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Running in the Heat & Humidity

What the Summer Weather Does to Your Body and How to Improve Athletic Performance

Take caution against sunstroke and dehydration, photo courtesy Scoll22

If there’s one thing runners in Texas understand, it’s heat. Some seem to tolerate it more than others, but too much of it can be deadly. Have you ever wondered exactly how and why it affects you like it does? I mean the real nuts and bolts explanation? Dr. McEntire has condensed a huge body of knowledge into this concise bulleted presentation. Read on; you’ll learn how to cope with hot weather running and perform better on race day.

Thermoregulation^1-3

Responsible for maintaining adequate heat production and sufficient heat dissipation
Normal skin temp: 33°C (91°F), range 32-35°C
Normal core body temperature: 37°C (98.6°F)
In the heat:

Skin is only region directly vasodilated by heat stress
Skin temperature increases before core temperature
Skin blood flow must then increase
Sweating begins within 2 seconds

Exercise^4-6

Core temperature increases during exercise in relationship to exercise intensity
Heat production is 15-20 times greater than rest
- ~70% of energy produced is released as heat instead of energy for muscles
Causes increase in core temperature by 1°C for every 5 min of exercise w/o heat loss
- Thus heat production exceeds heat loss
Heat & humidity:
- HR increases up to 10 bpm when temps are 75-90°F
- HR increases up to additional 10 bpm when humidity is 50-90%
- Performance decreases by ~20% when temps are above 80°F

Heat dissipation^{1, 7}

What areas of body are most important for heat dissipation?
- Forehead
- Upper limbs
- Trunk
- Lower limbs
Heat is transported by blood from muscles to skin by:
- Sweating (evaporation)
  - Primary mechanism for heat dissipation
  - 2/3 of evaporated fluids comes from skin (1/3 from respiration/breathing)
  - 1.6-4 million sweat glands
  - Sweat an average of 1.4 L/hr (max 3 L/hr)
  - High humidity
    - Air can’t absorb sweat so sweat drips from body
- Increased skin blood flow (convection)
  - Transfer heat from core to skin
  - Stimulates sweat response
Core temperature can reach dangerous levels within 10 min w/o adequate dissipation

Heat related illnesses

~240 people/year die⁸
3 types^4-6
- Heat cramps-sharp stabbing pain typically in legs or diaphragm
  - Caused by electrolyte deficiencies/imbalances
  - Treatment-stop running, fluids/electrolytes, cool body
- Exertional hyperthermia-
  - Symptoms
    - Core temp 39-40°C (102.2-104.0°F)
    - Excess sweating causing fluid volume depletion (6-10% of body weight)
    - Headache, nausea, vomiting, fatigue
    - High HR
  - Treatment
    - Seek medical help-can progress quickly
    - Fluids/electrolytes
    - Cool body via immersion
    - Elevate feet above level of heart
- Exertional heatstroke
  - Symptoms
    - All of the above
    - Core body temperature > 40.5° (104.9°F)
    - Mental changes-confusion, disorientation, loss of consciousness
    - Seizures, coma
    - Organ failure
    - Death
  - Treatment
    - IMMEDIATE medical treatment!!
    - Cool body-ice/immersion
    - No fluids if unconscious/severely disoriented/seizing, etc.
Risks factors for developing a heat related illness⁹
- Low fitness level (<40 ml/kg/min)
- Dehydration
- Unacclimated
- Overweight/Obese (BMI >27)
- Illness
- Prior heat illness
- No sleep
- Heavy clothing
- Medications/supplements
- Air temp >33°C (91°F) and/or low air velocity

Prevention

Adjust pace for heat (~2-3 min slower per mile)
Take walk breaks regularly & often
Hydration

Critical in 24 hours before a run
Thirst is not an indicator of dehydration but increased HR and dark urine are
Body absorbs fluids best when cold (40’F is ideal)
Rule of thumb= 4-8 oz. of fluids every 15-20 min
Calculate fluid needs/losses (nude wts best)

Needs during run
- Wt post – Wt pre + Fluids Consumed = Total fluid lost
- Total fluid lost divided by number of miles ran =amt of fluid needed per mile (ie 122 lb – 124 lb + 40 oz = 38 oz lost; 38 oz/5 miles = 7.6 or ~8 oz/mile to avoid dehydration)
Rehydration
- Wt pre – Wt post = lbs lost x 16 oz (ie 124lbs – 122 lbs = 2 lbs lost; 2 x 16 oz = 32 oz needed to rehydrate)

Do NOT drink so much water that you flush out (pee) or have too little electrolytes

Fluids with electrolytes, eat small amts of foods w/sodium 12 hrs before
Hyponatremia-abnormally low sodium levels, excess water moves into cells, causing them to swell/burst
- Symptoms can mimic heat illness
  - Elevated core temperature
  - Nausea/vomiting
  - Headache, confusion, irritability
  - Fatigue
  - Muscle weakness
  - Seizures
  - Loss of consciousness, coma¹⁰

Acclimation^{7, 11, 12}

~2 weeks of moderate intensity exercise, 30-100 min in duration in the heat
Kids need ~ 2x the number of sessions
Adaptations can be lost in as little as 10 days
Results in:
- More sweat production but less sodium lost in sweat
- Better control of core body temperature fluctuations
- More efficient heat dissipations mechanisms

Time of day

Early morning or evenings
Run when your shadow is taller than you are!
Avoid running between 12-3pm

Clothing

Light-colored, breathable, moisture-wicking fabric
Sunscreen
8 oz cotton t-shirt can soak up 4x its wt in sweat making it weigh 2 lbs-yikes!
Switch from a hat to a visor

About the author: Serina McEntire, PhD, is an Exercise Physiologist in the Dept of Emergency Medicine at the University of Pittsburgh. She has more than 10 years experience in the health and fitness industry working with a wide range of clients. She also coaches a half marathon training group.

References:

Rowell, L.B., Human Circulation: Regulation During Physical Stress. 1986, New York, NY: Oxford University Press. p. 174-212.
Johnson, J.M. and D.W. Proppe, Cardiovascular adjustments to heat stress., in Handbook of Physiology. Environmental Physiology. 1996, Am. Physiol. Soc.: Bethesda, MD. p. 215-243.
Rowell, L.B., Cardiovascular adjustments to thermal stress., in Handbook of Physiology. The Cardiovascular System. Peripheral Circulation and Organ Blood Flow., J.T. Shepherd and F.M. Abboud, Editors. 1983, American Physiological Society: Bethesda, MD. p. 967-1023.
Barrow, M.W. and K.A. Clark, Heat-related Illnesses. American Family Physician, 1998. 58(3): p. 749-756, 759.
Armstrong, L.E., D.J. Casa, M. Millard-Stafford, D.S. Moran, S.W. Pyne, and W.O. Roberts, American College of Sports Medicine position stand. Exertional heat illness during training and competition. Med Sci Sports Exerc, 2007. 39(3): p. 556-572.
Armstrong, L.E., Y. Epstein, J.E. Greenleaf, E.M. Haymes, R.W. Hubbard, W.O. Roberts, and P.D. Thompson, American College of Sports Medicine position stand. Heat and cold illnesses during distance running. Med Sci Sports Exerc, 1996. 28(12): p. i-x.
Shibasaki, M., T.E. Wilson, and C.G. Crandall, Neural control and mechanisms of eccrine sweating during heat stress and exercise. J Appl Physiol, 2006. 100(5): p. 1692-1701.
CDC. Heat-related illness and deaths-United States, 1994-1995. MMWR Morb Mortal Wkly Rep., 1995. 44: p. 465-468.
Armstrong, L.E., D.J. Casa, M. Millard-Stafford, D.S. Moran, S.W. Pyne, and W.O. Roberts, American College of Sports Medicine position stand. Exertional heat illness during training and competition. Med Sci Sports Exerc, 2007. 39(3): p. 556-572.
http://www.bing.com/health/article/mayo-126647/Hyponatremia?q=hyponatremia
Armstrong, L.E. and C.M. Maresh, Exercise-heat tolerance of children and adolescents. Pediatr Exerc Sci, 1995. 7: p. 239-252.
Johnson, J.M., Physical training and the control of skin blood flow. Med Sci Sports Exerc, 1998. 30(3): p. 382-386.

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Running in the Heat & Humidity

What the Summer Weather Does to Your Body and How to Improve Athletic Performance

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