C Chabert

E Hermand

Pr Olivier Hue

Ultra-endurance events in tropical environments and countermeasures to optimize performances and health Running head: Aerobic performance in tropical climate

Keywords: Performance, tropical climate, heat, humidity, triathlon, ultra-endurance

Physical performance in a tropical environment, combining high heat and humidity, is a difficult physiological challenge that requires specific preparation. The elevated humidity of a tropical climate impairs the thermoregulatory mechanisms by limiting the rate of sweat evaporation.

Hence, a proper management of whole-body temperature is required to complete an ultra-endurance event in such an environment. In these long-duration events, which can last from 8 to 20 h, held in hot and humid settings, performance is tightly linked to the ability in maintaining an optimal hydration status. Indeed, the rate of withdrawal in these longer races was associated with lower water intake, and the majority of finishers exhibited alterations in electrolyte balance (e.g. sodium).

Hence, this work reviews the effects on performance of high heat and humidity in two representative ultra-endurance sports, ultramarathons and long-distance triathlons, and several countermeasures to counteract the impact of these harsh environmental stresses and maintain a high level of performance, such as hydration, cooling strategies and heat acclimation.

Influence of heat on performance and health

It is now well established that aerobic performance and health can be deeply impacted by a hot and humid climate [START_REF] Racinais | Consensus recommendations on training and competing in the heat[END_REF]. In particular, long-duration races, such ultra-marathons or long-distance triathlons, are highly concerned by these environmental stresses due to a much longer exposure to these conditions compared to most events, from short-distance runs or triathlons to marathons.

1.1. Ultra-endurance events: ultramarathon 1.1.1. Ultra-endurance in the heat Ultramarathon is a running event which lasts more than 6 h and usually presents a high dénivelé, from hundreds to thousands meters elevation [START_REF] Zaryski | Training principles and issues for ultra-endurance athletes[END_REF]. In the past decade, trail running races have become a major ultra-endurance sport. Since the first edition of the 'Ultra Trail du Mont Blanc' (160 km and +10,000 m of elevation), the number of finishers has grown from 67 in 2003 to 1685 in 2017. This specific effort involves several physiological and psychological parameters that are modulated by environmental factors. Ultramarathon races are performed in a variety of off-road terrains, elevation profiles (positive: D+ and negative: D-changes) and distances, all of which may greatly complicate the understanding how environmental conditions affect performance. Marathon races present more standardized characteristics, facilitating the comparison of performances in order to study the role played by heat [START_REF] Adams | Thermoregulation during marathon running in cool, moderate, and hot environments[END_REF]. The analysis of winners' times in several marathons, evidenced that the optimal ambient temperature for running performance is between 10°C and 12°C for elite athletes [START_REF] Maughan | Distance running in hot environments: a thermal challenge to the elite runner[END_REF]. Above this temperature, data from 136 marathons performed between a wet-bulb globe temperature (WBGT) of 5 and 25°C showed a negative correlation between WBGT and running performance (Figure 2) [START_REF] Ely | Impact of weather on marathon-running performance[END_REF]. Interestingly, the same authors also showed that the performance of slower runners, exposed to heat for a longer time than faster runners, are more affected by higher temperatures. In ultramarathon distances, a similar effect of a hot environment on performance was observed in a study that analysed running times in the Western States Endurance Run (161 km and 6000 m D+) [START_REF] Parise | Influence of temperature and performance level on pacing a 161 km trail ultramarathon[END_REF]. The authors followed 50 runners who completed the race in two consecutive years (2006 and 2007) to investigate the effect of temperature changes, from 7-38°C in 2006 to 2-30°C in 2007 (figure 3). Regardless of their level, athletes were 8% slower during the 'hot' year (2006) than during the 'cooler' (2007), and the withdrawal rate was 14% higher in 2006. Slower runners did not appear to be more impacted by the hot environment, but this observation was probably due to the longer time running at a comfortable temperature (i.e. during the night).

Hydration requirements during ultra-endurance exercise in the heat

During muscle contraction, only ~25% of the substrate energy is converted to mechanical work, with the other ~75% released as heat. An effective thermoregulation during exercise relies on the balance between the absolute mechanical work generated by the athlete and the heat dissipating process. Ultramarathons, characterized by a low and steady relative exercise intensity over very long periods, do not significantly challenge the heat balance in temperate environments. However, maintaining mechanical load over a longer time may challenge heat balance in athletes, especially when competing in hot and/or humid environments. Furthermore, environmental heat stress (i.e. temperature) reduces dry heat loss, due to the small (or even negative) temperature gradient between the skin and air, which progressively puts sweat evaporation as the only mechanism available for the organism to dissipate endogenous heat [START_REF] Brotherhood | Heat stress and strain in exercise and sport[END_REF][START_REF] Taylor | Challenges to temperature regulation when working in hot environments[END_REF]. Indeed, in 30°C conditions, a dry heat loss of 75 W is observed whereas it induces a dry heat gain of 75 W at 40°C, increasing the sweating rate required to maintain body core temperature [START_REF] Gagnon | The evaporative requirement for heat balance determines whole-body sweat rate during exercise under conditions permitting full evaporation[END_REF]. Thus, ultramarathons performed in a hot environment require greater water intake to compensate increased water losses. During the Badwater Ultramarathon of 2012 (217 km and 4000 m D+), the temperature oscillated from 10.1°C at night to 46.6°C during the day. The follow-up of 4 runners who completed the race in 36±3 h showed a mean water intake of 34±13 [START_REF] Brown | Food and Fluid Intake During Extreme Heat: Experiences From The Badwater Ultramarathon[END_REF]. On average, each athlete drank 0.93 L.h -1 , whereas on completion of the Biel Ultramarathon in Switzerland (100 km and 645 m D+), in a temperate environment varying from 8 to 28°C, the runners consumed 0.65 L.h -1 [START_REF] Knechtle | Low prevalence of exercise-associated hyponatremia in male 100 km ultra-marathon runners in Switzerland[END_REF]. When the distances of these two races were linked to elevation changes (100 m D+ is equivalent to 1 km distance), the adjusted distance for the Biel Ultramarathon was 106.45 km and 257 km for Badwater. Hence, the Badwater runners drank almost twice as much water as the Biel runners (0.13 L.km -1 and 0.069 L.km -1 , respectively), despite a higher mechanical workload developed during the Biel Ultramarathon due to its shorter distance and lower elevation change (6.6 min.km -1 vs. 8.5 min.km -1 for Badwater). The characteristics of the 2007 Peninsula Ultra Fun Run (PUFfeR) of South Africa (80 km and 1000 m D+) were close to those of the Biel Ultramarathon, but the participants were subjected to lower environmental temperatures (8-20°C). The runners drank less during this more temperate race than during the Biel Ultramarathon (0.028 L.km -1 and 0.069 L.km -1 , respectively) [START_REF] Tam | Changes in total body water content during running races of 21.1 km and 56 km in athletes drinking ad libitum[END_REF]. The analysis of the hydration status of 16 ultramarathon runners (161 km, 7000 m D+) in a hot environment (4.8-37.8°C) showed that finishers (n=6) had drunk significantly more water at the 48 th km than non-finishers (n=10) [START_REF] Stuempfle | Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace[END_REF]). However, the authors were unable to determine whether beverage intake was directly linked to finishing capacity or if it was due to other factors, such as runners' experience or endurance capacity. A second study was focused on a 160km foot race (positive elevation not known) with temperatures that peaked at 38°C, and followed only runners who had completed at least one previous ultramarathon among the top 50% [START_REF] Glace | Food intake and electrolyte status of ultramarathoners competing in extreme heat[END_REF]. The authors showed that, despite their homogenous level and experience, non-finishers also drank significantly less than finishers (-35%, p<0.01). These data confirm the tight relationship between event temperature and water intake, and that hydration strategies remain essential for health and performance. Another study showed an average 0.19 L.km -1 sweat loss in runners during the 2011 Gwada Run, multi-stage race (6 days, 142 km) held in tropical conditions (30 ± 2.4°C and 82 ± 4% RH) [START_REF] Hue | Thermoregulation, hydration and performance over 6 days of trail running in the tropics[END_REF]. Yet water intake (1.5±0.3 L per stage) was probably distorted by the shortness of the stages (from 16 to 21 km), which enabled athletes to tolerate transient dehydration during the races (-4.2±0.9 L per stage).

Triathlon

Ironman and Challenge triathlon series are trademark brands for long-distance triathlons consisting of a 3.8-km swim, a 180-km cycling leg and a closing full marathon (42.195 km). The best professional triathletes complete the distance under 7 h and 50 min, whereas time of age group athletes may vary from 9 to 13 h and more, making this race one of the longest endurance races.

The duration depends on several factors, including terrain (e.g. course route, technicity, elevation) and environmental conditions. For the latter, hot and humid climate impacts the performance and the rate of withdrawal, as observed in the Kona (Hawaii) Ironman World Championships, held every October [START_REF] Giambelluca | Evapotranspiration of Hawaiʻi: final report to the US Army Corps of Engineers-Honolulu District and the Commission on Water Resource Management[END_REF][START_REF] Stiefel | A comparison of participation and performance in age-group finishers competing in and qualifying for Ironman Hawaii[END_REF]: top age-groupers were slower in Hawaii than in their qualifier races [START_REF] Stiefel | A comparison of participation and performance in age-group finishers competing in and qualifying for Ironman Hawaii[END_REF],

and abandon rate can reach 10%, a high proportion considering the higher fitness level of athletes participating in this particular event, more resistant to heat stress [START_REF] Cheung | Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress[END_REF]18]. Similarly, more than 25% of the athletes did not reach the finish line at the inaugural Ironman Vichy in 2015, which was held in unusually hot conditions (31~35°C), in contrast to North American races which present a minimum 95% finish rate (Britt 2011).

During long-distance triathlon, heat stress specifically impacts each successive leg: swimming, cycling and running.

Swimming

Most long distance triathlons allow neoprene wetsuits to be worn for the swim leg as long as water temperature does not exceed 24.5°C. Between 24.5°C and 28.8°C, athletes who choose to wear a wetsuit are allowed to participate, but are not eligible for age-group awards or for qualifying slots for the Ironman World Championships; over 28.8°C wetsuits are prohibited (Ironman 2018). Enhancing swimmer's buoyancy, the neoprene wetsuit reduces ventilation and O 2 consumption ( O 2 ) at a given swimming speed, and therefore decreases metabolic heat production [START_REF] Trappe | Physiological responses to swimming while wearing a wet suit[END_REF].

However, wetsuits also reduce heat dissipation capacity and may lead to a higher core body temperatures and subsequent dehydration [START_REF] Kerr | Hyperthermia during Olympic triathlon: influence of body heat storage during the swimming stage[END_REF][START_REF] Laursen | Core temperature and hydration status during an Ironman triathlon[END_REF]. Hence, although proscribing wetsuits in warm water may impair swimming performance, it also might decrease the existing risk of hyperthermia [START_REF] Wakabayashi | Thermal Insulation and Body Temperature Wearing a Thermal Swimsuit during Water Immersion[END_REF]. However, interestingly, Kerr et al. showed in a simulated Olympic triathlon that core temperature was not modified by wearing a wetsuit, thought to limit thermoregulatory mechanisms through heat-insulating properties of neoprene [START_REF] Kerr | Hyperthermia during Olympic triathlon: influence of body heat storage during the swimming stage[END_REF], as excess heat was still transferred to the periphery, leading to a higher skin temperature. Some field studies confirmed that core temperature remained around 38°C in well-trained athletes wearing a wetsuit in 20.5°C water during a 3.8 km swim [START_REF] Laursen | Core temperature and hydration status during an Ironman triathlon[END_REF], and in triathletes completing a long-distance Ironman, whose core temperature was continuously monitored [START_REF] Laursen | Hyperthermic fatigue precedes a rapid reduction in serum sodium in an ironman triathlete: a case report[END_REF] . In 29.5°C water, another study showed that the core temperature in moderately trained athletes increased by an average 0.7°C but remained under 38°C during a nonwetsuit half-Ironman (Figure 1). Thus, to this day, no study evidenced a critical elevation of core temperature during the swim leg of long-distance triathlon events. Moreover, while 13 of the 14 documented deaths in triathlon races from 2006 to 2008 occurred during the swim leg, pre-existing cardiovascular abnormalities seem to have been the major factor, ruling out hyperthermia as a potential cause [START_REF] Harris | Sudden Death During the Triathlon[END_REF].

Cycling

During cycling, some of the heat produced by the muscles is dissipated via air convection around the body. This, however, depends on a sufficient temperature gradient between the skin and ambient air. Convection mechanisms are therefore limited in hot conditions when air temperature exceeds 35°C. Heat is also dissipated during cycling via sweat evaporation, itself depending on the pressure gradient of water vapor between air and skin surface. Like convection, this mechanism is reduced in a humid environment, making performance more challenging as thermoregulatory efficiency is dramatically reduced [START_REF] Nybo | Cycling in the heat: performance perspectives and cerebral challenges[END_REF]. In a tropical climate (31-33°C, 70-75% RH), one-hour of pedalling at a submaximal intensity increases core temperature, heart rate, sweat rate and water loss [START_REF] Saat | Effects of short-term exercise in the heat on thermoregulation, blood parameters, sweat secretion and sweat composition of tropicdwelling subjects[END_REF][START_REF] Saat | Effects of exercise in the heat on thermoregulation of Japanese and Malaysian males[END_REF][START_REF] Voltaire | Influence of a hot/wet environment on exercise performance in natives to tropical climate[END_REF]. These results can be extrapolated to describe the physiological impact of 4-6-h events, especially if the cycling course includes hills, where the diminished air speed reduces heat dissipation by convection [START_REF] Nybo | Cycling in the heat: performance perspectives and cerebral challenges[END_REF]. Laboratory-controlled studies showed that increasing ambient temperature impaired cycling capacity when relative humidity was clamped at 70% [START_REF] Galloway | Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man[END_REF][START_REF] Maughan | Influence of relative humidity on prolonged exercise capacity in a warm environment[END_REF]. During a 9-day (2.5 h/day) cycling race held in Guadeloupe (31.1°C, 75.6% RH), tympanic temperatures measured immediately after the stages never exceeded a 2°C rise compared to resting pre-race values [START_REF] Hue | Heart Rate, Thermoregulatory and Humoral Responses During a 9-Day Cycle Race in a Hot and Humid Climate[END_REF]. In contrast to the intrinsic variability associated with stage racing in a peloton, triathletes usually adopt a much steadier pace during non-drafting racing to "save the legs" for the following marathon [START_REF] Wu | Pacing strategies during the swim, cycle and run disciplines of sprint, Olympic and half-Ironman triathlons[END_REF]. Interestingly, Baillot et al showed a negative correlation between body mass and the evolution of core temperature throughout the cycling leg, illustrating greater heat storage and inertia in larger athletes [START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF]. Thus, a larger body presents a larger surface which allow to preserve heat exchange at high velocity, even in a hot and humid environment [START_REF] Nielsen | Olympics in Atlanta: a fight against physics[END_REF].

In summary, the higher velocity and the intrinsic non-drafting nature of cycling in triathlon promotes heat dissipation by convection only if air temperature is lower than skin's. In a thermally challenging environment, sweat evaporation remains the main process for heat dissipation but is limited by the high humidity of a tropical environment. Nevertheless, triathletes appear to show adequate thermoregulatory capability to prevent excessive hyperthermia, mainly through intensity management and pacing strategies [START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF].

Running

Of the three sports in the triathlon, running may carry the greatest risk of hyperthermia for triathletes competing in a tropical climate. Several factors and stressors come into play. Firstly, although running is faster than swimming, air possesses neither the heat capacity nor the transfer celerity of water, hence limiting the quantity of heat energy dissipated from the body. Secondly, unlike in cycling, the lower speed of running does not allow excess heat dissipation via air convection around the body [START_REF] Nielsen | Olympics in Atlanta: a fight against physics[END_REF]. These factors may have a strong negative impact on running performance [START_REF] Kenefick | Thermoregulatory function during the marathon[END_REF], particularly in a marathon [START_REF] Ely | Impact of weather on marathon-running performance[END_REF]. Furthermore, this negative impact is amplified under two additional factors: (i) a heavier body mass, at a given pace, requires more energy for running locomotion and therefore produces a larger quantity of metabolic-related heat; and (ii) heat exchange by convection and evaporation is limited in heavier athletes running in a hot/humid environment, who display a greater imbalance between heat production and dissipation, even at slower paces [START_REF] Marino | Advantages of smaller body mass during distance running in warm, humid environments[END_REF].

While the core temperature data of professional triathletes in the Hawaii Ironman or other hot/humid locations are not available, a few studies in moderately to very well trained age group athletes pointed out that they did not suffer from heat-related illness and excessive increases in core temperature [START_REF] Laursen | Core temperature and hydration status during an Ironman triathlon[END_REF] (Figure 1). Laursen et al showed, in 7 out of 10 well trained triathletes racing a high-level sub-10-hour performance at Ironman Western Australia in temperate but humid conditions (23.3°C, 60% RH), that the average core body temperature remained close to 38°C [START_REF] Laursen | Core temperature and hydration status during an Ironman triathlon[END_REF].

Despite a much hotter and more humid environment in Guadeloupe (27.2°C, 80% RH), the average body temperature of moderately trained athletes during a half-Ironman run was 38.2°C and none of them reached 40°C [START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF]. These values are coherent with the relatively lower intensity and absolute workload at which most long-distance triathlon races are performed, in contrast with shorter events at a higher intensity and thus inducing a higher heat production [START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF][START_REF] Siegel | Ice slurry ingestion increases core temperature capacity and running time in the heat[END_REF]. The key role of exercise intensity on core temperature was confirmed in dryer conditions in a study showing that the fastest athletes exhibited the highest core temperatures [START_REF] Coso | Relationship between physiological parameters and performance during a half-ironman triathlon in the heat[END_REF].

Countermeasures to optimize performance and health

Hydration

Limiting dehydration during triathlons and ultramarathons in hot environments seems to be essential in order to maintain a mechanical workload over 6~7 hours. The key role of hydration strategy in a tropical climate is critical since it has been shown that combined hyperthermia and dehydration are worse than hyperthermia alone [START_REF] Sawka | Hydration effects on thermoregulation and performance in the heat[END_REF][START_REF] Sawka | Does dehydration impair exercise performance?[END_REF]. Despite contradictory observations on thermoregulatory mechanisms and core temperature [START_REF] Latzka | Hyperhydration: thermoregulatory effects during compensable exercise-heat stress[END_REF][START_REF] Ross | Effects of lowering body temperature via hyperhydration, with and without glycerol ingestion and practical precooling on cycling time trial performance in hot and humid conditions[END_REF], it seems that hyperhydration can delay the development of dehydration [START_REF] Latzka | Hyperhydration: thermoregulatory effects during compensable exercise-heat stress[END_REF][START_REF] Latzka | Hyperhydration and glycerol: thermoregulatory effects during exercise in hot climates[END_REF][START_REF] Sawka | Influence of Hydration Level and Body Fluids on Exercise Performance in the Heat[END_REF]. However, hyperhydration did not lead to enhanced performances during a laboratory-controlled 46 km cycling time trial by elite cyclists [START_REF] Ross | Effects of lowering body temperature via hyperhydration, with and without glycerol ingestion and practical precooling on cycling time trial performance in hot and humid conditions[END_REF] or a 60 min run by trained runners [START_REF] Scheadler | Glycerol hyperhydration and endurance running performance in the heat[END_REF]. As hyperhydration results in non-negligible added body weight, it may not be an advantage for running performance, greatly impacted by extra weight [START_REF] Teunissen | Effects of independently altering body weight and body mass on the metabolic cost of running[END_REF]. In addition, it does not enhance thermoregulation in a hot and humid climate and augments the risk of hyponatremia [START_REF] Noakes | Water intoxication: A possible complication during endurance exercise[END_REF].

In triathlon, although hydration depends very much on beverage availability provided by race organization, descriptive papers report that no triathlete has suffered from dehydration symptoms in hot and humid events, despite occasional significant water losses [START_REF] Laursen | Core temperature and hydration status during an Ironman triathlon[END_REF][START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF][START_REF] Coso | Relationship between physiological parameters and performance during a half-ironman triathlon in the heat[END_REF][START_REF] Speedy | Fluid balance during and after an ironman triathlon[END_REF]. This suggests that hydration strategies used by athletes meet the body's water requirements [START_REF] Kenefick | Drinking Strategies: Planned Drinking Versus Drinking to Thirst[END_REF].

Maintaining an optimal hydration level is more complicated in ultramarathon. Indeed, analysis of the hydration status of ultramarathon runners in two different races showed that finishers drank significantly more water than non-finishers, even if the runners' experience was taken into account [START_REF] Glace | Food intake and electrolyte status of ultramarathoners competing in extreme heat[END_REF][START_REF] Stuempfle | Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace[END_REF]. In contrast, slower runners became over-hydrated because they feared dehydration, potentially leading to hyponatremia [START_REF] Hew-Butler | Statement of the Second International Exercise-Associated Hyponatremia Consensus Development Conference, New Zealand[END_REF][START_REF] Noakes | Drinking guidelines for exercise: what evidence is there that athletes should drink "as much as tolerable", "to replace the weight lost during exercise" or "ad libitum[END_REF]. Hyponatremia however, is not only due to overhydration, but may be the consequence of insufficient sodium intake during the race [START_REF] Glace | Food intake and electrolyte status of ultramarathoners competing in extreme heat[END_REF][START_REF] Stuempfle | Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace[END_REF]. In a hot environment, hyponatremia affected 30-50% of ultramarathon finishers, reflecting the inadequate or incorrect hydration strategies of numerous runners [START_REF] Glace | Food intake and electrolyte status of ultramarathoners competing in extreme heat[END_REF][START_REF] Hoffman | Hyponatremia in the 2009 161-km Western States Endurance Run[END_REF][START_REF] Lebus | Can changes in body mass and total body water accurately predict hyponatremia after a 161-km running race?[END_REF]. Thus, additional sodium intake during ultramarathons could contribute to faster performances by stimulating thirst, increasing voluntary fluid intake, enhancing intestinal glucose and water absorption, optimizing extracellular and intracellular fluid balance, and potentially mitigating the occurrence of clinically significant episodes of hyponatremia [START_REF] Maughan | Fluid replacement and exercise stress. A brief review of studies on fluid replacement and some guidelines for the athlete[END_REF][START_REF] Shirreffs | Fluid and electrolyte needs for training, competition, and recovery[END_REF][START_REF] Speedy | Hyponatremia in ultradistance triathletes[END_REF][START_REF] Vrijens | Sodium-free fluid ingestion decreases plasma sodium during exercise in the heat[END_REF].

Heat acclimation

Heat acclimation (HA) before a race is an efficient way to limit the decrease of performance in a hot environment [START_REF] Armstrong | The Induction and Decay of Heat Acclimatisation in Trained Athletes[END_REF][START_REF] Nielsen | Heat stress and acclimation[END_REF], with physiological adaptations ranging from the fifth day [START_REF] Chalmers | Short-Term Heat Acclimation Training Improves Physical Performance: A Systematic Review, and Exploration of Physiological Adaptations and Application for Team Sports[END_REF] to a two-week span [START_REF] Chalmers | Short-Term Heat Acclimation Training Improves Physical Performance: A Systematic Review, and Exploration of Physiological Adaptations and Application for Team Sports[END_REF][START_REF] Voltaire | Effect of fourteen days of acclimatization on athletic performance in tropical climate[END_REF]. These adaptations of HA lead to better cardiovascular function (output, stroke volume, heart rate) and a decreased core temperature at rest and during exercise, partly due to an enhanced sweat rate and an expanded plasma volume [START_REF] Armstrong | The Induction and Decay of Heat Acclimatisation in Trained Athletes[END_REF]. HA is also known to reduce sodium loss by sweating [START_REF] Kirby | Plasma aldosterone and sweat sodium concentrations after exercise and heat acclimation[END_REF]. However, HA does not fully restore long to ultra-long endurance performance to the level reached by athletes in temperate conditions [START_REF] Voltaire | Effect of fourteen days of acclimatization on athletic performance in tropical climate[END_REF].

If early arrival at the race location is not possible, HA in an environmental chamber remains an alternative strategy that may help preserve ultramarathon performance. Costa et al examined the effects of six 2-h sessions of running at 60% of VO 2max on a motorized treadmill in an environmental heat chamber at 30°C or 35°C [START_REF] Costa | Heat acclimation responses of an ultra-endurance running group preparing for hot desert-based competition[END_REF]. From the third session onward, mean heart rate was lower at 30°C, whereas mean heart rate and thermal comfort were lower at 35°C. The authors concluded that two bouts of running at 60% of VO 2max in a 30°C air temperature conditions were sufficient to induce heat acclimation in ultra-runners, which may enhance their performance in such environments.

Hydration policy and implementation

In a triathlon field study, oral salt supplementation improved half-Ironman performance through faster cycling (p<0.05) and showed a similar trend in the running leg (p=0.06), with reduced sweat rate and limited electrolyte deficit [START_REF] Coso | Effects of oral salt supplementation on physical performance during a half-ironman: A randomized controlled trial[END_REF]. Thus, the consensus recommendation is to ingest 0.5 to 0.7 g.L -1 .h -1 for long endurance races [START_REF] Duvillard Sp Von | Fluids and hydration in prolonged endurance performance[END_REF] and up to 1.5 g.L -1 .h -1 for athletes prone to develop muscle cramping [START_REF] Bergeron | Heat cramps: fluid and electrolyte challenges during tennis in the heat[END_REF]. However, adding salt to the consumed water is not sufficient to avoid hyponatremia if athletes overhydrate during a race, as 73% of severe symptomatic hyponatremia found after an Ironman [START_REF] Speedy | Hyponatremia in ultradistance triathletes[END_REF].

Hoffman et al observed no advantage to sodium-enriched beverages during a 161-km ultramarathon performed by all levels of athletes in heat (38°C) [START_REF] Hoffman | Hydration strategies, weight change and performance in a 161 km ultramarathon[END_REF]. This result may have been due to a variation in sodium intake from solid food and/or to a better tolerance of faster runners to hypohydration and hyperthermia [START_REF] Hue | Thermoregulation, hydration and performance over 6 days of trail running in the tropics[END_REF][START_REF] Baillot | Fluid replacement strategy during a 27-Km trail run in hot and humid conditions[END_REF].

Cooling

Cooling strategies such as cold drink ingestion or cold-water immersion in order to reduce the thermal stress may enhance performance in the heat. This effect has been observed in cyclists (Figure 4) [START_REF] Burdon | The Effect of Ice Slushy Ingestion and Mouthwash on Thermoregulation and Endurance Performance in the Heat[END_REF][START_REF] Riera | Precooling does not Enhance the Effect on Performance of Midcooling with Ice-Slush/Menthol[END_REF][START_REF] Trong | Ingestion of a cold temperature/menthol beverage increases outdoor exercise performance in a hot, humid environment[END_REF] and runners, illustrated by a longer time to exhaustion [START_REF] Siegel | Ice slurry ingestion increases core temperature capacity and running time in the heat[END_REF][START_REF] Siegel | Pre-cooling with ice slurry ingestion leads to similar run times to exhaustion in the heat as cold water immersion[END_REF][START_REF] Yeo | Ice slurry on outdoor running performance in heat[END_REF]. Wearing cooling garments prior to or during exercise has proven to be performance-effective in hot and humid climate [START_REF] Arngrïmsson | Cooling vest worn during active warm-up improves 5-km run performance in the heat[END_REF][START_REF] Hasegawa | Wearing a cooling jacket during exercise reduces thermal strain and improves endurance exercise performance in a warm environment[END_REF][START_REF] Luomala | Adding a cooling vest during cycling improves performance in warm and humid conditions[END_REF], but remain difficult and impractical to use, and are often banned in official long-distance events.

Other studies have focused on more direct cooling strategies, but the exercise durations were shorter than ultra-endurance events. Nonetheless, they show interesting results that may be extrapolated for long and ultra-long events. Spraying or pouring water over the face and/or body can improve performance in tropical conditions. For example, pouring cold water over the skin will reduce skin temperature by absorbing the peripherally-transferred heat, before dripping off the body, and transiently improve thermal comfort. Cooling the head in this manner resulted in a 51% increase in cycling time to exhaustion at 75% VO 2max [START_REF] Ansley | The effects of head cooling on endurance and neuroendocrine responses to exercise in warm conditions[END_REF], with similar effects recently observed in running [START_REF] Stevens | Running performance in the heat is improved by similar magnitude with pre-exercise cold-water immersion and mid-exercise facial water spray[END_REF]. Neck cooling during a 90-min running trial in a hot environment (30.4°C and 53% RH) increased the distance covered by 7.4% with no change in rectal temperature [START_REF] Tyler | Neck cooling and running performance in the heat: single versus repeated application[END_REF]. A similar effect was found with menthol ingestion, as it activates cold dermal sensors [START_REF] Stevens | Menthol: A Fresh Ergogenic Aid for Athletic Performance[END_REF]. Mixed into a cold beverage, menthol did not lower core body temperature but had a positive effect on thermal sensation and running/cycling performances over various distances in tropical climate [START_REF] Trong | Ingestion of a cold temperature/menthol beverage increases outdoor exercise performance in a hot, humid environment[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]. Indeed, Stevens et al showed that a menthol mouth rinse every kilometre (25 mL at a concentration of 0.01%) during a running time trial in the heat improved 5-km performance time by 3% [START_REF] Stevens | Running performance and thermal sensation in the heat are improved with menthol mouth rinse but not ice slurry ingestion[END_REF]. A cumulative effect of menthol and ice slurry or cold water was observed on performances during a 20-km cycling trial in hot environment (30.7±0.8°C and 78±0.03% RH) [START_REF] Trong | Ingestion of a cold temperature/menthol beverage increases outdoor exercise performance in a hot, humid environment[END_REF]. Ultimately, menthol mouth rinsing and ingesting influence thermal perception and thermal comfort, which in turn might contribute to enhance performance in hot climates [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] Stevens | A Comparison of Mixed-Method Cooling Interventions on Preloaded Running Performance in the Heat[END_REF].

Cooling policy and implementation

According to the existing literature, pre-cooling by ice or cold water has been successfully studied during short duration exercise (<60 min), yet too brief to be a key factor of performance during ultra-endurance trials [START_REF] Siegel | Pre-cooling with ice slurry ingestion leads to similar run times to exhaustion in the heat as cold water immersion[END_REF][START_REF] Stevens | Running performance in the heat is improved by similar magnitude with pre-exercise cold-water immersion and mid-exercise facial water spray[END_REF]. Conversely, percooling by cold water or ice slurry ingestion during exercise seems to be a very interesting intervention to improve performances in a hot environment and possibly increase fluid intake, particularly for well-trained athletes. Indeed, most high-level runners exhibit the greatest increase of internal temperature, probably due to higher thermogenesis consecutive to higher workloads. Faster trail runners (27 km) [START_REF] Baillot | Fluid replacement strategy during a 27-Km trail run in hot and humid conditions[END_REF] and multi-stages trail runners (127 km on 6 days) [START_REF] Hue | Thermoregulation, hydration and performance over 6 days of trail running in the tropics[END_REF] also present a greater post-race dehydration, which could be reduced by cold water due to its effect on voluntarily increasing water absorption during exercise.

The higher temperature and greater dehydration observed in faster trail runners also seem to indicate that performance in heat is associated with a better tolerance to hyperthermia in elite athletes [START_REF] Mckeag | American College of Sports Medicine[END_REF]. .

Conclusion

The maintenance of performance in long endurance events such triathlon and ultramarathon held in a hot environment is a considerable challenge which requires meticulous preparation and management before and during the race. To limit performance decrements induced by these harsh environmental conditions, several countermeasures have been proposed, such as cold-water or ice slurry ingestion, external precooling (before exercise) and precooling (during) and/or menthol use.

However, the limited duration of exercise performed in these studies does not allow a direct application to an ultra-endurance or ultra-triathlon context. Thus, further studies will be needed to validate these approaches in ultra-endurance events.

Currently, one of the most relevant strategies to employ for performing in the heat is to consume adequate fluids: maintaining water availability along different race courses is thus crucial due to the detrimental effects of dehydration on fluid balance and the subsequent heat loss via sweating. Individual sweat rates are highly dependent on athlete's morphology, the intensity level of exercise and the environmental temperature and humidity. Sweat losses are accompanied by electrolyte losses, mostly sodium, potentially contributing to hyponatremia if not compensated: exogenous intake of sodium-enriched beverages is the most effective strategy to reduce this risk. In addition to the traditional compounds found in most sport drinks (e.g. glucose, magnesium,), ultramarathon beverages should contain about 0.7-1.2 g.L -1 of salt when performing in the heat. However, sodium-enriched beverages remain insufficient if athletes overdrink by fear of dehydration during the race, placing management of water intake as a key determinant of performance.

Heat acclimation remains a relevant strategy that may enhance ultra-endurance performance in hot environments: most of the acclimation benefits occur in the first 14 days of exposure, and a short-term protocol of 5 days will already induce significant early adaptations. Individual and mean values (±SD) of core temperature at each stage of the Guadeloupe half-Ironman held in tropical climate. T1: just before the race; T2: after the swim phase; T3: after the cycle phase; T4: at the end of the run phase. Temperatures were obtained from telemetric intestinal temperature devices ingested at least 6 hours before the race. Modified from [START_REF] Baillot | Hydration and thermoregulation during a half-ironman performed in tropical climate[END_REF]. Nomogram representing the impact of WBGT on the relation between marathon finishing times according to quartiles and relative performance decrement in comparison with WBGT at 5°C. Modified from [START_REF] Ely | Impact of weather on marathon-running performance[END_REF]. β and † denote that block performance was affected by Time Period (β: P<0.007) and the Time Period x Drink Temperature interaction ( †: P<0.004), respectively.

Additional information

Modified from [START_REF] Trong | Ingestion of a cold temperature/menthol beverage increases outdoor exercise performance in a hot, humid environment[END_REF].

Figure 1 .

 1 Figure 1.

Figure 2 .

 2 Figure 2.

Figure 3 .

 3 Figure 3. Proportion of runners abandoning the Western States Endurance Run (161 km; 6000m D+) race at each checkpoint along the course (A), and relationship between finish times for the 2006 (7-38°C) and 2007 (2-30°C) edition (B). Modified from [6].

Figure 4 .

 4 Figure 4. Trial times for 5 successive blocks (4 km cycling + 1.5 km running,) with the ingestion of Neutral water (orange), Cold water (blue) and Ice-slurry (green). Mean ± SD. a Neutral water vs. Ice-Slurry/Menthol (P<0.05). b Cold vs. Ice-Slurry/Menthol (P<0.05).

Funding

This work was supported by the « Programme Opérationnel -Fonds Européen de Développement Régional »(PO-FEDER) under grant 2015-FED-213, GP0008037.


Pr./M.D L Fourcade

Borel Benoit
email: benoit.borel@unilim.fr

Evolution of muscular oxygenation parameters during walking test in pre-term children : a pilot study

Keywords:

Evolution of muscular oxygenation parameters during walking test

ABSTRACT

OBJECTIVE: Premature children tend to be less involved in physical activities than their fullterm contemporaries. This behaviour could be partly explained by pulmonary impairment but also by a muscular impairment. NIRS technology could offer interesting way of investigation of muscular adaptation. The aim of the study was to explore parameters of peripheral muscular oxygenation, coupled to gait characteristics, between premature (PT) and full-term (FT) children during a 6-minute walking test (6MWT). No statistically significant difference was found for other NIRS parameters. Lastly, the analysis of gait parameters highlighted a group effect for walking speed (p<0.05) and stride length (p<0.01).

CONCLUSION: Premature children showed decreased walking performance and greater change in peripheral muscular oxygen extraction, associated with lower walking speed and stride length. This may point out a muscular maladjustment and reduced functional capacities for PT children. These phenomena could be responsible of a greater muscular fatigue.

INTRODUCTION

In the last decade, numerous factors, like obstetrical (uterine malformation), generic (increase of multiple pregnancies) or maternal (addiction, pregnancy disease or socioeconomical factors) risk factors, can be responsible of the ratio (from 6 to 11%) of premature births worldwide, e.g. children born before 37 weeks of gestation [START_REF] Tikanmäki | Physical Fitness in Young Adults Born Preterm[END_REF][START_REF] Torchin | Epidemiology of preterm birth: Prevalence, recent trends, short-and long-term outcomes[END_REF]. The degree of prematurity and birth weight are key factors in the risk for long-term impaired development [START_REF] Maclean | Altered breathing mechanics and ventilatory response during exercise in children born extremely preterm[END_REF][START_REF] De | A u u v v f m b h. Wh m y c c , x c reveal[END_REF]: children born before 28 weeks of gestation are considered as extremely preterm, between 28 and 32 weeks great preterm and late preterm for those born after 34 to 36 weeks of gestation. Prematurity leads to multiple consequences and, in particular, causes an interruption of lung development, impacting the normal pulmonary vascular and alveolar development [START_REF] Narang | Review series: What goes around, comes around: childhood influences on later lung health? Long-term follow-up of infants with lung disease of prematurity[END_REF].

The major respiratory pathology developed in great prematurity is bronchopulmonary dysplasia (BPD), affecting up to 40 % of preterm (PT) children with birthweight inferior to 1000g (4). This pathology is defined as the need of oxygen supply during more than 28 days following birth with severity assessed at 36 weeks for infants born at gestational ages of less than 32 weeks [START_REF] Simpson | Lung function trajectories throughout childhood in survivors of very preterm birth: a longitudinal cohort study[END_REF]. It is characterized by fewer and larger alveoli and a reduction in pulmonary vascularization. As a result of BPD, there is a decrease in gas exchange efficiency, higher dead space and gas trapping, along with lower pulmonary function and higher airway resistance [START_REF] Maclean | Altered breathing mechanics and ventilatory response during exercise in children born extremely preterm[END_REF].

During exercise, children with BPD show ventilatory limitations and a higher oxygen uptake for the same power output than full term (FT) children [START_REF] Kriemler | Aerobic and lung performance in premature children with and without chronic lung disease of prematurity[END_REF]. Moreover, long term consequences seem to be important as adult survivors of preterm birth, with or without BPD, globally exhibit mechanical constraint to increase tidal volume during near-maximal exercise, associated with severe dyspnea and leg discomfort [START_REF] Lovering | Ventilatory and sensory responses in adult survivors of preterm birth and bronchopulmonary dysplasia with reduced exercise capacity[END_REF]. Prematures show lower values of maximal oxygen uptake ( O 2 max) and forced expiratory volume in one second (FEV1) than FT controls [START_REF] Edwards | Effect of preterm birth on exercise capacity: A systematic review and meta-analysis[END_REF][START_REF] Malleske | Pulmonary sequelae and functional limitations in children and adults with bronchopulmonary dysplasia[END_REF]. As a consequence, PT children are less involved in physical activities [START_REF] Tikanmäki | Physical Fitness in Young Adults Born Preterm[END_REF][START_REF] Et | Pu m y G Exchange and Exercise Capacity in Adults Born Preterm[END_REF][START_REF] Lowe | Physical activity outcomes following preterm birth[END_REF]. Still, little information is available to explain how impaired lung function in PT children alters exercise capacity [START_REF] Maclean | Altered breathing mechanics and ventilatory response during exercise in children born extremely preterm[END_REF].

Chronic Obstructive Pulmonary Disease (COPD) model of deconditioning could be an interesting hypothesis for explaining the link between impaired lung function and exercise capacity alteration in PT children. In fact, several studies report muscular dysfunction secondary to respiratory impairment in COPD population [START_REF] Maltais | An Official American Thoracic Society/European Respiratory Society Statement: Update on Limb Muscle Dysfunction in Chronic Obstructive Pulmonary Disease[END_REF][START_REF] Ramon | The dyspnoea-inactivity vicious circle in COPD: development and external validation of a conceptual model[END_REF]. Strengthened by the fact that BPD children exhibit a high risk of COPD in later life [START_REF] De | A u u v v f m b h. Wh m y c c , x c reveal[END_REF][START_REF] Narang | Review series: What goes around, comes around: childhood influences on later lung health? Long-term follow-up of infants with lung disease of prematurity[END_REF][START_REF] Simpson | Lung function trajectories throughout childhood in survivors of very preterm birth: a longitudinal cohort study[END_REF][START_REF] Maltais | An Official American Thoracic Society/European Respiratory Society Statement: Update on Limb Muscle Dysfunction in Chronic Obstructive Pulmonary Disease[END_REF], the impairment of lung function, as mentioned above, could have consequences on muscle function of PT children. In COPD patients, this muscular dysfunction is associated with alterations of morphological and structural skeletal muscle, atrophy, loss of type I fibers and mitochondrial dysfunction and represents a major element of COPD functional impairments [START_REF] Allaire | Peripheral muscle endurance and the oxidative profile of the quadriceps in patients with COPD[END_REF]. Nowadays, one way for muscle function investigation is related to non-invasive techniques like Near Infra-Red Spectroscopy (NIRS) [START_REF] Maltais | An Official American Thoracic Society/European Respiratory Society Statement: Update on Limb Muscle Dysfunction in Chronic Obstructive Pulmonary Disease[END_REF]. NIRS technology has been previously used in neonate and premature to measure tissue oxygenation and perfusion in regions of interest, e.g. the brain, kidneys or peripheral muscles, in early states of shock [START_REF] Höller | Peripheral Muscle Near-Infrared Spectroscopy in Neonates: Ready for Clinical Use? A Systematic Qualitative Review of the Literature[END_REF]. It has also been used in children with muscular pathologies and chronic heart disease, in whom different profiles of muscular oxygenation were evidenced, compared to healthy subjects [START_REF] Allart | Evaluation of muscle oxygenation by near-infrared spectroscopy in patients with Becker muscular dystrophy[END_REF][START_REF] Moalla | Training effects on peripheral muscle oxygenation and performance in children with congenital heart diseases[END_REF]. This method already reveals a decrease in oxidative capacities in lower leg muscle that contributed to exercise intolerance in these patients [START_REF] Adami | Reproducibility of NIRS assessment of muscle oxidative capacity in smokers with and without COPD[END_REF]. Similarities between BPD and COPD suggest that PT children may present a high risk of exhibiting the same muscular disorders, also responsible for the reduced functional capacities reported in this population [START_REF] Tsopanoglou | Functional capacity during exercise in very-low-birth-weight premature children[END_REF]. However, muscular oxygenation in PT children during physical activity of daily living, like walking task, has not been studied yet.

The objective of this study was to compare parameters of peripheral muscular oxygenation using NIRS between PT and FT children during a 6-minute walking test (6MWT). We hypothesize that PT children present impaired muscular oxygenation parameters, compared to FT ones.

Methods

Population

This cross-sectional pilot study was conducted on two populations: i) an experimental group with extreme and great PT children; and ii) a control population of FT children. PT children

w c u h 'H Mè E f ' (HME) L m g (F c ), f w by h 'C 'Ac M c -S c P c c ' (CAMSP) u h x h b h y. Th c
population was composed of FT children recruited in several recreational centres in Limoges.

Patients were eligible aged from 3 to 6 years old. Inclusion criteria for PT children included an age of birth before 32 weeks of gestation and a birth after 37 weeks of gestation for the full-term population. Exclusion criteria were moderate PT children, major neurological disabilities (cerebral palsy) or cardiac pathologies that would interfere with protocol realization and any refusal of participation by the child or parents. Parents were signed an informed consent and authorization before the participation of their children to this protocol.

The study was carried out as per the standards set by the Declaration of Helsinki.

Protocol design

The study period took place over a 3-month period. At their arrival children were asked to sit and the NIRS probe was placed on the right medial gastrocnemius (Figure 1) [START_REF] Kutsuzawa | Oxygenation state of the calf muscle during the 6-min walk test in patients with COPD[END_REF]. They were asked to walk as fast as possible once the start had been given and this for 6 minutes

Walking performance and gait parameters

A 6MWT was conducted in accordance with American Thoracic Society recommendations [START_REF] Singh | An official systematic review of the European Respiratory Society/American Thoracic Society: measurement properties of field walking tests in chronic respiratory disease[END_REF][START_REF] Holland | An official European Respiratory Society/American Thoracic Society technical standard: field walking tests in chronic respiratory disease[END_REF]. Children were asked to walk back and forth along a 30-meter flat corridor as quickly as possible for 6 minutes. The covered distance (6MWT, in meters) was then recorded upon the completion of the test. During the 6MWT realization, gait parameters were measured using an OPTOGAIT system (Microgate, Bolzano, Italy) ( 26), placed at the beginning of the walking path 2 meters after the starting line in order to eliminate the acceleration and deceleration phase. Speed (m.s-1), cadence (st.min-1), stride step (cm) and stride time (s)

were collected at each passage.

Statistical analysis

Unless otherwise stated, data are expressed as mean ± SD.

Data analysis was performed, for all the tests described below, with the help of Prism software (software version 6.0c, GraphPad, San Diego, CA, USA).

For each statistical test performed, significant level was set for p<0.05. The time-course variations of NIRS parameters (i.e. ∆HbO2, ∆HHb and ∆HbTot) are illustrated in Figure 3. ∆HbO2 c u g h 6MWT, indicating that peripheral muscle shows a greater deoxygenation (defined as a fall of HbO 2 ) at the onset of the 6MWT. This was followed by a plateau phase at the third minute, with no difference between the two populations (p>0.05).

On the contrary, we observed an increase of ∆HHb, then reaching also a plateau at the third minute (cf. Figure 3). The peripheral oxygen extraction is enhanced at the beginning of exercise and will then stabilize after 3 minutes of moderate intensity activity. There is a significative difference between the two populations starting at the third minute of the 6MWT (p<0.05), with greater oxygen extraction (i.e. increase of ∆HHb) for PT group in comparison with FT group. This variation is maintained until the end of the test.

Lastly, there is no difference for ∆HbTot between the two populations during the 6MWT. The major result of the current study is that great preterm children highlight a significantly lower walking performance, in comparison with full-term population, with a difference of 56 meters (-9%) between the mean walking distance of both groups (p<0.05). The lower performance reported for PT population has been previously observed in PT children with very low birthweight [START_REF] Edwards | Effect of preterm birth on exercise capacity: A systematic review and meta-analysis[END_REF][START_REF] Tsopanoglou | Functional capacity during exercise in very-low-birth-weight premature children[END_REF]. Several hypotheses could be raised up for explaining this decreased performance in PT group. First of all, difference in anthropometric factors could explain this difference. FT children show significantly higher height, associated with higher stride length normalized to height. Therefore, with the same exercise time, higher body dimensions could help FT children to reach higher distance. Secondly, this lower functional capacity could be the expression of lung impairment and a global desadaptation to exercise in PT population. This could also be explained by a lesser muscle mass, as the significant difference for body weight between both populations (p=0.012) could be an indicator of hypotrophy, one of major characteristics of prematurity. This hypotrophy could lead to a decreased muscle adaptability to exercise and therefore to an increased muscle fatigability. In order to investigate the potential impairment of muscle function in PT population, we decide to use a non-invasive monitoring method (NIRS), that uses near-infrared light for exploring muscular oxygenation and its adaptation to exercise. Peripheral muscular oxygenation measured with NIRS has been previously used in PT children to detect early states of shock and in different pathologies with muscular disorders [START_REF] Allart | Evaluation of muscle oxygenation by near-infrared spectroscopy in patients with Becker muscular dystrophy[END_REF] and could be an interesting method for muscular evaluation in PT population.

The variations of muscular oxygen parameters observed in the two populations from a general point of view are consistent with the NIRS data measured in adults and children [START_REF] Moalla | Training effects on peripheral muscle oxygenation and performance in children with congenital heart diseases[END_REF][START_REF] Kutsuzawa | Oxygenation state of the calf muscle during the 6-min walk test in patients with COPD[END_REF][START_REF] Boushel | Monitoring tissue oxygen availability with near infrared spectroscopy (NIRS) in health and disease[END_REF][START_REF] Leclair | Reproducibility of measurement of muscle deoxygenation in children during exercise[END_REF].

In both g u , w k g k f mu u uc f ∆HbO2 c f ∆HHb, reflecting a temporary inadequacy between O2 consumption and delivery [START_REF] Moalla | Muscle oxygenation and EMG activity during isometric exercise in children[END_REF][START_REF] Van Beekvelt | In vivo quantitative nearinfrared spectroscopy in skeletal muscle during incremental isometric handgrip exercise[END_REF][START_REF] Murthy | Forearm muscle oxygenation decreases with low levels of voluntary contraction[END_REF].

After 3 minutes of walking, a plateau phase is then observed, as noted before: O2 delivery is then sufficient to compensate for metabolic requirements [START_REF] Delorey | Relationship between pulmonary O2 uptake kinetics and muscle deoxygenation during moderate-intensity exercise[END_REF]. H w v , ∆HHb c more in the PT population after 3 minutes of walking and until the end of the 6MWT. In previous studies, a greater O2 consumption was observed in BPD children during exercise [START_REF] Maclean | Altered breathing mechanics and ventilatory response during exercise in children born extremely preterm[END_REF]. Cardiac and muscular pathologies, such as Becker Muscular Distrophy (BMD), also have a negative impact on O2 extraction, due to an insufficient blood delivery. O2 extraction is h m z ∆HHb h c r to compensate disorders in O2 delivery [START_REF] Moalla | Training effects on peripheral muscle oxygenation and performance in children with congenital heart diseases[END_REF][START_REF] Barroco | A practical approach to assess leg muscle oxygenation during ramp-incremental cycle ergometry in heart failure[END_REF].

The oxygen supply in children with BMD would be slower (cardiac insufficiency or capillarity default) and related to the degree of muscular and functional deficit [START_REF] Allart | Evaluation of muscle oxygenation by near-infrared spectroscopy in patients with Becker muscular dystrophy[END_REF]. Alteration of muscular perfusion and impaired oxygen supply would contribute to exercise intolerance and premature fatigue observed in these children [START_REF] Moalla | Training effects on peripheral muscle oxygenation and performance in children with congenital heart diseases[END_REF]. However, this higher O2 consumption could also be associated to mechanical ventilatory constraints [START_REF] Kriemler | Aerobic and lung performance in premature children with and without chronic lung disease of prematurity[END_REF], that could induce greater work of breathing for PT population and may explain higher O2 consumption during exercise in PT population.

The skeletal muscular oxygenation parameters show a higher oxygen extraction during exercise in PT children compared to FT, reflecting the metabolic state of the activated muscle.

Prematurity has numerous consequences for child development. Cardiovascular disorders are rarely evocated in BPD children to explain impairments in muscular oxygenation [START_REF] Et | Pu m y G Exchange and Exercise Capacity in Adults Born Preterm[END_REF][START_REF] Vardar-Yagli | Pulmonary and extrapulmonary features in bronchopulmonary dysplasia: a comparison with healthy children[END_REF][START_REF] Jacob | Exercise ability in survivors of severe bronchopulmonary dysplasia[END_REF].

A previous study of Lewandowski et decreased alveolarization and pulmonary microvascular development in their lungs [START_REF] Kriemler | Aerobic and lung performance in premature children with and without chronic lung disease of prematurity[END_REF]. Gaz exchanges are impaired and As a consequence, physical activity leads to various physiological responses [START_REF] Tikanmäki | Physical Fitness in Young Adults Born Preterm[END_REF][START_REF] Simpson | Lung function trajectories throughout childhood in survivors of very preterm birth: a longitudinal cohort study[END_REF][START_REF] Kriemler | Aerobic and lung performance in premature children with and without chronic lung disease of prematurity[END_REF].

In healthy subjects, muscular performance partially depends on the proportion of the different types of muscular fibers in the activated muscle. Prematurity has negative impacts on the respiratory function similar to COPD patients in which muscle morphologic adaptations such as fat infiltration, fibrosis, inflammation, increased subcutaneous adipose, loss of type I fibers and mitochondrial density may hamper NIRS measurement of muscle oxidative capacity [START_REF] Maltais | An Official American Thoracic Society/European Respiratory Society Statement: Update on Limb Muscle Dysfunction in Chronic Obstructive Pulmonary Disease[END_REF][START_REF] Adami | Reproducibility of NIRS assessment of muscle oxidative capacity in smokers with and without COPD[END_REF]. Muscular physiological characteristics of PT children with BPD could be close to COPD patients. Indeed, [START_REF] Wenchich | Mitochondrial energy metabolism in very premature neonates[END_REF] showed a reduction of the number of mitochondrial proteins and activity of the PDH and COX enzymes in very PT neonates [START_REF] Wenchich | Mitochondrial energy metabolism in very premature neonates[END_REF]. The muscular oxygen metabolism is then impaired, and the subsequent ATP production is reduced. Skeletal muscle dysfunction and peripheral factors are major determinants of reduced exercise capacity and exercise intolerance, and could contribute to their reduced involvement in physical activities [START_REF] Moalla | Training effects on peripheral muscle oxygenation and performance in children with congenital heart diseases[END_REF]. Very PT children would be more prone to fatigue, as for a same moderate intensity physical activity, requiring a greater O2 extraction in activated muscle in order to respond to the metabolic needs.

Another potential explanation for the lower performance observed for PT population, related to the muscle function, could be the difference for gait parameters observed in this study.

Despite the absence of significant interaction for both factors (Time and Group), gait parameters seem to be influenced by group condition (PT vs. FT). In fact, walking speed and stride length are both lower for the PT group, in comparison with FT group, throughout the 6minute walking test. Walking speed could be directly linked to muscle function and contractile capacity but also to the efficiency of the walk for each child, PT children having a higher age for independent walk. Lastly, the length of lower limbs could influence the length of the stride, as confirmed by a significant difference for normalized stride length to height between the two groups. But this hypothesis requires a more precise assessment of sitting height, for the determination of the length of lower limbs. Overall, gait parameters, linked with NIRS adaptation results, are interesting and tend to highlight a global muscle dysfunction of lower limbs in PT.

Limitations

They are a few limitations to report for this study. First, children aged from 3 to 6 years old may present various degree of psychomotor maturity within this age range (Cf. Table 1).

Nevertheless, the mean age between the two populations was not significantly different, which allowed comparisons between the populations of PT and FT children. Furthermore, this range of age for PT children is representative of real-life population monitored and cared for in paediatric units.

Then, the NIRS technique is a technology that is sensitive to methodological considerations and evaluates oxygenation parameters in a limited area of a targeted muscle group. Also, muscle microvascular oxygenation is heterogeneous within the studied muscle [START_REF] Miura | Application of Multiple NIRS Imaging Device to the Exercising Muscle Metabolism[END_REF]; in order to avoid this issue, the probe disposition was standardized (Cf. Figure 1). In order to have a global approach, it w u h v b g c u h NIRS m u w h V O2 measurements and cardiorespiratory adaptations. Furthermore, NIRS only offers a peripherical point of view and NIRS measurement could be influenced by central factors linked to cardio-vascular characteristics (like blood flow and blood volume variation, haemoglobin or haematocrit levels. Therefore, this implies to couple NIRS assessment with other complementary measurement in order to provide a more global point of view.

In order to take account of these principal limits, it could be interesting to investigate muscle function of PT population with the help of complementary assessment tools (like oxygen uptake evaluation, muscle function evaluation (with bio-impedancemeter or force assessment)

or multi-site NIRS assessment). This would help a better and more comprehensive vision of prematurity effect on body function (pulmonary, muscle functions). It could be also interesting to consider long-term follow-up studies in order to observe a cohort of PT subjects throughout the growing process. Physical activity level and lifestyle could also be interesting to monitor in order to determine the effects of these parameters on exercise performance in PT population. Lastly, studies investigating the effect of intervention specifically focused on muscle function could also be interesting to design in order to observe potential effects on physical activity participation and muscle function evolution.

Conclusion

This study is the first to provide new and original data on peripheral muscular oxygenation in PT children with BPD. This work allows to investigate some hypothesis in order to understand why PT children, and in particular great PT, are less involved in physical activities. In this study PT children with BPD showed reduced functional capacities during the 6MWT compared to FT children, due to specific adjustments. A better understanding of PT impact could be interesting in order to design and optimize specific intervention strategies.

Proposition :

This study is the first to provide new and original data on peripheral muscular oxygenation in PT children with BPD. PT children with BPD showed reduced functional capacities during the 6MWT compared to FT children. This study supports the hypothesis that prematurity has pulmonary impact on childhood. Further studies are needed to clarify the strategy for respiratory monitoring theses children during childhood and adulthood.

Clinical messages

 Prematurity induces specific oxygenation adaptation during walking task  Better knowledge of exercise adaptation for PT population is of great importance for the optimization of interventions. These ones should include specific focus on muscle function, as this function seems to be directly impacted in PT condition. First, a straight line (ST) between the middle of the popliteal fossa and calcaneus was traced. Larger calf circumference (LCC) was then measured and traced. At the intersection between the LCC and ST the probe was moved medially from LCC/8 cm and finally secured around the lower leg.

  STUDY DESIGN: Prepubescent children performed a 6MWT. During the test, changes in muscular oxyhaemoglobin (∆HbO 2 ), deoxyhaemoglobin (∆HHb) and total haemoglobin (∆tHb) w m u w h NIRS ch gy, ubj c ' c v . G m were monitored with an OPTOGAIT system. RESULTS: 45 children (33 FT children and 12 PT children) participated in this study (mean age±SD: 4.9±0.7 and 4.6±0.9 years, respectively). Statistical analysis highlighted a decreased walking performance for PT children, with significantly lower walking distance (p<0.05) than FT ones (405.1±91.8m vs. 461.0±73.3m respectively, -9%). A concomitant increase of xyg x c ( v ∆HHb m c u ) w b v f m h h m u f h (p<0.05).

(

  6MWT). The starting line and the turning area were materialized by a line and a cone. The NIRS measurements began before the walking test once children were asked to stand up (allowing NIRS baseline determination). At the end of the test, children were asked to sit and the NIRS records were ended and collected ended. *** Insert Figure 1 near here *** Muscle oxygenation: NIRS setup and acquisition Changes in muscle oxygenation and blood volume during the 6MWT was measured with a NIRS device (PortaMon, Artinis, Netherlands) (22). The NIRS probe was placed on the right medial gastrocnemius as illustrated in Figure 1. NIRS acquisition was made through the Oxysoft software (version 3.0.97.1), with a 2 Hz sample frequency. Differential Pathlength Factor (DFP) was set on 4 (23). A 0.1 Hz low-pass filter was applied to the signal to remove physiological and instrumental noise. The relative concentrations in oxy-xyh m g b (∆HbO2 ∆HHb, c v y, μm .L-1) during the last 10 seconds of each minute of the 6MWT were then normalized by subtracting to them the mean value of the last 15 s of baseline, immediately before the beginning of the k

  ∆HbTot starts by slightly decreasing and then increases after 3 minutes of exercise. *** Insert Figure 3 near here *** Time-course evolution of gait parameters are resumed in Figure 4. The statistical analysis ff c f "T m " f c without factors interaction. Stride cycle time presents no significant of both factors and no significant interaction. *** Insert Figure 4 near here *** Discussion In this work, our main finding highlights a decreased walking performance in the PT population. Thus, prematures exhibit a reduced functional capacity, associated with a , during a standardized walking evaluation. Other results show no difference m f xyg u y (∆HbO2) b v um ( fu ∆HbT ). A f h u are really interesting as it offers original and novel knowledge about exercise adaptation of muscle function in PT children. In fact, to the best of our knowledge, no study previously used skeletal muscular NIRS monitoring during exercise in PT children. It provides then new fundamental insight on peripheral muscular oxygenation in premature children aged from 3 to 6 years old and on the consequences of PT birth.

Figure 1 :

 1 Figure 1 : NIRS setup on the lower limb of children

2 Figure 3 :Figure 4 :

 234 Figure 3 : Evolution of muscular oxygenation parameters during the 6MWT

  al. reports specific left cardiac adaptations due to prematurity[START_REF] Lewandowski | Preterm Heart in Adult Life: Cardiovascular Magnetic Resonance Reveals Distinct Differences in Left Ventricular Mass, Geometry, and Function[END_REF]. This could lead notably to reduced cardiac volume and reduced O2 delivery, that could explain the greater O2 extraction for PT population. However, in this study, the kinetics of oxyhaemoglobin (HbO2) do not differ between PT and FT children. This could indicate that the lower performance (distance) observed for PT children for the walking test could be mainly associated to a muscle dysfunction, explaining the greater O2 extraction. and

	there is no diff	c	∆HbT b w	h	w	u	. Furthermore, the pulmonary
	function is altered, particularly in children affected by BPD, who exhibit air trapping,

Table 1 :

 1 Population characteristicsValues are expressed as mean ± SD. Stride length was normalized to height of children, in order to consider significant difference for height between the two groups.

		FT (N=33)	PT (N=12)	P-value
	Parameters
	Age (years)	4.9 ± 0.7	4.6 ± 0.9	0.216
	Weight (kg)	18.6 ± 4.0	15.7 ± 2.&7	0.012
	Height (m)	1.1 ± 0.1	1.0 ± 0.1	0.011
	Body mass index (kg/m 2 )	15.1 ± 2.4	14.5 ± 1.6	0.426
	Birthweight (kg)	3.3 ± 0.6	0.9 ± 0.3	0.000
	Gestational age (weeks)	39.9 ± 1.5	27.6 ± 1.1	0.000
	Perinatal variables
	Independent walking age (months)	12.6 ± 2.0	17.8 ± 3.5	0.000
	Days with supplemental oxygen	/	42.5 ± 29.9	/
	Gait parameters
	Speed (m/s)	1.4 ± 0.3	1.3 ± 0.3	NS
	Cadence (step/m)	152.8 ± 18.6	151.6 ± 45.9	NS
	Stride length (cm)	105.3 ± 13.5	92.6 ± 13.0	NS
	Normalized Stride length	94.7 ± 8.4	86.5 ± 9.3	0.009
	Stride time (s)	0.8 ± 0.1	0.8 ± 0.1	NS

Acknowledgements

The authors want to thank all the children that participated in the study for their involvement and their participation. We also want to thank all the medical staff and persons who allow the realization of the study.

The different authors contributed at different levels to the different steps of the study conception, data acquisition, data analysis and manuscript writing and reviewing process. Z.O-J: study conception, data acquisition, data analysis, manuscript writing and reviewing A.P: study conception, manuscript writing and reviewing E.H: data acquisition, data analysis and reviewing L.F: study conception and reviewing B.B: study conception, data analysis, manuscript writing and reviewing

Conflict of interest: no author declares any potential, perceived or real conflict of interest.

There is no prior presentation of the current work, as an abstract or poster.

List of abreviations

Authors contributions