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Exercise in the heat II

Exercise in the heat II. Stephen Burns September 2019 Environmental Exercise Physiology SS3111. Overview. Introduction Definitions Behavioural strategies Aerobic fitness Heat acclimatisation Cooling strategies Clothing Summary Recommended reading. Introduction.

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Exercise in the heat II

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  1. Exercise in the heatII Stephen Burns September2019 Environmental Exercise Physiology SS3111

  2. Overview • Introduction • Definitions • Behavioural strategies • Aerobicfitness • Heatacclimatisation • Coolingstrategies • Clothing • Summary • Recommendedreading

  3. Introduction

  4. Minimising risk for exertional heatillness • Heat illness during exercise may take various forms: • Exercise associated muscle cramps (heat cramps) • Heatexhaustion • Heatstroke • What strategies can be implemented to sustain/enhance performance during training and competition in the heat, and minimize the risk of exertional heatillness? (Nichols, Current Reviews in Musculoskeletal Medicine, 7:355-365,2014)

  5. Availablestrategies • Behavioural • Aerobicfitness • Heatacclimatization • Coolingstrategies • Hydration

  6. Definitions

  7. Definitions • Acclimatisation: refers to physiological adaptations that occur in naturally hot ambientconditions. • Acclimation: refers to physiological adaptations that occur in artificially hot indoor environments. • Heat stress: refers to environmental (including clothing) and metabolic conditions that tend to increase bodytemperatures. • Heat strain: refers to physiological (e.g. body temperature) consequences of heat stress. • Compensable heat stress: exists when heat loss occurs at a rate in balance with heat production so that a steady-state core temperature can be achieved at a sustainable level for a requisiteactivity. • Uncompensable heat stress: occurs when the individual’s evaporative cooling requirements exceed the environment’s evaporative coolingcapacity.

  8. Behaviouralstrategies

  9. Cooling breaks and restperiods • Rest breaks provide an opportunity for body temperature to reduce via reduced heat production. • Several sporting bodies (ITF, ITF-WTA, FIFA – see lecture 2 for list) provide additional breaks for sportspersons when the heat stress is high (wet bulb-globetemperature). • This strategy is also useful for individuals in the military or in professions such as firefighting, ships stokers, steelworkers. • May not be a realistic strategy in many exerciseevents. From: Lim et al, Annals of Academic Medicine Singapore, 37:347-353,2008 Note: Data show a 12-km forced march in 10 male SAF recruits carrying an ~28-kg load under an ambient temperature of 30C and relative humidity of 65%. Marching was completed in three 45-min stages interspersed with a 15-min and 30-min rest after stage 1 and 2, respectively.

  10. Aerobicfitness

  11. Aerobicfitness • In a recent meta-analysis, aerobic fitness stood out as the most important mitigation strategy to favourably alter core temperature for endurance exercise in the heat. • Aerobic exercise training is best combined with heat acclimation/acclimatisation for full benefits for the individual. 11 From: Alhadad et al, Frontiers in Physiology, 10:71,2019

  12. Heatacclimatisation

  13. Habituation to hotenvironments • “In 1768, James Lind published the first report on the ability of humans to adapt to environmental heat. He reported that when relocating to East and West Indian climates, Europeans were at first adversely affected by the environment, but over a period oftime, • habituated and eventuallylived comfortably.” (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38,2015) James Lind (1716-1794) (Dunn, Archives of Disease in Childhood, 76:F64-F65, 1997) Lind J, An Essay on Diseases, 1788 (4th Edition). From: London School of Hygiene and TropicalMedicine

  14. Habituation to hotenvironments • Thermal comfort in resting individuals changes when acclimatized to a tropical (hot and humid)climate. • Comfort level is altered in relation to specific humidity (i.e. grams of water per kilogram of air), dry bulb temperature, and relativehumidity. (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38, 2015 – Adapted from Folk, Textbook of Environmental Physiology, Philadelphia: Lea & Febiger,1974)

  15. Physiological adaptations to exercise in hot environments • (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20- 38,2015) • Several important physiological adaptations occur with heat acclimatisation that improve thermal comfort and endurance performance. Improved sweating, reduced core temperature, reduced skin temperature and changes to the cardiovascular system are among the most importantchanges.

  16. Changes to sweat rate andcomposition • Adaptations to sweat rate and composition occur with heatacclimatization. • Body core temperature threshold for the onset of sweating isreduced. • The sweat rate (slope) isincreased. • Concomitantly, the body core temperature threshold for onset of cutaneous (skin) vasodilation is reduced, whereas skin blood flow sensitivity isincreased. • Sweat sodium and chloride concentrations are reduced. (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38,2015) HA, heatacclimation.

  17. Temperature and cardiovascularadjustments • Aerobic training in temperate environments can reduce physiological strain and improve exercise capabilities in theheat. • Heat acclimation is the preferred method ofadaptation. • Core and skin temperature are lowered withacclimation. • Heart rate reduces whilst stroke volume increases. Cardiac output improves and plasma volume increases with heatacclimation. (Sawka et al., Comprehensive Physiology, 1:1883-1928,2011) Note: Participants completed an initial exercise heat test (4 h at 35% of maximum aerobic power). Then 11-wks trainingof 1-h for 4-d/wk in temperate conditions. Finally, 8-d of exercise training for 4-h/d at 35% of maximum aerobicpower.

  18. Cardiorespiratory and performancechanges • Acclimation to the heat leads to improvements in maximum oxygen uptake, increases in lactate threshold, increases in maximal cardiac output and improvements in time trialperformance. • The magnitude of any heat adaptations depend on the intensity, duration, frequency, and number of heatexposures. • Note: Heat acclimation group received 10-d at 40C for 100-min at 50% of maximum oxygenuptake. • Control group completed same exercise for 10-d in a temperate climate. Both groups tested pre and post in hot and temperateenvironments. (Sawka et al., Comprehensive Physiology, 1:1883-1928,2011)

  19. Environment • Heat acclimatisation in dry heat improves exercise in humid heat and viceversa. • However, acclimatization in humid heat evokes higher skin temperatures and circulatory adaptations, potentially increasing maximum skin wettedness and therefore maximum rate of evaporative heatloss. • Athletes unable to travel to naturally hot ambient conditions (acclimatization) can train in artificially hot indoor environments (acclimation). • Training outdoors is more specific and allows athletes to experience the exact nature of heatstress. (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38,2015) 19

  20. Time course of adaptations to heatstress • Heat acclimation occursrapidly • – 75-80% of adaptations infirst 4-7days. • Considered: • short-term: < 7days • medium-term: 8-14days • long-term: > 15days • Exercise in the heat is the most effective method for developing heat acclimation but passive heat exposure can also result in someadaptation. (Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38,2015) • Remember: To achieve optimal adaptation, work rate and environmental conditions should closely replicate those of the competitionsetting.

  21. Decay • Heat acclimation is transient and gradually disappears if not maintained by repeated heatexposures. • Heart rate improvements are lost more rapidly than thermoregulatory responses. • There is no agreement on the rate of decay. Aerobic fitness and regular exercise appear to contribute to retaining the benefits of heat acclimation forlonger. (Flouris et al., European Journal of Applied Physiology, 114:2119-2128,2014) Note: Data show 14-d of heat acclimation followed by 14-d of no exercise in theheat. • However, the rate of decay is generally slower than its induction allowing maintenance of most benefits for 2-4 weeks. Moreover, (re) acclimatization during this period is faster than the firstacclimatization.

  22. Heat acclimatization strategies • From: Racinais et al, Scandinavian Journal of Medicine and Science in Sports, 25(S1):6-19,2015 • Heat acclimatisation strategies will vary depending on the stage of training and in relation to competitionperformance.

  23. Heat as a trainingstimulus • There has been recent movement toward examining exercise training in the heat as a stimulus to improveperformance. • Some studies have shown improvements in maximum oxygen uptake, time trial performance, lactate threshold, cardiac output and stroke volume with heatacclimation. • Athletes may consider using training camps in hot ambient conditions to improve physical performance both in-season andpre-season. • However, training quality should not becompromised. • Experienced athletes requiring a novel stimulus may benefit most. • From: Lorenzo et al, Journal of Applied Physiology, 25(S1):6-19,2015 • Note: Data showed 12 trained cyclists after 10-d heat acclimation vs 8 matched controls after 10-d in a coolenvironment.

  24. Cooling strategies

  25. Cold water immersion(CWI) • Heat loss to water is four times greater than that to air at the sametemperature. • CWI may take different forms e.g. prolonged duration in tepid water (22-30C; shorter duration in cooler water (17- 18C); cold shower or water spray/mist; recovery cooling between exercisebouts. • Most are effective at reducing skin temperature and often core temperature. Concomitant increases in performance have beennoted. • (Kay et al, Journal of Sports Sciences, 17:937-944,1999 • Once criticism is that blood flow to the active musculature may be reduced causing the athlete the need to re-warm up beforecompetition. • An alternative is selected cooling or part-body immersion of non-active bodyparts. Note: Data show 30-min of self-paced cycling under warm and humid conditions (31C and 60% RH) after either pre-cooling (water immersion) orcontrol.

  26. Coolinggarments • (Arngrímsson et al, Journal of Applied Physiology, 96:1867-1874,2004) • Note: Data show 5-km time trial core temperature after pre-cooling with ice vest or wearing t-shirt(control). • Ice-cooling jackets are a development from the earlier practice of using iced towels for cooling athletes before or duringexercise. • Cooling garments are practical in reducing skin temperature and can be worn during warm-up andrecovery. • Data shows they are effective in reducing thermal strain during early stages of exercise but they can improve exerciseperformance. 26

  27. Cold fluids and iceslurry • Cold fluid or ice slurry ingestion leads to internal heat transfer which may represent an additional method to the four avenues of heat transfer at the skinsurface. • However, ingestion of cold fluids appears to lead to transient reductions in skin blood flow and sweating which reduce evaporative and dry heat loss and negate the additional internal heatloss. • These strategies may be of most use in hot and humid climates where evaporative cooling is impaired or for athletes with physiological disruptions to sweating such as spinal cord injury or burn injuries as their capacity for skin surface evaporative heat loss islower. (Jay & Morris, Sports Medicine, 48(S1):S17-S29,2018) Note: Data show 200-m pacing over 10-km after ice slurry vs ambient temperaturedrink. 27 (Yeo et al, International Journal of Sports Medicine, 33:859-866,2012)

  28. Combining strategies • (Duffield et al, Journal of Strength and Conditioning Research, 23:2524-2532,2009) • Note: Data are from 30-min intermittent sprint conditioning test with pre-cooling (ice vests, cold neck towels and ice pack to quadriceps) or nocooling. • Combining techniques (i.e. both external and internal cooling strategies) has a higher cooling capacity than the same techniques used in isolation, allowing for a greater benefit on exerciseperformance. • For athletes this can be achieved by combining simple methods such as ingestion of ice slurry, wearing cooling vests, and providingfanning. 28

  29. Clothing

  30. Clothing • Dry clothing retards heat exchange. Evaporative heat loss occurs when clothing is wet! • Cottens and linens readily absorbmoisture. • Sweatshirts, rubber or plastic produce high relative humidity close to theskin. • Moisture-wicking garments optimally transfer heat and moisture. • Dark colours absorb light and add to radiant heat; light colours reflect heat rays away from thebody.

  31. Americanfootball • In American football the uniform retards heat loss and adds to the weight of the athlete. Note: Data show walking in shorts only (blue), with shorts and a pack equivalent in weight to an American football uniform (yellow), or wearing the uniform (orange). From: Exercise Physiology: Energy, Nutrition and Human Performance. McArdle WD, Katch FI, Katch VL 7th Edition, 2010. Lippincott Williams andWilkins.

  32. SummaryI • Strategies to sustain/enhance performance during training and competition in the heat, and minimize the risk of heat illnesses include changing behaviour, improving aerobic fitness, acclimatization, body cooling, and fluidintake. • Changes in behaviour can mitigate heat strain to the greatest extent. Whilst rest breaks are a useful strategy to reduce body temperature used in certain sports, the military and selected vocations they may not be a realistic option in manysports. • Aerobic fitness is suggested to be one of the most important measures to favourably alter core temperature for endurance exercise in the heat but improving fitness whilst training in hot and humid environments is morehelpful. • Heat acclimatization/acclimation can help reduce physiological strain and optimize performance in the heat. • Heat acclimatization should comprise repeated exercise-heat exposures over 1-2weeks.

  33. SummaryII • Heat acclimatization results in increased sweating, reduced core temperature, reduced skin temperature, changes to the cardiovascular system and improved enduranceperformance. • Cold water immersion reduces skin and core temperature but may reduce blood flow to active musculature – part-body immersion may be a betterstrategy. • Cooling garments reduce skin temperature and can be worn during warm-up and recovery to aid performance. • Cold fluid or ice slurry ingestion leads to internal heat transfer but also transient reductions in skin blood flow and sweating which negate any additional internal heat loss. Optimal use may be in environment where evaporative cooling is impaired or for athletes with physiological disruptions tosweating.

  34. SummaryIII • Combining cooling strategies is a useful strategy to aidperformance. • The type of clothing worn should be a consideration for athletes competing/training in theheat.

  35. Recommendedreading • Exercise Physiology: Energy, Nutrition and Human Performance. McArdle WD, Katch FI, Katch VL 8th Edition, 2015. Lippincott Williams and Wilkins. Chapter 25: Exercise and Thermal Stress. Pgs.615-642. • Exercise Physiology: Theory and Application to Fitness and Performance. Powers SK, Howley WT. 8th Edition, 2012. McGraw-Hill. Chapter 12: Temperature Regulation.Pgs.261-280.

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