Exercise & sport in the cold

Exercise & sport in the cold

Thermal Balance • Core temperature declines in the cold when heat loss exceeds heat production • Core temperature rises if heat gain exceeds heat loss • Humans can tolerate a decline in deep body temperature of 10°C but only an increase of 5°C (88F or 103F)

Long-term Cold Exposure • Hypothalamus acts as a thermostat • Initiates the body’s heat-regulating mechanisms: • Thermal receptors in the skin • Direct stimulation (detect change in blood temp) • Increases BMR, HR, respirations

Thermoregulation in Cold Stress: Heat Conservation & Production • Stimulation of cutaneous cold receptors constricts peripheral blood vessels, reducing the flow of warm blood to the body’s surface and redirecting it to the core • Epinephrine and norepinephrine increase heat production during cold exposure • Prolonged cold stress stimulates release of thyroxine (increases resting metabolism)

Heat loss • Circulation: At rest in the heat, heart rate and cardiac output increase while superficial arterial and venous blood vessels dilate to divert warm blood to the body shell • Evaporation: An effective thermal defense exists when evaporative cooling combines with a large cutaneous blood flow • Hormonal Adjustments: Sweating produces loss of water and electrolytes

Heat loss (cont.) • Why is it important to keep your head covered when out in the cold? • No vasoconstrictor fibers to the head; therefore the head always remains unaltered

Temperature Regulation • Shivering • Autonomic response to falling core temperature • Metabolic heat • No work is done, almost all energy contributes to generation of heat • Physical activity provides the greatest contribution in defending against cold

Core Temperature During Exercise • Heat generated by active muscles can raise core temperature to fever levels that would incapacitate a person if caused by external heat stress alone • Fatigue generally coincides with core temperatures between 100.4F/38C and 104/40°C, which impairs muscle activation directly from a high brain temperature that decreases the central drive to exercise

Exercise in the Cold • Core temperature becomes further compromised during chronic exertional fatigue and sleep loss, inadequate nourishment, reduced tissue insulation, and a depressed shivering heat production • Exercise energy metabolism sustains a constant core temperature in air as cold as -22°F

Exercise in the Cold • Wear several thin layers of clothing • Add or remove during exercise as needed • CLO units = measure of insulation of clothing • 1.0 clothing is ordinary business dress • Should be comfortable at 70F • N & S poles need 12 CLO • Sweating becomes an issue, especially when exercise is finished

Body Fat, Exercise, & Cold Stress • Differences in body fat content among individuals influence physiologic function in the cold during rest and exercise • Successful ocean swimmers possess a larger amount of subcutaneous fat than highly trained non-ocean swimmers; additional fat = insulation

Water Immersion • Water has 25% greater thermal conductivity than air & 350 times greater heat capacity than air • Greater the swimming intensity the colder the water. • Sensory deprivation tank: 92 F • Learning to swim: 86 F • Actively swimming: 80 F • Competitive swimmers: 77F

Muscular Performance in Cold • Every 2 degrees drop in F temp = 3% drop in muscle power • Normal skin temp: 91.4F • Extreme thermal discomfort < 77F • Extremity temp: • 73F = clumsiness • 68F = impact on skin has to be 6x greater than normal for usual sensation to be registered • 59F = loss fine manipulative performance

Muscular Performance in Cold(cont.) • Joint synovium becomes more viscous creating stiffness • Joints are needed in order to survive • If you are stranded, it’s not a bad idea to start walking to stay warm. • When fatigued, stopping to rest or eat/drink is detrimental • Unable to generate enough heat & losses are greater

Optimal Exercise Temps • Endurance on stationary bike = 52F • Distance runners = 57F • Climbers = anything above 0F

Cold Stress and Children • Large ratio of body surface area-to-mass is a liability during cold stress  body heat dissipates rapidly • During exercise in the less stressful cold-air environment, children rely on: • Augmented energy metabolism • More effective peripheral vasoconstriction in the limbs • Brown fat = thermogenesis • Why more in kids?

Cold Stress and Children • A child’s distinctly large ratio of body surface area-to-mass facilitates heat loss in a warm environment but becomes a liability during cold stress because body heat dissipates rapidly • During exercise in the less stressful cold-air environment, children rely on: • Augmented energy metabolism • More effective peripheral vasoconstriction in the limbs • Brown fat = thermogenesis • Why more in kids?

Injuries from Cold • Frostbite • Ice crystals form in skin • Temp below 30.2F • Recent studies suggest not to be too aggressive in cuttingoff the black tissue • Once “bit”, reoccurrence is easy • Tissue is painful and hard, difficult to heal • Hypothermia • Cold Shock

Frostbite • Predisposing factors to frostbite: • Alcohol use  vasodilator = accelerated heat loss • Low physical fitness • Fatigue • Dehydration • Poor peripheral circulation • Susceptible areas: • Face, ears, fingers/hands, feet/toes • Tingling, numbness, burning

Injuries from Cold • Frostbite • Hypothermia • Cold, water, wind • Occur during any season • Sense of cold & shiveringnumbness & uncontrollable shiveringspeech slurs, thought process slowserraticmvmts, skin swells/blue unconsciousnessdeath • Cold Shock

Injuries from Cold • Frostbite • Hypothermia • Cold Shock • Ice cold water, fatal within 5-10 minutes • 7 steps: • Peripheral vasoconstriction •  HR, CO •  systolic pressure • Immediate hyperventilation •  muscular activity, inability to swim 100m •  breath/hold time • Drowning, death by hypothermia uncommon

Survival times in water • Naked  30 min • 1.0 CLO  60-80 min • 5mm wet suit  120-150 min • Dry suit  300 min

Acclimatization to Cold • Humans possess much less capacity for adaptation to long-term cold exposure than to prolonged heat exposure

Acclimatization to Cold (cont) • Cold adaptation occurs with regular, prolonged exposure • Body regulates at a lower core temperature • Repeated cold exposure of hands/feet increases blood flow through these tissues • Shivering occurs at a lower body temperature • Improved ability to sleep in the cold • Changes in peripheral blood flow distribution • 5 – 2 minute cold immersions will reduce “cold shock” response by half

Wind-Chill Index • Air currents on a windy day magnify heat loss because the warmer insulating air layer surrounding the body continually exchanges with cooler ambient air • The wind-chill temperature index provides a useful way to understand the dangers from winter winds and freezing temperatures and provides frostbite threshold values

Exercise & sport in the cold