Lecture 16 Exercise in teh heat Flashcards
How does climate change in europe affect exercise in the heat?
Currently the evidence is in workers in countries that we would expect to be associated with hot climates. Central and South American countries close to the equator. Large periods of time in the sun.
However, this might start to increase in importance in Europe, particularly as we experience increasing temperatures related to climate change. This figure highlights the increase in frequency of annual temperature anomalies with 4 of the hottest years occurring in the last 5 years.
* Figure showing average temperatures.
* Red is higher than normal temperatures
What happens when we exercise in the heat?
- Redistribution of blood flow
- Hyperthermia ^ core body temp
- Hypohydration
- Substrate depletion/ availability?
- Lots of things going on here
- Was in Mexico in the middle of the ay
- And humidity was high which stopped sweating.
- Hyperthermal is exacerbated by hypohydration.
- Bring dehydration means there is less fluid available for sweating.
- When we are hot, instead of giving to muscles it shifts to sin and surface to dry
and get rid of all the heat.
Also increased usage of carbo, reduction of substrate availability.
Using more and not delivering more means that we have decrease in substrate availability. - Think about safety and was it safe to run this race?
Explain body temp regulations
- Body temperature is maintained within a narrow range
– Humans are homeothermic - In extreme cases, body temperature will deviate from the normal range (36.1-37.8°C)
– E.g. heavy exercise, fever from illness and extreme hot or cold conditions
Explain the heat balance
M Metabolic heat
W useful energy to muscles - about 75% of work is produced as heat – about 25% efficient
R Radiation surface radiates heat, absorb it from objects – skin constantly radiates heat – at rest about 60% of heat loss
K conduction – touching of surfaces – usually negligible during exercise
C convection – movement of gas or liquid over a surface – constantly removes metabolic heat
E evaporation – major method of heat loss, requires sweat to evaporate from the skin, can’t be just removed – 80% total heat loss. Clothing adds resistance
* Looking at how it is measured
- Radiation : you give ff radiant heat that other object gives off
- Biggest contributor to radiant heat in the sun
- Conduction = solid to solid contact
- In terms of the cycling on right they are travelling faster so will have lots of convection cooling
- It’s difficult to do this in the lab.
- Cycling is traveling vast so will have lots of convection cooling.
- Being able to simulate a 30-40km wind in a chamber is very difficult as we don’t get the same airflow
- Biggest loss is through evaporation.
- The process of sweat / fluid evaporating from the skin.
- We are normally 255 efficient 75% turns in to heat gain.
What do we measure?
- Core temperature – Tc
- Skin temperature – Tsk
- Whole body sweat rate
- Local sweat rate: if we have a helmet on etc
- HR
- VO2/ VCO2
- Plasma volume changes: change in blood volume of we are sweating are we loosing blood volume.
- Performance
- Thermal comfort
- RPE
- Blood flow
How do we measure core temperature?
- Thermometer
- Ear temperature is the most easily accessible one.
- Rectal probe
- Ingest pill the night before and can track the temperature until the pill has passed.
What is maximal core temperature
- There is a theory that there is a maximal temperature.
- There is evidence that we can go above these extreme temperatures.
- Its I extreme cases and is very individual
- Shows data from the world championships in cycling.
- Some of the medallist reaches 41.5, they are a whole degree over wat would be classed as heatstroke
- And over 2 degrees over what we would have to stoop at.
- We would stop them well before they reached maximal performance.
- Very individual response
- And it’s very hard to quantify as we have a safety gap where we don’t wsnt t put athletes under too much pressure.
How do we measure temperature of the skin?
- Thermal imaging cameras
- Temperature gun.
How do we control temperature: Thermoregulatory control?
- Internal body temperature is closely regulated
- Regulated by the preoptic-anterior hypothalamus
- Thermoreceptors detect changes in temperature
- Works on a negative feedback loop
- Controlled by the POAH
- Thermoreceptors detect change and relay back to central receptors located in hypothalamus
- detection in temp
- We get 2 main reactions vasodilation and vasoconstriction
- Signals through the sympathetic nervous system
- Vasodilation increases skin blood flow, decreases blood to main organs and also to working muscles i.e. Jonny Brownlee’s legs give way
- Means that we can dissipate heat
- But we then loose heat from other areas.
- Increases sweat evaporative losses
What are the physiological responses to exercise in the heat.
- Cardiovascular function (big effect)
– Skin arterioles vasodilate ↑ skin blood flow
– ↑ Cardiac output : we want to be able to maintain cardiac output during performance (SV X HR)
– If we are seeing a shift in blood flow we need to increase the cardiac output an to do this we need increases stroke volume or heart rate.
– ↓ Stroke volume (sweat) ↑ HR (CV drift)
– When we sweat we looks the plasma in the blood, so we have less stroke vole so less cardiac output
– So therefore we will need to increase the heart rate but increasing HR has a limit.
– Heart cannot provide sufficient blood flow to heart and skin : cant just keep increasing heart rate, once we reach the limit an stroke volume is going down then performance will decrease.
– ↓ Performance - ↑Sweating ↓ Blood volume ↓Cardiac output
- Rectal temp increase as we can not effectively cool
- HR increases as SV decreases.
- Cardiac output decrease after. Awhile as stroke volume has to decrease an performance decreases.
- Cardiac output cant be maintained.
- To see how it affetcs us in the heat there are a series of studies shown below.
Explain how we can determine heat stress through a wet. bulb globe thermometer
- Takes into consideration dry heat and wet heat (humidity)
- The value this give stends to be lower than the temperature we re working with throughout the day
- So we need to understand the measurement.
How can fluid intake help duirng exercise in the heat?
- Fluid intake maintains or reduces the decrease in plasma volume arising from fluid lost as sweat
– Helps maintain stroke volume and cardiac output
– Therefore maintain or prevent reduction in performance
Can it be used to increase performance
Would it have altered skin blood flow? Hepatic blood flow to get liver glycogen?
How can we use heat acclimatisation to help prepare for exercise in the heat?
- Heat acclimation – Physiological changes occur over a short period of time ( such as training camp)
- Heat acclimatisation – Physiological changes because of living in a hot climate for months/years
- Becoming adapted.
- By performing a period of heat acclimation athletes can become develop beneficial physiological changes and adaptations that may help maintain or prevent reductions in performance
– The heat acclimation period can vary in duration, methods, intensity
– Often performed before competitions or military deployment
– Typically done over a 5-14 day period
– Can vary in terms of methods
– Performed before comp of before military performance.
What adaptations do we get from heat acclimatization?
- Showing all the adaptations that we get and the time frame.
- We have short term which plateau after a short period and long term heat accumulation which last longer
- Plasma volume expansion means that we have a bigger stroke volume so therefore we can increase cardiac output.
- Short term and long term heat accumulation
- Drop in heat rate as we have increase in the fluid
- If we have plasma fluid expansion then we can have a lower
- Then also we will be. A lot more comfortable in the heat.
What are the methods of heat acclimatisation?
- Bassline testing : 30 mins of exercise in the heat monitoring core temp HR and perceptual measured also, we are getting a response,
- Then there are mainly 3 options
- Fixed duration: 90 mins in the hear mixed workload.
- Is a progressive: exercise until you reach core temp of 38.5, then we go up and down manipulating the workload, so if on bike will go to 300 watts toll we meet the temperature and then bring it up and down.
- Or exercise until reach 38.5 however long that take
- Don this for short or long term, and the repeat the baseline testing with the same measurements.
- Need to consider thing for the athlete.
- Do it regardless of the weather
- As a coach or an athlete, we need to consider things such as endurance training
- We need to think about how we don’t normally go by temperature as a means of testing.
- Doing this for 14 days before the Olympics in the heat it won’t really work
- Must work out how we can do heat accumulation training during the week.
- So many considerations about when we fit this in, and how we fit it in alongside all of the other training.
- Also need to think about altitude and heart
- Getting heat adaptations and altitude adaptations.
- Thinking about can we do both at the same time
- It’s a big management and very difficult to do.
