ± 0.6	-1.4 ± 1.0	-1.2 ± 0.6

Captions

Table 1

Mean values (± SD) of rate of perceived exertion (RPE, from 6 'Very, very light' to 20 'Very, very hard'), feeling scale (FS, from -5 'Very bad to +5 'Very good), thermal sensation (TS, from -3 'Very cold' to +3 'Very hot'), thermal comfort (TC, from -3 'Very uncomfortable' to +3 'Very comfortable'), thermal acceptability (TA, from -1 'Clearly unacceptable' to +1 'Clearly acceptable'), ingested drink and percentage of weight loss. *P<0.05, **P<0.01: vs. CTL ++P<0.01: vs. CLD †P<0.05: vs. MENT-AMB