Module 5 - Environmental exercise physiology

• Warm muscles produce more power & force
• Muscles increases 2-3 degrees C in endurance & resistance exercise
• Hot muscle can produce less force & fatigue faster
• Muscle temperature depends on: Time, Intensity, Clothing, Ambient Temperature (Temp of environment)

  • Mostly time/duration & intensity
• One off endeavours improved by hotter climate, but repeated sprinting (RHIE) impaired w/ increasing ambient heat stress

  • Muscle temp affecting power, strength output
• Endurance Performance

  • Heat impairment
  • Cool environment usually a benefit
  • Optimal temp muscle cooler than resting
  • Effect not as big such findings ‘suggest’
• Optimal temperature for endurance exercise depends on:

  • Medium (water or air)
  • Exercise intensity
  • Clothing (help or impair)
  • Body composition & size
  • Exercise mode
  • Environment doing it in (different optimal temps)
  • Air resistance has effect → more convection cooling
  • Individual differences
  • Capacity to sweat
  • Different heat tolerance
• Optimal temp at rest:

  • ~26 C for Air
  • ~34 C for Water
• Optimal temp = non linear (inverted U shape):

  • Perform worse in more cooler and hotter conditions
  • Optimal temp somewhere bw/ the 2
• Problem with heat:

  1. Thermoregulation reduces exercise capacity
  2. Thermoregulatory power is finite & limited by other effects of exercise on the body
  3. Heat stroke can be fatal - if not able to thermoregulate
• Importance of Maintaining Heat Balance (Tolerance)

  • ~42 C - Some athletes tolerate
  • ~40 C - Tolerance limit for many athletes
  • Increase cell permeability
  • ~39 C - Tolerance limit for moderately trained (esp if clothed)
  • Normal cut off point
  • Cells can lose some of their integrity
  • ~38 C - Peak skin blood flow & sweat rate
  • Tolerance limit some folk
  • ~37 C = Resting zone for sedentary vs athletes/acclimated
  • Will maintain slightly lower core temp
  • Note: if no heat dissipated, core temp would rise by ~100 C in a active day
• Resting zone temp for sedentary vs athletes/acclimated = 37 degrees C
• Peak skin blood flow & sweat rate. Tolerance limit some folk = 38 C
• Tolerance limit for many athletes. Increase cell permeability = 40 C
• Body temp sensed by receptors in:

  • Skin
  • Spinal cord
  • Hypothalamus (mainly)
  • Increase sweat & blood flow
• Body temperature is governed by:

  • Heat production:
  • Work rate (mostly)

  • Duration
  • Pattern
  • Efficiency (lesser extent)

  ~ Balanced by heat loss ~

  • Heat Loss:
  • Physical (& behavioural) aspects
  • Environment (can't change)
  • Clothing
  • Movement
  • Physiological responses
  • Skin blood flow
  • Sweating
  • Initial status:
  • Body Temperature (core temp)
  • Body Composition
• Most heat gain from metabolism (66% of heat produced)
• Heat Gain if air temp > skin temp
• Heat Gain (Air temp > skin temp):

  • Metabolism
  • Radiation
  • Convection
  • Conduction (less so)
• Heat Loss if air temp < skin temp
• Heat Loss (Air temp < skin temp):

  • Evaporation
  • Skin (primarily)
  • Resp tract
  • Radiation
  • Convection
  • Conduction (lesser extent)
• Exercise is the major heat stressor; is easily quantified:

  • 75-100% of energy release in exercise “wasted” as heat; only ~0-25% yields work
  • So, Heat Production Rate = Metabolic Rate - Work Rate (Lab 3 & 5 give this)
  • Thus, heat production directly related to work rate (& rise in core temp)
  • Similar for arms or legs (at matched work rates)
• Offloading heat effectively requires evaporating sweat off skin
• Sweat causes dehydration

  • Dehydration decrease heat loss & exercise tolerance
• Environment is a heat sink
• Capacity of environment to absorb heat depends on:

  • Water vapour pressure (absolute humidity)
  • Radiant temp (direct solar)
  • Air movement (relative to skin surface)
  • Resistance to heat transfer (clothing, posture/position)
  • Ambient temp
• Hot and/or humid environments are stressful

  • Harder to offload heat; esp if low evaporative capacity (very humid)
  • Harder to evaporate into air w/ lot of water in
• Ineffectively sweating/not evaporating = not cooling
• Conflict in Demand for Blood Flow:

  • Muscle - increased need of blood flow
  • Skin - increase need of blood flow
  • To offload heat to prevent hyperthermia
  • Problem - to maintain arterial pressure (MAP)
  • To adequately prefuse other tissues (esp Brain)
• Physiological of exercise in heat:

  • Warmer periphery
  • Increase skin blood flow & sweating (can cause dehydration)
  • Increase blood osmolarity
  • Solutes in plasma get more concentrated!
  • Bigger decrease Blood Volume/SV (esp “central BV”)
  • Increase HR & decrease SV (they partly cause each other), to maintain cardiac output (CO) & MAP
  • Increase stress hormones
  • Increase muscle glycogen depletion
  • Increase [Lactate] muscle & blood
  • Increase RPE
  • Increase thermal discomfort
• Exhaustion @ ~38 C if heat intolerant, or ~40 C+ if good athlete

  • Some people less tolerant
• Cardiovascular strain in heat:

  • Decrease blood pressure (arterial & central venous)
  • Decrease blood flow to several tissues & organs
  • Decrease blood flow to skin, muscle & also brain
  • All together: increase HR, decrease VO2 max & sustainable thresholds
• Increase HR with increase temp
• Muscle glycogen depletion in heat stress
• Major personal factors governing performance in heat:

  1. Aerobic fitness
  2. Heat acclimatisation
  3. Initial physiological status

  4. Hydration
• Aerobic fitness personal factor in heat:

  • Accustomed to producing & offloading heat
  • Mainly via increase CV & heat loss capacities (Sweating & Skin blood flow)
  • Sweating: lower onset temp for sweating
  • Start conserving more sodium
  • Greater adaptation to sweat more to remain cooler at given ex intensity + better skin blood flow
  • Small benefit from increase work efficiency
  • If fitter & max is higher
  • Can drop down to 70% VO2 max
  • Have bigger ceiling to reach = benefit
  • Aerobic fitness personal factor in heat:solute & relative ex intensity
• Heat acclimatisation personal factor in heat:

  • Mostly same reasons as for fitness; esp increase plasma volume
  • Acclimation (= artificial, periodic exposures) requires several days of exercise + heat exposure
• Initial physiological status personal factor in heat:

  • Thermal - temp elevated or lower
  • Metabolic - good glycogen reserve?
  • Hydration
  • Illness - slight rise in temp
• Hydration personal factor in heat:

  • Low body water (Hypohydration) impairs several processes:
  • Cardiovascular function - more CV strain
  • Reduced sweat production
  • Cognition impaired
  • Amount of internal heat sink
  • Can wipe out benefits of short term increase in fitness or acclimation
  • Hydration level can wipe out those short term increases of aerobic fitness of acclimation
• Other personal factors affecting performance in heat:

  • Recent illness
  • Medication
  • Body size - small & light best
  • Age - older CV impaired
  • Sex - more Hb in males
  • Fatigue & sleep deprivation